Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: ilyaa on March 10, 2014, 04:46:46 PM

Title: sound city clean-up
Post by: ilyaa on March 10, 2014, 04:46:46 PM
this might be a long one....

its a Sound City Concord combo

(schematic attached)

i inherited this amp from a friend - "if you can fix it, you can have it!" - another mutual friend had worked on it once upon a time with no success.

its already a complicated, cramped mess in there, but add to that the fact that i think the previous 'tech' may have tried out some voodoo of his own, and i really have no idea what's going on.

with a limiter in-line, the pilot lamp lights (dimly) but that's about it. no heaters no nothing. so im stuck at the first step trying to figure out: is the amp getting power? the light bulb does NOT light at all.

here are some brief measurements/observations so far:

1) something weird with the mains switch: the pilot light is hooked up DIRECTLY to the same pin as the "N" mains connection (rather than being on the other side of the switch, as it says in the schematic). on the other side of the switch is a blue wire that goes to a node that goes somewhere into the PT.

2) with the amp turned on (but on standby, of course), and one DMM lead hooked up to the "N" side of the mains, i can read ~120VAC all over the place: both main filter cap terminals, tube heater pins....??

3) the main filter caps have little lumps on the bottom with some white looking flaky residue. i assume that's bad!

4) there seems to be some kind of modification on the output of the bridge....an additional 47nF cap and 22K resistor and another diode....doing something i dont know.

i will go in and do a more careful trace to see what's going on and report back. its pretty confusing and hard to pick apart but ill start with the power section and see if i can at least figure that out.
Title: Re: sound city clean-up
Post by: DrGonz78 on March 11, 2014, 01:49:06 AM
with a limiter in-line, the pilot lamp lights (dimly) but that's about it. no heaters no nothing. so im stuck at the first step trying to figure out: is the amp getting power? the light bulb does NOT light at all.

What is the wattage rating on the bulb? This is important to know and the wattage of the amp per rating too.

Edit: Ignore my response here... I misread... Although from ground reference do we have voltage on any of the heater pins? (Perhaps your feeling about the way the pilot light is wired has something to do with this...?)

1) something weird with the mains switch: the pilot light is hooked up DIRECTLY to the same pin as the "N" mains connection (rather than being on the other side of the switch, as it says in the schematic). on the other side of the switch is a blue wire that goes to a node that goes somewhere into the PT.
So part of the pilot light goes to neutral where is the other side of the lamp connected? Pictures say a 1000 words...

2) with the amp turned on (but on standby, of course), and one DMM lead hooked up to the "N" side of the mains, i can read ~120VAC all over the place: both main filter cap terminals, tube heater pins....??
This is alarming as ever!! I would assume that the filter caps should not have AC voltage readings other than a small ripple. Is there a reference to ground? You should not be reading voltages in this manner and it is very dangerous. Meaning do not connect any lead to Neutral as your "Ground reference". Is the amp safety grounded or does it have the isolation cap(i.e. Death cap)?

3) the main filter caps have little lumps on the bottom with some white looking flaky residue. i assume that's bad!
It not a good sign... Although they still may function, but they should be changed. First diagnose the fault on the amp and that may include the caps.

4) there seems to be some kind of modification on the output of the bridge....an additional 47nF cap and 22K resistor and another diode....doing something i dont know.
Restoring the amp to stock will be very important to really get the amp working again. I mean it may be easier to get it back to stock as a way to truly repair it.
Title: Re: sound city clean-up
Post by: DrGonz78 on March 11, 2014, 05:30:54 AM
1) something weird with the mains switch: the pilot light is hooked up DIRECTLY to the same pin as the "N" mains connection (rather than being on the other side of the switch, as it says in the schematic). on the other side of the switch is a blue wire that goes to a node that goes somewhere into the PT.

I did not look at the schematic at first when I answered you in previous post. I understand better now what you are saying there and that it is bypassing the switch. Or should I say it is in front of the switch... Since the switch is double throw it may make no difference as it is cut from the circuit either way when it is turned off. Still I would figure out where it is supposed to be soldered (correctly) after the switch as the schematic shows, good call.

****Edit****
One more thought on this one...
2) with the amp turned on (but on standby, of course), and one DMM lead hooked up to the "N" side of the mains, i can read ~120VAC all over the place: both main filter cap terminals, tube heater pins....??
I was thinking you might have been reference ground on certain parts of the amp. For example, the caps do go to ground on the negative side, so they were your ground reference. You were then seeing the 120vAC at the neutral connection since the amp was on.
Title: Re: sound city clean-up
Post by: Roly on March 11, 2014, 12:12:32 PM
As you may have already noticed, this amp contains valves (toobs if you must) and requires a few differences in approach to a solid state amp.  The good news is that as valve amps go this one is pretty conventional.  The bad news is that unlike a solid state amp it isn't going to behave at all well under reduced voltages.

Normally the first thing I do with any repair is check the mains plug, lead, and chassis grounding.  American mains practice differs from the 240VAC L.E.N. world so I'll have to leave that to those with American mains experience, DrGonz &c.  I will say however that the mains pilot light should be wired to the transformer side of the mains switch, not the mains side.  As this amp has been fiddled with I would restore the mains wiring as per the circuit.

Check the fuses F1 and F2 are the right value and intact.

A major difference you have already encountered - it has a heater circuit which draws significant power, and in this case it is fitted with EL34's (6CA7's) which are absolute heater hogs.  The first thing to do is gently remove the two output valves by gripping the base and gently rocking them out of their sockets, then putting them somewhere safe.

And talking of "safe" you have to be well aware that the main HT (B+) supply is delivering several hundred volts to the output stage.  While is is unlikely to kill you if you are in reasonable health, you will find getting connected to it a very disagreeable experience indeed.

This particular amp is fitted with HT bleeders (R55 and R56, R54 and R5?).  Make it a practice after each power down to put your meter on the HT line and watch it sink (the Standby switch can be left on).

Visually inspect R51 and R52, 1k connected to pins 4 of the output valves.  These very frequently burn up in amps using EL34's.

Check the diodes in the main bridge rectifier.

If these are okay then switch Standby off clip your meter across the HT supply at the main filter caps C34 and C35, and using a large limiting lamp, say 100W, power the amp up and observe the voltage; it should be high, perhaps around 350-400V, but may be higher or lower.

Now turn the Standby switch on and check that you are still getting around the same HT voltage.  You should also check that you are getting minus 40-50V to chassis across C34, the bias supply.

Turn off and watch the HT decay so you get an idea of how long it takes to discharge to a safe level, say around 20-30V.

At this point you can remove the limiting lamp and power the amp directly from the mains, again watching the HT voltage (which will be somewhat higher).  All the valves should light up.  Note that the preamp valves may light very brightly at the bottom for a second or so - this is perfectly normal.  Check that both sides of each preamp valve lights up.  For any that don't try gently rocking the valve in its socket.  any that still don't light should be gently rocked out of their socket and both heaters checked with your ohm meter.  If any of the heaters tests open that's the end of the line for that valve.

