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Author Topic: sound city clean-up  (Read 18545 times)

Roly

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Re: sound city clean-up
« Reply #45 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.
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 #46 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?

DrGonz78

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Re: sound city clean-up
« Reply #47 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.
“A person who never made a mistake never tried anything new.” -Albert Einstein

Roly

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Re: sound city clean-up
« Reply #48 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.
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 #49 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?

phatt

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Re: sound city clean-up
« Reply #50 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.

Roly

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Re: sound city clean-up
« Reply #51 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.

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 #52 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??

DrGonz78

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Re: sound city clean-up
« Reply #53 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. 
« Last Edit: April 25, 2014, 04:04:56 AM by DrGonz78 »
“A person who never made a mistake never tried anything new.” -Albert Einstein

DrGonz78

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Re: sound city clean-up
« Reply #54 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.

“A person who never made a mistake never tried anything new.” -Albert Einstein

ilyaa

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Re: sound city clean-up
« Reply #55 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.

Roly

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Re: sound city clean-up
« Reply #56 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.
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 #57 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....
« Last Edit: June 23, 2014, 01:55:24 PM by ilyaa »

Roly

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Re: sound city clean-up
« Reply #58 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.
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 #59 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!