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Author Topic: sound city clean-up  (Read 16983 times)

ilyaa

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Re: sound city clean-up
« Reply #30 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.)
« Last Edit: March 24, 2014, 03:57:39 PM by ilyaa »

g1

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Re: sound city clean-up
« Reply #31 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?

DrGonz78

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Re: sound city clean-up
« Reply #32 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

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?
« Last Edit: March 25, 2014, 06:51:05 AM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

ilyaa

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Re: sound city clean-up
« Reply #33 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.

g1

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Re: sound city clean-up
« Reply #34 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.

ilyaa

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Re: sound city clean-up
« Reply #35 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.

g1

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Re: sound city clean-up
« Reply #36 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.

ilyaa

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Re: sound city clean-up
« Reply #37 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

g1

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Re: sound city clean-up
« Reply #38 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.

ilyaa

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Re: sound city clean-up
« Reply #39 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.....

Roly

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Re: sound city clean-up
« Reply #40 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.



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:
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: sound city clean-up
« Reply #41 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!
« Last Edit: April 10, 2014, 03:48:04 AM by ilyaa »

Roly

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Re: sound city clean-up
« Reply #42 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. 


Dead EL34 above, good one below
http://www.ozvalveamps.org/valveabuse.htm

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


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!.}
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: sound city clean-up
« Reply #43 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.....

g1

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Re: sound city clean-up
« Reply #44 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.

 

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