The ECC83 is a 12AX7 (still made), while the ECC81 is a 12AT7 (not still made and rather harder to find).  Don't get these mixed up (write the type on the chassis next to each socket).  Mixing them up won't do any harm, but it will lead to some rather strange results.

Set the tone controls for half way, 5/10, Reverb to minimum.

Feed a signal in and signal trace it along its respective channel to the Phase Inverter (V5A&B) and to the grids of the output valves, pin 5.  Be careful, pins 3 and 4 have HT on them.

That's enough for now, should keep you busy for a while.

Note: a valve output stage must never be driven without a suitable load, speaker or dummy load, securely connected, otherwise the output transformer may well be damaged.
Title: Re: sound city clean-up
Post by: g1 on March 11, 2014, 12:23:06 PM
  Don't connect your black probe to neutral, connect it to the chassis (ground).
Is it a three prong plug and is it wired correctly?
Don't worry about the pilot lamp if it is turning on and off with the power switch, as long as one side of the lamp is switched (hot side) you should be ok.
  Is the voltage selector set correctly?
  The caps that look bad may need to be replaced with higher voltage versions due to the increase in modern line voltage.
 The diode etc. off the bridge may be a revised bias arrangement.  Leave it in place till you have drawn it out and know what is going on with the circuit.
You have a dim pilot lamp, what is the AC across the PT primary and PT secondary (into the bridge) ?
Title: Re: sound city clean-up
Post by: ilyaa on March 12, 2014, 03:59:07 AM
quick question before i get into it (thanks again, Roly!):

the mains fuse (3A) tests okay (short) with meter but has a little bubble on it - that okay?

is that just a variety of slow blow?
Title: Re: sound city clean-up
Post by: DrGonz78 on March 12, 2014, 06:07:08 AM
Yeah it looks like a little solder blob right in the center of fused wire right? It's all good no worry about that  :dbtu:
Title: Re: sound city clean-up
Post by: ilyaa on March 14, 2014, 04:48:35 AM
ookay - so:

no voltages on the amp side of the PT.

i tried to untangle the mess thats been made of the wiring coming in from the mains. the mains plug and lead itself seems all good, with the earth wire properly grounded to chassis.

first issue is the mains switch:

the white wire from the mains plug (is that the L connection?) goes to a brown wire, via the mains switch, that then goes to the mains fuse.

the black wire from the mains plug (the N connection?) attaches directly to one side of the pilot light, and then goes to a blue wire, via the switch, that goes into the transformer (bottom of the primary,  i assume, at least per the schematic). should i rewire the pilot light onto the transformer side of the switch?

the other side of the pilot light goes to a green/yellow wire that then hooks up with an orange wire that disappears into the transformer (top of the primary, depending on the mains voltage switch?).

IF i take an AC reading with my DMM, i can only get 120VAC reading from the BLACK wire to chassis. nothing if i read the white wire to chassis. is that normal?

i get ~35VAC from the pilot light to chassis (???).

second (bigger) issue:

the reason im not getting anything inside the amp is because the mains voltage selector switch does not seem to be working. there is NO connection between it and the mains fuse. it looks like there should be, but either the switch broke internally or it needs to be turned. i have to say it scares me a little bit - seems like if i turn it the wrong way im misinforming the power transformer as to the voltage its going to get....would that be catastrophic? im not sure what to do. should i take a picture of the switch??

heres a description: the center pin connects to the fuse. around it there are 6 pins. a pair is connected together and connects via a fat yellow wire into the transformer. another pair is connected together and goes to the green/yellow wire that hooks up to the other side of the pilot light and ALSO to another wire that goes into the PT. NONE of these pins have a connection to the center pin (DMM tested)

i get 0VAC from the mains fuse to chassis.



Title: Re: sound city clean-up
Post by: DrGonz78 on March 14, 2014, 06:19:49 AM
the white wire from the mains plug (is that the L connection?) goes to a brown wire, via the mains switch, that then goes to the mains fuse.
Yes that is Line voltage "Hot". That will be fused.

the black wire from the mains plug (the N connection?) attaches directly to one side of the pilot light, and then goes to a blue wire, via the switch, that goes into the transformer (bottom of the primary,  i assume, at least per the schematic). should i rewire the pilot light onto the transformer side of the switch?
Like G1 said don't worry about that at this point. The light attached there is not gonna cause any problems since there is a double throw switch. Do it later and focus on the main problem, the transformer.

IF i take an AC reading with my DMM, i can only get 120VAC reading from the BLACK wire to chassis. nothing if i read the white wire to chassis. is that normal?
Yes normal and safe :-) Neutral to ground should be Zero or very very close to zero volts AC. If you ever measure voltage from neutral to ground, then call an electrician.

second (bigger) issue:

the reason im not getting anything inside the amp is because the mains voltage selector switch does not seem to be working. there is NO connection between it and the mains fuse. it looks like there should be, but either the switch broke internally or it needs to be turned. i have to say it scares me a little bit - seems like if i turn it the wrong way im misinforming the power transformer as to the voltage its going to get....would that be catastrophic? im not sure what to do. should i take a picture of the switch??

heres a description: the center pin connects to the fuse. around it there are 6 pins. a pair is connected together and connects via a fat yellow wire into the transformer. another pair is connected together and goes to the green/yellow wire that hooks up to the other side of the pilot light and ALSO to another wire that goes into the PT. NONE of these pins have a connection to the center pin (DMM tested)

i get 0VAC from the mains fuse to chassis.
Yes take a picture of this and let the crew here see. This scares me too. Got to see where the switch is located and how it might be broken. Do not power it up until you get instruction from senior techs on the site.

Except that part about the mains fuse voltage... So on one side you have 120vac and 0vac on the other? Are you sure that fuse is good?
Title: Re: sound city clean-up
Post by: Roly on March 14, 2014, 07:28:59 AM
WTF were they doing stuffing around with the mains side wiring I wonder?

Quote from: ilyaa
should i rewire the pilot light onto the transformer side of the switch?

Not urgent, but before you finish it really should be wired on the inboard side of the mains switch.

Quote from: DrGonz78
This scares me too. Got to see where the switch is located and how it might be broken. Do not power it up until you get instruction from senior techs on the site.

Too, too.

There are a few things about this mains side mess that I don't like, but in my world using green+yellow stripe for anything other than a mains safety ground is an absolute nono and likely to get you dragged out the back and severely beaten.  :trouble   There are not too many absolutes in the world of electrics, but in the 240VAC LEN part of the world this is one of them.  It seems to have been used as part of the mains wiring and as such is called a "death trap" because it is saying "I'm safe" when it isn't.

I'm with the Doc on this one, post pix, do not power up (we'd hate to lose you at this point after so much Win!).

In the meantime (with the amp well isolated) have a look at the resistance of the leads coming out of the mains side of the tranny.  The circuit shows a "C" for common, then taps for 105V, 115V, 225V and 245V.  These should have resistances to C that increase as their voltage increases, so you should be able to map these tappings to cable colours which is bound to be a help, and also prove the tranny primary is still intact.  At least on 115V mains an incorrect setting won't cause the damage applying 240V to a lower tap would cause here.

