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!

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....

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.

fixing this one up for a friend -

incidentally i at one point had 3 of these identical amps in my garage -  a popular one in my milieu, i guess....

anyway, this one was doing something weird and i traced the problem to the phase inverter (solid state differential amp), which was giving me wonky voltages. replaced the transistors and all voltages are back to normal.

amp sounded fine - nice and loud - but had an unpleasant kind of congestion and loss of volume still when cranked up.

noticed the bias voltage for the PI (which should have been 330V) was only at 260 or so. saw that the last resistor in the PS filter network on the main cap was not, in fact, 68K but 87K, for some reason. change that out and voltage is closer (now at about 300V) - the first resistor in the HT filtering is 1K (instead of 400), so that might be it, but ill leave that as is for now.

the amp is making power. the slightly low HT voltages seem an unlikely suspect. i have noticed, looking through the amp, that many of the smaller coupling/DC blocking caps have a white residue on them as if they have suffered a tiny explosion. is this a sure sign of age/wear? should any caps with this kind of sign of stress be replaced? just wondering if that's a clear indicator or one of those 'maybe you can change it but it's probably okay' kind of things.

thanks!

k rewired it seems all good - thanks guys!

two follow up questions:

1) does it matter where the switch is? because of the way things were set up in there, it was easier to wire the switch on the neutral side of the transformer primary, not the hot side. that doesnt really matter, right?

2) the amp has a ground switch that was wired to the death caps. can someone give me a run down of why this switch is there and whether or not, with a probably wired 3 prong cord, it could serve any useful purpose? i know plenty of 3 pronged gear still has one for eliminating ground loops and that kind of thing. i disabled it completely, though, because im not sure how i would wire it in a useful way. insight?

and a side note: when i was using the amp before, with the two pronger, the chassis was hot! meaning, it was shocking me when i touched the amp! and even with the ground switch flipped and the amp not shocking, there was still a differential somewhere because then the mic hooked up to my PA would shock me if I held the bass. thank goodness the new wiring setup fixed that.

im fixing up an acoustic 150b head

works great - just had some caps that had desoldered themselves and other ones that seem leaky - just doing a basic clean up and recap

this amp actually sounds awesome! a real growler

anyway, the power cord is kind of deteriorated and also only 2 prong

just wondering: what is standard procedure for retrofitting an older SS amp with a three prong cable?

im particularly confused because the cable it has now is polarity indifferent (equal sized prongs) -

guidance please!

think it was the caps around the parametric EQ!

changing Q105 had no real effect - but the sound disappeared after replaced C108 and C115. like i said in a previous post, i was getting about ~1.5VDC on the negative end of either of those caps when that area should be a DC dead zone (right?). after i replaced them - no more VDC and no more noise! think the electrolytic caps were my problem.

so far so good - does this fix make sense?

if those caps were leaky, they would have been letting some DC into that EQ circuit, but in this particular case, that DC, seeing as it was the result of a leak, would look like a fluctuating voltage (right?) and be treated as AC by the amp - amplified and turned into the strange rustling noise i heard. it was early in the preamp, so by the time it got to the power amp the tiny leak was audible.

maybe progress, maybe not.

i dont think its the pots or sliders - its not that kind of rustle, its much more of a light and repetitive thumping - not scratchy at all. but ill try and scrub them again.

(yeah, i meant emitter of Q105 - but i think the noise is there before, just much much smaller, and that's where its amplified enough to be noticeable)

hot didnt make any change, but after i sprayed Q105 with some aerosol (from a deoxit container - all i had) the noise went nuts for a second, but who knows, maybe it was the contact cleaner component of it!

anyway, i changed out Q104 and Q105 - no change, noise still there.

C112, 116, 107 i changed, too.

then i went on a DC hunt and found some odd things: i was getting ~7V on the negative end of C125 and C124. there should not be any DC there, right?! switched them out for new caps and no DC there anymore. i was also getting ~1.5V on the negative end of C108 and C115. that should be DC dead-zone, too, right? i switched them out and it looks better but still about 10 mV in that area....

after turning the amp on after that no noise yet, but its an intermittent problem that usually pops up once the amps been on for a while, so who knows. ill report back....

ack i dont like doing this like this - seems so slapdash. the only things im certain off:

its coming from the preamp
it gets noticeable around Q105
it seems like a DC problem

last thing: emitter of Q105 is only at 14V - should be at 20. maybe R125 has drifted low? or R129 low? earlier this amp had a problem where R104 had drifted high and cutoff the very first transistor in the preamp. amp stopped working until i put an ~8M across it and corrected the bias.

this amps giving me guff again - have to say im getting a little tired of it.

when on (no input signal) the amp makes a low end kind of rustling noise - almost sounds like rats have made a home in the speaker cones. its, unfortunately, an intermittent problem. sometimes the amp will be on for a while and not make the noise and sometimes the noise will get louder and louder as time goes on.

what ive managed to figure out/try so far:

1) i think its coming after the volume, because the volume has no effect on it (it can even be all the way down and the noise still persists)
2) it might come before the main EQ, because the treble/middle/bass knobs seem to have a *slight* impact on it
3) it definitely comes before the graphic EQ, because that has a huge impact on it. if i turn the graphic EQ all the way down the noise *almost* disappears and if i crank it the noise becomes thunderous
4) pretty sure its in the preamp, because if i send the Line Out to another amp the noise comes with it
5) scoping around in there i dont get too much info.
    scope on the speaker out when its happening: what you'd expect, just some low frequency looking     noise
    DC volts on the speaker out when its happening: a little bit, jumping around between like 15 and 70 mV
   scoping around inside the preamp when its happening: i dont get anything at all really UNTIL the collector of Q105, when the flat line on the scope starts to move up and down (not AC-style, but looking like shifting DC) - it moves along with the rustle. as i move further along the preamp the movement gets more significant but stays the same - just that flat line moving up and down.

my thoughts:

poor filtering feeding the preamp transistors an unsteady/fluctuating bias thats leaking into things like a low frequency AC? only thing is ive checked the main big caps and they seem pretty solid - although of course they are old.....

bad/leaky caps in the preamp? particularly C108, C112, C115, C116, C118, C119, C122, C124, C125, and C127.

am i going down the right path? i dont want to disappoint everyone and resort to shotgunning these caps around but in the absence of a clear indicator of the fault im not sure what to do. unless the shifting DC on the scope points to something specific.

on a side note: im curious about the speaker outs on this amp. with the speaker unplugged, the amp shows like 25V on the speaker outs, but once i plug it in that goes down to near 0. that normal?

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??

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?

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?

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.....

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!

first of all, that mod video is insane. i can hardly believe its real.....

what might have burned the screen resistor? normally is that a sign of a bad tube, pulling too much current?

Roly, what you say about standby switching makes a lot of sense - I never thought about it that way before. I read some of the other links and see that the standby myth has plenty of debunkers.

a related question: what about the standby switch and cap DIScharging? with that sound city ive been working on, i see that if i turn the standby switch off (open) before i switch the amp off the caps drain quite slowly. BUT if i switch the amp off and leave standby on (closed) they drain much more quickly. i guess thats because with the standby switch closed, the caps can drain through all kinds of stuff throughout the amp, but with it open, they can only drain through those rather large bleeder resistors. ive also noticed though, that if i leave it closed when i turn the amp off and let the caps drain, they start to charge back up if i open it again (even if they have drained to 0V....)....whats charging them, in this case??