Always remember to check the mains plug is unplugged (not just switched off) and the HT/B+ line is fully discharged before diving inside.


{It sounds like an idiot has been at this one ilyaa, so be on the lookout for boobytraps, IED's, and pure idiocy; be suspicious, be very suspicious - double check everything, make no assumptions, particularly on the mains side}
Title: Re: sound city clean-up
Post by: g1 on March 14, 2014, 01:27:33 PM
  It sounds like the white and black are reversed at the switch.
Black should connect to brown (to fuse) when switch is turned on, white should connect via switch to blue.
  The orange wire may be the transformer case, which is shown on the schematic as connecting to the green earth wire of the mains.

  With some pictures and transformer primary resistance readings, it should be easy to bypass the voltage selector switch for test purposes,  or hard wire for 120V operation.
Title: Re: sound city clean-up
Post by: ilyaa on March 15, 2014, 02:05:16 AM

EDIT: i removed the stupid links! for pix see this site: http://dropcanvas.com/nl0yl

Quote
  It sounds like the white and black are reversed at the switch.

i thought so, too....isnt the Black wire supposed to be L(ine)? remember, the 120VAC readings im getting are BLACK wire to chassis (meaning I am reading from the bottom of the pilot light to chassis). I get 0VAC from the white wire (mains fuse) to chassis.......????

Quote
using green+yellow stripe for anything other than a mains safety ground is an absolute nono

if you look at the pictures marked "Standby wiring" you will see that the green/yellow striped wire was cut from that bundle and possibly spliced (????) onto the pilot light side, where it now goes to the pilot light....its hard to tell whats going on and why. where SHOULD that wire (or a wire of that sort) go to/from in the normal amp world?

Quote
have a look at the resistance of the leads coming out of the mains side of the tranny

with the amp unplugged, i did this, but the results were not encouraging. first of all, there are no leads that are not connected to something - where do the other (245V/225/105,etc) taps go normally when they are not in use? the only transformer taps that were readily available were the ones attached to the mains selector switch and two blues ones (as in the file "PT 1"), one lighter blue and one darker. the darker one goes back to the mains switch - switching to the black mains lead and the pilot light - and the lighter one to the grounding selector switch (the file "grounding cap"). with my DMM hooked to the node where these two blue wires connect to a thick black wire from the PT (common point on primary winding?), i get 0 ohms/dead short to BOTH sides of the mains selector switch - the side that goes to a fat yellow wire into the PT and the side that goes to a thinner red wire that goes into the PT (that node also goes the green/yellow stripe that goes back to the pilot light).

i am getting a short between the fat yellow and the thin red wires going into the PT.

my hope, i guess, is that the mains selector switch is shorting things together, not the PT! remember, i get NO connection between the outer nodes of the mains selector switch and the middle pin (which goes to the mains fuse).

there are a few other pix that are more for your interest at this point, seeing as the amp has bigger fish to try. one other thing i will mention is that (as per the picture "fuse_to_ground") i disconnected the wire that goes from the fuse to the grounding selector switch, thinking that, since the amp has a 3 prong mains plug, i didnt need that. death cap, right?? why would an amp that was built stock with a 3 pronger have that, anyway?



Title: Re: sound city clean-up
Post by: g1 on March 15, 2014, 11:54:19 AM
 I'm seeing a "file unavailable" message for the pictures.
Title: Re: sound city clean-up
Post by: Roly on March 15, 2014, 12:19:30 PM
I'm seeing a "file unavailable" message for the pictures.
Ditto.


The circuit shows all the primary leads except Common going to the voltage selector.  One side of the pilot light goes to the highest voltage tap, 245V.

It is quite unlikely that the tranny primary has turned into a melted zero ohm glob.

Quote from: ilyaa
since the amp has a 3 prong mains plug, i didnt need that. death cap, right?? why would an amp that was built stock with a 3 pronger have that, anyway?

That sounds reasonable to me; and yes, why?
Title: Re: sound city clean-up
Post by: J M Fahey on March 15, 2014, 03:27:08 PM
I can see the pictures but being a google service maybe they require you to have a Google+  account or similar
Title: Re: sound city clean-up
Post by: ilyaa on March 15, 2014, 06:18:06 PM
can you download/open this file:

https://drive.google.com/file/d/0B96d2ZNKqmsHWUljZG1wU1ROT0E/edit?usp=sharing

?

if not ill upload them a different way (easiest this way but you know......lazy way isnt the only way....)
Title: Re: sound city clean-up
Post by: DrGonz78 on March 16, 2014, 02:32:40 AM
That file is not working either... I was able to download some of the files the other day, but now they won't download. I logged into a gmail account and downloaded a couple. Now it won't download at all... If you have gmail account you can still view the pics but downloading is hit or miss, mostly miss. Try dropcanvas it rocks!!
Title: Re: sound city clean-up
Post by: ilyaa on March 16, 2014, 03:44:37 AM
jesus what a simple solution to such a pain in everyones....

http://dropcanvas.com/nl0yl

thanks, Gonz!!

(i edited my previous post with the correct corresponding file titles)
Title: Re: sound city clean-up
Post by: DrGonz78 on March 16, 2014, 07:15:26 AM
So yeah that mains power selector switch is totally missing the switch part huh? I have attached a version of the PT that someone drew out, pretty sure this might help. The orange wire tap should be the 115v or 105v on this diagram. Confusing part there is I see red and yellow taps but where does that orange wire in the middle go? The whole switch is wired up in a manner that makes no sense to me.

Man this thing is so confusing! All the close up pics are nice but a fuller picture will help see how all the wiring is connected. Honestly all the AC wiring in the amp needs to be redone and I am pretty sure you are already starting that task. Others here will definitely know more than I will, good luck!  :tu:
Title: Re: sound city clean-up
Post by: g1 on March 16, 2014, 08:31:41 PM
  The voltage selector is a 2 piece switch, like the old style Marshall switches.  The piece you are missing looks like this:
http://www.mojotone.com/amp-parts?search=Switches+Marshall+Old+Style+Impedance+Selector+Switch
http://www.amplifiedparts.com/products/P-H502
  It will be very easy to bypass, but as Dr.Gonz stated, you need to post some pictures where we can see everything at once, not just little bits at a time.
Title: Re: sound city clean-up
Post by: ilyaa on March 16, 2014, 09:53:40 PM
http://dropcanvas.com/#oykJ406u48IJ9y

those work okay?

EDIT: this link works now - fixed it
Title: Re: sound city clean-up
Post by: ilyaa on March 17, 2014, 12:17:07 AM
ohhhh

now i see! i thought that switch ddint make sense.....i thought it was an internal thing - of course its not working!! i wonder if it's always been missing that part........

alright ill just hard wire for 115V, then! let me know if you guys see anything the pix i should take note of....and whats the best way to hard wire it??
Title: Re: sound city clean-up
Post by: Roly on March 17, 2014, 02:48:56 AM
  The voltage selector is a 2 piece switch, like the old style Marshall switches.  The piece you are missing looks like this:
http://www.mojotone.com/amp-parts?search=Switches+Marshall+Old+Style+Impedance+Selector+Switch
http://www.amplifiedparts.com/products/P-H502
  It will be very easy to bypass, but as Dr.Gonz stated, you need to post some pictures where we can see everything at once, not just little bits at a time.

Oh that non-captive thing.  They are always falling out and getting lost.  Generally bad idea.
Title: Re: sound city clean-up
Post by: ilyaa on March 21, 2014, 02:02:15 AM
check it out:

ran into someone with a perfect condition sound city concord combo!

he let me take some pix (i couldnt take it fully apart and kind of had to rush....) but heres what i got:

http://dropcanvas.com/#Mw8P8TI4N7mKe4

what i saw and gonz' picture dont quite jive....

it seems like the fat yellow wire going into the PT is the 115V tap (seeing as thats the way both my amp and the working one are wired). there are no orange wires going into the PT. the only orange wires are the ones around the mains fuse. also, doesnt it seem like the red wire is the bottom, rather than the top of the primaryy, seeing as its also connected to green/yellow (safety)?

in the absence of a working selector, how can i hard-wire it?
Title: Re: sound city clean-up
Post by: g1 on March 21, 2014, 11:21:36 AM
  Sorry I can not look at 100's of megs worth of pictures as I don't have unlimited high speed internet.
  If you are unable to read schematics you should not be doing this.
If you are able, then you should be able to follow the drawing.
Try to tell us exactly where wires are going, rather than telling what colors, the color scheme of this wiring is horrible.
  If the yellow of the PT primary is the 115V tap, then you want to connect the "out" of the fuse holder directly to the 115V tap, bypassing the voltage selector.

The only wire on the PT primary that should go to chassis is the core/case wire.  Look on the schematic at the middle of the PT, at the bottom it shows a wire going over to chassis ground.  Is the green/yellow (safety) you mentioned connected to chassis? 
Disconnect the red wire and measure the resistance to the case of the PT, does it measure as a short (0 ohms) ?
Title: Re: sound city clean-up
Post by: g1 on March 21, 2014, 02:04:44 PM
  Looking a bit more, one of the taps connected to the voltage selector should also go to the pilot light.  Is that where the grn/yellow connected to the red is going?  I think the red is the 245V tap and the grn/yl it connects to should go to the pilot lamp.

Do not trust any wire colors, someone has rewired this amp using AC cord.  Does the 3rd prong on the AC plug (round pin) connect to a wire that goes to the chassis?
Title: Re: sound city clean-up
Post by: DrGonz78 on March 22, 2014, 03:01:22 AM
Strange thing about these amps is that I am starting to think the AC cord wiring is how they were made...

http://www.chambonino.com/work/soundcity/city3.html (http://www.chambonino.com/work/soundcity/city3.html)

http://www.chambonino.com/work/soundcity/city7.html (http://www.chambonino.com/work/soundcity/city7.html)

I would bet that green/yel wire that connects to the RED 245v tap is going to one side of the pilot lamp.
Title: Re: sound city clean-up
Post by: Roly on March 22, 2014, 09:08:22 AM
Quote from: DrGonz78
...green/yel wire that connects to the RED...

AAAARRRGGGhhhhh ... {I feel ill}   :duh
Title: Re: sound city clean-up
Post by: phatt on March 23, 2014, 07:12:15 AM
I once worked on a really old open backed Amp which had exposed mains terminals right behind the upper rear panel board.

The handle was long gone so I bent down to lift the Amp onto bench and felt the terminals of what was the transformer lugs (mains side).  :o Sent shivers down my spine, Thank God it was unplugged at the time.

If I recall twas a Roland GA50 or something like that. :-X
There is some scary stuff out there so wise to take extra care no matter how big and pro the brand name plate looks.  8)
Phil.
Title: Re: sound city clean-up
Post by: Roly on March 23, 2014, 10:18:19 AM
I've managed to do something like that when it wasn't unplugged (or switched off at the outlet).  :o  Fa-zat! - eee-haaa   xP
Title: Re: sound city clean-up
Post by: ilyaa on March 24, 2014, 03:51:20 PM
okay ive made some progress:

bypassed the switch and connected the mains fuse. now the PT is doing what it should. i get ~500V on the main filter caps and the power tube plates. i get about ~-68V volts on the bias winding which drops to ~-42V through some resistors and stuff to power tubes' control grids.

the preamp tubes are lighting up BUT i cant seem to read any heater voltage, either on the preamp tubes or on the power amp tubes (those tubes are currently NOT in the amp, btw). why/how are preamp tubes lighting up w/o heater voltage? all of the heater pins are measuring ~5 ohms to ground....

thats where im at now - trying to track down the heater wiring.

now that ive sorted through some of the wiring, i see that there is a brown wire and an orange wire that go NOWHERE (dont worry just three pics!):

http://dropcanvas.com/#1529c4668Pi1J4

in the picture titled "my amp" you see two wires coming out of the secondary of the PT that go to nowhere nodes. i disconnected them, checked and the BROWN wire is 0 ohms to transformer casing/chassis ground. the orange wire shows high (~500K - 1M) resistance to the power taps on the primary side of the PT, but they are unstable, going up and down. might this orange wire be heater ground?

in the pictured titled "working amp" there is a brown wire coming from the secondary of the PT that goes to the cathode of the biasing diode. if that brown wire goes to ground/PT casing, why would it go to that side of the biasing diode? furthermore, where is the orange wire that i have in my amp?? its missing completely.

notice also that in the "working amp" there is an additional gray wire going from the secondary of the PT to a point on the chassis. im thinking this might be the heater ground? in my amp there is only one (brown) wire going to the chassis from the secondary of the PT.

in "my amp 2" you see there is a 56k resistor that goes from the cathode of the biasing diode to ground. that, of course, is nowhere to be found in the working amp. there is ~40V across this resistor.....there other end of this resistor, and thus also the biasing diode, is wired through a 47nF cap to the middle of the bridge. is this resistor stealing my heater voltage?

should i redo the wiring of my amp to match the working amp? if so, how can i figure out what the brown and orange wires coming from the secondary of my PT are? where is the heater ground?? are the two brown wires mixed up? if so, how can i tell/test them to see which is which? what's weird is the bias voltage is correct, indicating that that part of the amp is wired correctly....

(note that on Gonz' diagram the heater wires are red - mine are green. his HT taps are black and black, mine are black and red.)
Title: Re: sound city clean-up
Post by: g1 on March 24, 2014, 08:34:52 PM
 Do not rewire to match the other amp.  As you can see, it has a different PT with different color wires.  And the picture Dr.Gonz posted was an example from another model so those colors don't match either.
  Have you checked that there is nothing connected to the underside of any turrets?
You have heater voltage, otherwise the tubes would not light up.  You have to measure it across the 2 green wires, not to ground.  They may be using resistors to ground from each side of the heater winding rather than a center-tap, check that there is around 6.3VAC between the heater greens and forget about it for now.
  The bias winding of the PT likely fried.  That may be why the brown is disconnected and the other bias circuit has been connected to the bridge.
  Do you measure any AC voltage from the disconnected brown or orange to ground?
Title: Re: sound city clean-up
Post by: DrGonz78 on March 25, 2014, 06:22:02 AM
G1 is right about my attempt to post an example of wiring diagram and it is clearly not the right one. Also I meant to ask if there are any parts numbers on the PT? TG9875 is the one listed on the schematic, can you confirm this?

So I stumbled upon this Ebay link selling a TG9875 and I think this could help too.
http://www.ebay.com/itm/Sound-City-50-Watt-Power-Transformer-/191046508634?pt=LH_DefaultDomain_0&hash=item2c7b42605a&nma=true&si=qAWI9%252FBxuSL4qGq%252FabjvM%252FBQ5b0%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557 (http://www.ebay.com/itm/Sound-City-50-Watt-Power-Transformer-/191046508634?pt=LH_DefaultDomain_0&hash=item2c7b42605a&nma=true&si=qAWI9%252FBxuSL4qGq%252FabjvM%252FBQ5b0%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557)

So it is only wired up for 117v or 220v which totally makes sense. Primary: Yellow is 117v || Red is 220v || Black is Common || White is Ground. 

That is what the guy online posted but It kinda makes good sense until we look at the secondary wiring. Yes Black and Red B+ voltage. He says two browns are Bias voltage 34v and white is the ground. I see on yours brown as a ground on the secondary. So you have 2 browns(bias windings?) 1 orange?(???) 1 black(B+) 1 red(B+) and 2 Green(filaments) on your secondary right? Really just posting all this info up for points of reference and clarifications.

Have you checked that there is nothing connected to the underside of any turrets?
This definitely needs to be checked to understand the PT secondary function better.
Edit: Reading your latest post you said the brown and orange wires lead to nowhere nodes. So they truly are not connected on the bottom to anything else right? But as G1 asked is there any voltage measured from ground on the orange or brown wires?
Title: Re: sound city clean-up
Post by: ilyaa on March 25, 2014, 03:03:13 PM
whoops - i was measuring DC on the heaters by accident - yes, ive got ~6VAC there.

the brown wire gives me ~30VAC to ground
the orange wire gives me ~230VDC to ground

the PT is, in fact, TG9875.

nothing on the bottom of the turrets.

Quote
Yes Black and Red B+ voltage. He says two browns are Bias voltage 34v and white is the ground. I see on yours brown as a ground on the secondary. So you have 2 browns(bias windings?) 1 orange?(???) 1 black(B+) 1 red(B+) and 2 Green(filaments) on your secondary right? Really just posting all this info up for points of reference and clarifications.

that sounds all right to me!

G1's idea - that the bias winding is bad and that the voltage is taken from the bridge instead - seems to make sense maybe....

one of the secondary brown wires is connected to ground and the other one to nothing.
Title: Re: sound city clean-up
Post by: g1 on March 25, 2014, 09:24:59 PM
  On the orange wire you are looking for AC, not DC.  Check orange to ground and also check orange to black of secondary.
  It's possible the orange is a CT of HV winding that is not used in this amp.

 The brown bias winding could be bad, or may not have provided enough bias range, so someone put in an alternate bias circuit, leave it for now.
Title: Re: sound city clean-up
Post by: ilyaa on March 25, 2014, 10:22:46 PM
orange to ground gives me ~230VDC, like i said, but my meter reads 500VAC. i know its probably getting confused by for whatever reason putting a cap in series does not work - it reads 0VAC when i do that....

when i read orange to black of secondary i get ~320VAC.
Title: Re: sound city clean-up
Post by: g1 on March 26, 2014, 11:20:26 AM
 Check AC volts from orange to red of secondary, should read same as orange to black.
  Then, with power off and caps drained, check resistance of orange to red and orange to black.  Also check resistance from either heater green to ground.
Title: Re: sound city clean-up
Post by: ilyaa on March 26, 2014, 01:23:24 PM
orange to black: ~300VAC
orange to red: ~50VAC

orange to black: ~30 ohms
orange to red: ~10 ohms

heater to ground: ~<10 ohms
Title: Re: sound city clean-up
Post by: g1 on March 27, 2014, 12:14:02 PM
 So the orange wire is a lower voltage tap on the HV winding, you can ignore it.  I suppose if you wanted a lower B+ voltage you could use the orange instead of the red.
  Also, it seems the heater winding is grounded somehow, so no worry about that either.
  As far as the bias winding (brown), further reading suggests it is a little low for the range of bias needed, so this is why some people mod the units with the alternate bias circuit like you have.  So you can ignore the browns and stick with the bias circuit you have.
  Did you reverse the black and white at the power switch yet?
If so, I think you can probably start normal troubleshooting of the sound problems.
A complete listing of DC voltages at all the power supply points and at the tubes would be a good place to start.
Title: Re: sound city clean-up
Post by: ilyaa on April 09, 2014, 04:24:45 AM
bet you guys guys thought i had given up!!

on the contrary, some semblance of a victory:

powered up by bypassing the mains selector - DC voltages looked good everywhere. im not 100% exactly the range i should be seeing on all the various pre-amp tubes, but suffice it to say all the plates had high-ish voltages and all the cathodes had low-ish voltages, except for the PI which had about 80VDC at the cathodes. power tube voltages looked good, with about 450V on the plates and -50 on the grids with the bias turned all the way down (with tubes pulled at this point) - i put in a lil sine wave and traced it through and, sure enough, on the output of the PI i had about 80V p-p from about 100mV on the way in. now the preamp tube configuration in this amp is new to me, and the order is not intuitive (the PI is the second tube in....even though on the scheme its V5....???!), so i might end up having to come back here if there is a problem.

popped the tubes in and checked the see if the bias circuit was working and sure enough it was! i had the light bulb limiter in at this stage and it started to get brighter as i increased the control grid bias voltage. that seems right - tubes are actually pulling current so the amp must supply it. wondering if even trying to bias them with a limiter in-line is a dumb move, but i am still nervous something is going to blow up.

anyway, put in sound and sound came out! (i was worried the OPT might have been bad)

so ill try it out tomorrow with a guitar and we'll take it from there.....
Title: Re: sound city clean-up
Post by: Roly on April 09, 2014, 06:37:43 AM
Quote from: ilyaa
bet you guys guys thought i had given up!!

Well no.  Actually from your history here so far I thought you were having a long ponder about  it.

Quote from: ilyaa
im not 100% exactly the range i should be seeing on all the various pre-amp tubes, but suffice it to say all the plates had high-ish voltages and all the cathodes had low-ish voltages
:lmao:

This is a bit more cluttered than I was looking for, but hopefully it will do as an example.

(http://www.turneraudio.com.au/basic-tube-1_files/schem-basic-6sn7-sig-amp.gif)

The points to note are the local anode supply voltage (300V), the anode load (47k), and the anode voltage (140V).

In this case the drop across the anode load is 300 - 140 = 160 volts.  Apply Ohms Law and we get a current of;

I = E/R

160/47 = 3.4mA (volts and k ohms gives mA)

Note there is nowhere else for this current to go except through the valve and cathode resistor, so the voltage on the cathode (the bias voltage) again applying Ohms Law (which we do an awful lot);

Vk = I * R

3.4 * 1.5 = 5.1V (mA times k ohms gives volts)

In our more common 12AX7 stage we typically find anode resistors of either 100k or 220k, and the valve current of between 0.5 and 1.5mA, so the cathode resistor is around 1k to 2k2 to give a bias of about 1 to 1.5V.

With a 300V supply we want half that on the anode for best available signal swing, so with a 100k anode load that means a drop of 150V which in turn means 1.5mA through the 100k, and of course through the valve and cathode resistor.  1k and 1.5mA will give 1.5V of bias.

The heartbeat of this sort of stage is that the anode voltage is about half the local supply (generally a few volts above half), so if you have a supply of 300 volts you expect to see something around 160V on the anode.  If it's low the valve is drawing too much current (or the anode load resistor has gone high, or the output coupling cap is leaky, or the cathode bypass cap is leaky/shorted).

If the anode voltage is high then not enough current is flowing, and the normal reason for this is that the valve section is worn out, the cathode has no more emission work function left (although it could be due to a heater circuit fault, poor heater connection at the socket).

But either way, high, too little current, or low, too much current, tells you what to go looking for next.


No, you can't set the OP stage bias correctly with a limiting lamp in series.  Turn the bias setting so that you have the maximum negative voltage on the grids, power the amp up without the limiting lamp, making certain you have a load securely connected at all times, then when it has warmed up and settled you can adjust the bias for the desired cathode current/anode power dissipation.


{I notice that you call making the output stage bias more positive "increasing" the bias.  While this makes sense it generally isn't how tech think about it, that "increasing" the bias means more voltage.  That this happens to be more negative voltage is perhaps perverse, but we tend to think of a voltage that is further away from ground as "increasing", and that making the bias more positive is "decreasing" the bias which makes the valves draw more current, "more" bias voltage = less current.}


If I seem a bit rough on you at times it's actually because I think you have good prospects for becoming a real tech, you have nutted out quite a bit of stuff on your own, such as the dummy load, but sometimes I think you need a good poke because you have demonstrated you are capable of working stuff out but just ... kinda ... can't be bothered (or had a hard night, or summat).  So when I ginger you up it's not because I think you are a hopeless clod, quite the opposite.  You're obviously keen and adept, but you still need to internalise the relationship between current, voltage and resistance until it is second nature.   :dbtu:
Title: Re: sound city clean-up
Post by: ilyaa on April 10, 2014, 03:29:21 AM
no need to apologize - benefiting from the knowledge on this forum has been an immense boon - i feel like ive made huge strides in just a few weeks!

hmmm that diagram is quite helpful!

i can go through the amp now with some knowledge of what i should expect to see.

played it today and its certainly sounding pretty good! nice, sharp clean tone, but a little brittle and dull sounding. seems like i should be able to get a little more harmonics and saturation out of it....as far as easily identifiable problems - the EQ sliders seem to be kind of wonky - its banged up real bad and ive read about those going bad, so ill open them up and see what's going on. there is no reverb tank which stinks - maybe ill put one in. but i suspect there might be something in the preamp/PI section of the amp thats responsible for the lifelessness. as soon as i get an 8 ohm load together ill do some real power/gain tests and see what's up. in the meantime, what's the real difference between ecc83s and ecc81s? the PI tube has a bit of a ghosty haze around the bottom - sign of fatigue?

but otherwise im on the right track, i think! it's working and sounds alright so far - which is miles from where i started!
Title: Re: sound city clean-up
Post by: Roly on April 10, 2014, 02:18:47 PM
Yes, you have made considerable progress and shown flashes of brilliance (it's just that to be a good tech you have to be brilliant all the time, right guyz?   8| ).

I go fishing through Google images because it is often the case that someone somewhere has already done a drawing that illustrates what I'm trying to explain.

It is much easier to get a dead amp going again than it is to track down some vague problem about "tonality", and it is easier to deal with tonality problems with your own gear than with a clients' gear.

If somebody comes in a says that their amp isn't as "creamy" (or whatever) as it used to be I tend to hide under the bench 'coz it's really hard to know what the hell they are talking about; HOWEVER you often find that once up on the bench there is something obvious and serious wrong with it - like worn out OP bottles or a blown speaker.

Some clients are great and can give you a highly detailed and accurate description of the problem, but the majority aren't and can't.  So most of the time you start on a job only about 80% sure that the problem is actually in the amp and not in the associated leads, pedals, guitars, or operators.  This means that you listen very carefully to what they tell you, then take it with a large grain of salt - it's important, but it could be quite wrong.

Quote from: ilyaa
what's the real difference between ecc83s and ecc81s?

One is currently made and the other isn't?   ;)

Serriosly - a.k.a. 12AX7 and 12AU7; voltage gain.  12AX7 has a typical stage gain of around x30 (abs max x60), 12AU7 about x10.  There is also a 12AY7 and 12AT7 that have gains in between, all pin compatible.

Quote from: ilyaa
the PI tube has a bit of a ghosty haze around the bottom - sign of fatigue?

A grey-to-silver discolouration, particularly that has "shadows" of the internal structure, is caused by electrons impinging on the inside of the glass and has no harmful effect, but does give a clue to how many years a valve has been in service.  Valves start clear, then get a smoky look over time that develops into a dark grey-silver.  This should not be confused with the getter which is bright silver.

Creamy white discolouration of the bright silver gettering is a sure sign of gas, and cracked bottles will normally show white getters instead of silver. 

(http://www.ozvalveamps.org/generic/6l6cloudygetterc.jpg)
Dead EL34 above, good one below
http://www.ozvalveamps.org/valveabuse.htm (http://www.ozvalveamps.org/valveabuse.htm)

Preamp valves such as the 12AX7 are normally gettered in the crown, not around the base.

(http://www.ozvalveamps.org/goldentone/crashedgt3290225259aggh.jpg)
The preamp valve dead centre looks intact but the white getter in the top gives it away as cracked.
{Mind you the 6DQ6 left rear has really had it - it ain't got no glass left!.}
Title: Re: sound city clean-up
Post by: ilyaa on April 15, 2014, 04:41:49 PM
im happy to say this amp sounds great! i recorded something with it today and im digging it.

couple of things:

1) there is a gain switch that i had neglected - its labeled 'sensitivity' and is on the back of the amp. i saw it when i first opened the amp up but then forgot all about it UNTIL i spotted it on the schematic again. turns out when switched to the higher gain it kicks things into gear a bit - cool!

2) the bass eq slider on the second channel was broken (open) so i replaced it (with a pot unfortunately - where the hell am i supposed to get a slider like that these days anyway?) and now that channel sounds much better - it was totally bass-less before.

3) im curious about that second channel. the only discernible difference between the two (from the schematic) is that the first one has a 100uF bypass cap on the first preamp tubes cathode while the second channel has a 2uF cap in that place. normally if the cap were in-line with the input (like in a guitar pedal) itd have a direct effect on frequency rolloff - bigger cap would let more low end content into the circuit. will it have any effect on the signal (frequency-wise/tone-wise) at all - what role is AC playing on the cathode end of the tube? if i had to guess id say the bigger cap would be sending more low end AC to ground - so the second channel, with the 2uF, might have better bass response? is it maybe the 'bass' channel and the first one the 'lead' channel?

4) the second channel also has a reverb circuit - currently disconnected because there is no tank. the reverb slider does have an effect on the signal though - makes it louder and even a bit overdriven, but with some weird barely noticable hum underneath as i turn it up. i havent looked to see what EXACTLY is disconnected and (potentially) re-wired, so ill check that out and we can come back to that issue if we have to.

5) is V3A common to both channels? final stage before the PI? im having a bit of a hard time understanding how the signal is traveling out of V1 and V2. it seems like there is a path for signal from V1 (meaning channel 1) to go into the reverb circuit, as well. is that right?

6) i took some power measurements and this thing looks great! its putting out about 50V p-p unclipped into 8 ohms, so thats almost 40 watts! woo!

7) this is just out of curiosity, but can explain the resistor networks at the input jack? i knnow its common to most amps to have something like this and my gut tells me it has to do with dealing with a guitars output impedance, but the wiring configuration itself seems a bit mysterious to me.....
Title: Re: sound city clean-up
Post by: g1 on April 15, 2014, 09:01:45 PM
  For the cathode bypass cap, you have the right idea, but it works opposite.  The ac voltage (signal) on the cathode resistor creates negative feedback.  By allowing the low frequencies to ground through the cap, there is less negative feedback and more bass.  So the larger the cathode bypass cap, the more gain for low frequencies for that triode stage.
V3A is common to both channels.  The path through R21 can go both ways, it allows Ch.1 signal to the reverb, but also routes Ch.2 dry signal into V3A.
Title: Re: sound city clean-up
Post by: Roly on April 16, 2014, 02:08:17 PM
Quote from: ilyaa
2) the bass eq slider on the second channel was broken (open) so i replaced it (with a pot unfortunately - where the hell am i supposed to get a slider like that these days anyway?) and now that channel sounds much better - it was totally bass-less before.

If at all possible you repair the slider.

3) - almost right.  The cathode bypass cap does influence the stage frequency response.  The traditional wide band value was a 25uF/25V, but for "lead" channels this was reduced, often to a faction of a microfarad.  Unbypassed the cathode resistor provides AC as well as DC negative feedback, fully bypassed you get Maximum Available Gain (MAG) from the stage.  By selecting some intermediate value you get higher gain at higher frequencies because they are bypassed, but at lower frequencies the reactance of the cap is higher, less bypassing, more NFB, and less gain, so the stage becomes more toppy.

4) - the reverb control is another input to the channel mixing point (R24, R21, R25, R26) and since this is a passive mixer there will be some control interaction - changing the reverb injection will also alter the main channel signal levels (R45, VR7); and as there is no reverb tank the grid of V3-B is not connected to anything, that would be where your hum is coming from.  Just try grounding this grid for the moment.

5) - it has a Voltage Amplifier Stage (VAS) ahead of the PI, V3-A.  Yes it was typical to take the feed to the reverb from one of the input channels, one being clean with no Fx, the other the reverb and trem channel.  While what @g1 says about R21 is true, the value of R20 compared to R24 (mis-marked as C24) and R21 means that you aren't going to get much signal from that source compared to from V2-B.

7) - the first function of a resistor between the input socket and the first grid is as an RF stopper to prevent radio frequency pickup from AM radio stations, CB's, old AM taxi two-ways, and the like.  An arrangement like this was normally fitted to provide two different input impedances and sensitivities, highish Z and full gain for guitar, and a lower Z and attenuated input suitable for connecting a tape recorder output or some other line level signal which could be a fair bit hotter than a guitar direct.

J1 has 470k across and just a 68k in series with the grid, so that will present 470k load to the guitar with no attenuation.  A plug into J2 will change the contacts so that R2 is now in circuit instead of being bridged, and the previous series 68k (unnumbered) is now connected to ground via the contacts of J1 ('tho it isn't obvious here) resulting in an input resistance of 2x 68k = 136k, and 2:1 attenuation.  Variations of this arrangement are quite common.
Title: Re: sound city clean-up
Post by: ilyaa on April 20, 2014, 02:34:29 PM
hm im trying to repair the slider but its hard - it cracked and the conductive black strip has a little crack in it that im not sure how to bridge....got some copper tape but i dont know if itll do the trick.

quick question:

amp sounds good and is making power, like i said, but im wondering if i should replace the filter caps - the voltages all look good and i measured the ripple - only about 1.5V p-p - which seems fine, but they definitely have physical signs of distress - little bumps on the bottom side where they seemed to burst out a little.

should they be replaced? is age a good enough reason? and signs of distress? or should i be able to measure an actual problem to warrant replacement? people talk a lot about the massive change that filter caps have on tone and stuff - any technical commentary on that?
Title: Re: sound city clean-up
Post by: DrGonz78 on April 20, 2014, 04:40:58 PM
For the slider what is the resistance? Take a pic to show us the slide pot so as to provide better help. There are times we can find replacements or what not. I might even have some slider pots around here that might work...

If the filter caps are over 30 years old and they have bubbles then it is a great idea to replace with new ones. It is better to worry about keeping the amp alive rather than some idea that the filter caps change tone in any significant fashion.
Title: Re: sound city clean-up
Post by: Roly on April 21, 2014, 01:05:56 AM
Quote from: ilyaa
should they be replaced? is age a good enough reason? and signs of distress?

Yes, yes, and yes.

If replacing the filter caps makes a radical difference to an amp then it had pretty serious problems to start with.  99% of the stuff you read on the net from non-techs working on their amp is horsefeathers and can be safely ignored.  As you proceed repairing amps you will encounter a strong subjective effect that a repaired amp sounds "better than ever" but this is at best a boiled frog response from somebody who didn't realise just how bad their amp had got with neglect, and it also makes them feel better about the money spent - but it ain't real.  Just smile, nod knowingly, and put the bread in your pocket.
Title: Re: sound city clean-up
Post by: ilyaa on April 21, 2014, 01:31:01 AM
should i replace with same value (capacitance)? or go up if i can for better filtering? i imagine the difference wont be big (these are already 200uF....)

any recommendations on wheres good to get these big cans?
Title: Re: sound city clean-up
Post by: phatt on April 21, 2014, 03:19:32 AM
No time to check the circuit in question,, meantime 200uF already sounds big enough to me.
You can go bigger BUT be aware that with some rectifier circuits you can blow things up by doing so.
I'd research that first, others here will know more.
Phil.
Title: Re: sound city clean-up
Post by: Roly on April 21, 2014, 10:38:01 AM
I'd stay in the same ballpark.  The two 200uF in series give 100uF effective and that should be more than ample for a pair of EL34's - many such amps use less.  There is no real advantage in going much higher in value, and as phatt says, you may run into charging surge problems.  It's possible your supplier has a near value such as 220uF and these will be just fine.  Just make sure the voltage rating is equal or higher.  Don't forget the other decoupling caps down the HT line, C33, 25, 15 and 14; and you may care to do the cathode bypass electros while you are at it.

Title: Re: sound city clean-up
Post by: ilyaa on April 24, 2014, 05:08:47 PM
slider pics:

http://dropcanvas.com/#997fdp0XjSmBB9

its 500K

3.25" long

you can see where the conductive strip is cracked in the pix....

anybody got one laying around??
Title: Re: sound city clean-up
Post by: DrGonz78 on April 24, 2014, 08:08:56 PM
I will check and see what I have around the shop. Also did you try to put a small piece of solid core wire to fill in the gap and just a tiny quick solder over it? It looks like there is little to no carbon at that end of the track so it must be low resistance at that point anyway. 
Title: Re: sound city clean-up
Post by: DrGonz78 on April 25, 2014, 04:04:22 AM
Yeah I looked around through some spare parts and found what I thought might be the right size. Right size but unfortunately it's only a 100k slide pot.

I think bridging a connection on that crack will work though. For one thing the break is a good place and the track is nice solid metal material. Just don't heat up solder iron very much while trying to bridge the crack. Good luck.

Title: Re: sound city clean-up
Post by: ilyaa on June 15, 2014, 04:29:46 AM
Haven't tried fixing the slider yet but been using the amp and all seems fine.

Yesterday I noticed though that both transformers are getting pretty hot after I play it for twenty minutes or so. Don't think I've noticed that with other amps. Normal? Hot to the touch.
Title: Re: sound city clean-up
Post by: Roly on June 15, 2014, 06:14:51 AM
If it's working okay then that is probably normal if you can still touch them without burning your fingers.  Too hot to touch for even one second, or smells of cooking varnish, are warning signs, but if they don't appear to be in thermal runaway and stabilise after a good long thrash then they should be okay.

If you have a DMM with a thermocouple or one of those IR non-contact thermometers you can actually measure the temperature of the cores and confirm that they are not continuing to heat up, particularly on idle.
Title: Re: sound city clean-up
Post by: ilyaa on June 22, 2014, 02:05:14 AM
hmm i dont know - thermanl runaway might be a possibility - ill try and track down a thermometer or find a way to get an accurate read on the temperature, but if played for more than twenty minutes it gets pretty hot - i cant keep my finger on it longer than a couple seconds. and this is BOTH the transformers - in fact, the power transformer, although hot, does not get nearly as hot as the output transformer. which seems strange - seems like if anything the power transformer should be heating up from the HT its providing

ill pop it open and see if anything looks funky - any suggestions where to start looking? or is the problem likely to be internal to the transformer itself?

im thinking maybe its an intermittent short somewhere in the windings....
Title: Re: sound city clean-up
Post by: Roly on June 24, 2014, 05:59:46 AM
Nah.  If you can hold your finger in it for a second or more, then its just working hard.  Can't touch it is too hot.

Overall the power transformer is more efficient than the output transformer, but they also have different core sizes for their different power levels.  Because the OPT is a broadband device and not monotonic like the mains tranny, it tends to have more iron losses in the core, more saturation due to it being hammered into non-linearity (pretty much the whole point of valves really) so it gets hotter.

If you have a short in the windings it will sound quite weak, unable to make watts.  I don't think I've struck an intermittent one yet, they typically flash over between winding layers, carbon/metal track, shorted turns, and that it baby - rewind or bust.
Title: Re: sound city clean-up
Post by: ilyaa on June 24, 2014, 04:43:44 PM
Kay I guess that's good news - I thought it would have been strange for it to work despite any kind of internal problems. Maybe just an inefficient transformer then, heat-wise. I'll keep an eye on it but thanks for now!
Title: Re: sound city clean-up
Post by: ilyaa on July 03, 2014, 04:49:28 PM
v4 turned ghostly white!

since there is no reverb tank to speak of, if i just pull v4 out and play without a tube in that socket, will the amp be mad?
Title: Re: sound city clean-up
Post by: Roly on July 04, 2014, 06:38:40 AM
Quote from: v4 turned ghostly white!
v4 turned ghostly white!

A sure sign of an air leak.

When valves are manufactured they are first pumped down with a vacuume pump connected to the seal off point (the top tip in 12AX7's and the like, inside the base with octal types).

They are then sealed off and an induction heater used to "fire" a gas "getter", generally barium, inside the envelope which leaves a silver deposit on the inside of the glass somewhere.  This patch continues to mop up any residual gas that escapes from the assembly during its life.  If the valve gets a gross leak, say around one of the leadouts, or simply by being broken, the remaining barium reacts with the oxygen in the air to form barium oxide which happens to be white.

(http://www.ozvalveamps.org/generic/6l6cloudygetterc.jpg)

A pair of 6L6's, a good one in front, a duff one behind.

Note: broken valves should be treated as toxic waste because they contain materials such as barium getters and thorated (radioactive) cathodes.
Title: Re: sound city clean-up
Post by: g1 on July 04, 2014, 12:27:22 PM
if i just pull v4 out and play without a tube in that socket, will the amp be mad?
Should be fine, preamp supply voltage will probably come up a little.
Title: Re: sound city clean-up
Post by: ilyaa on July 04, 2014, 01:04:17 PM
roly,

are you saying its a physical issue with the valve itself? another valve in that same socket has done the same thing - what non-physical/mechanical things could cause it?
Title: Re: sound city clean-up
Post by: nashvillebill on July 04, 2014, 05:08:02 PM
Yes, the tube (valve) has lost its integrity physically.  A tiny crack in the glass-- at the base, or around a pin perhaps.

If this is the second one to experience the exact same failure, then I'd look very carefully at the entire mechanical area around the tube.  How is it being inserted, how is it being clamped, is something hitting it or pressing on it.
Title: Re: sound city clean-up
Post by: Roly on July 05, 2014, 05:12:07 AM
are you saying its a physical issue with the valve itself? another valve in that same socket has done the same thing - what non-physical/mechanical things could cause it?

Well it is possible to break a bottle with thermal shock, sudden gross red-plating causing the glass to crack;

(http://www.ozvalveamps.org/dvalve.jpg)
(note the white getter in the top)

... but as Bill says, this is basically a mechnical problem (or perhaps a very unlikely co-incidence, they do happen), but I'd be giving the whole area of the valve socket/can/retainer a really good scrute, nothing caught down one of the socket contacts, pins actually going into and not alongside a contact (bent pins), something stressing one of the contacts underneath, ...too much force...?

Nothing, particularly metal, should be tight on the glass (shields excepted of course).
Title: Re: sound city clean-up
Post by: ilyaa on July 05, 2014, 03:40:54 PM
ah yeah i see both tubes that have turned white have a crack in a similar place alongside some of the pins - ill check the socket closely - might be bent in one side putting too much pressure on the glass