k its all good -

i guess i just had to "tune" it using the tremolo bias pot -

i turned the pot with my scope on the preamp output -
at some settings there was no tremolo at all, until i hit a sweet spot where it looked great, after which if i kept turning it disappeared again - i was not able to find a setting that was 'perfect' - no matter where i set it the depth knob seemed to not really give me a full range of tremolo, only activating the effect once i hit 5 or 6 - but perhaps thats just the nature of this effect!

anyway, amps working great now, thanks, guys!!

sounds good - ill order some tubes and report back once ive replaced everything.

that sneaky 832!

in the process of trying out which tubes were good and which were bad i blew a fuse (doh! why didnt i have the limiter inline?) and after some searching around (now with the limiter in place) found that D9 and D10 had gone bad/shorted out....the tube that did the job kind of sparked inside before everything went south....im hoping this was just a bad batch of tubes (who knows what condition they were in before this amp came to me) - without power tubes in all voltages and stuff are normal -

what are those diodes doing, anyway??

(they are not in the original schematic i posted - see a slightly different schematic attached)

ooookayyy so things went in a slightly different direction -

i took another look at the tube and it reads "ECC 832" - i was assuming it was an 833 but no!! a quick internet search told me that the ecc 832 - or 12DW7 - sometimes has a non-standard pinout.....

the schematic asks for a 12AU7 - i dont have one on hand so i put it an actual 12ax7 and now the voltages on both PI plates are the same - but they are high - about 340V each - assuming this is because 12au7 is lower gain so the 12ax7 maybe running at higher quiescent current than schematic values - right??

but at least the imbalance seems to be solved!

in fact, ALL the tubes that should be ecc833 are ecc832.....sneaky sneaky! who was in here before me?!
but no other voltages seem funny........

i put in the 7027 power tubes that the amp came to me with and they started red-plating....uh oh....is that usually a tube problem or an amp problem? i only had one other 7027 on hand so i put it in alone and it seemed to be okay - lonely but not red-plating at least - i can use 6L6's in here, correct?

Quotedirectly into the effects loop and I did get a clean tone at a low/medium volume.

you mean into the effects return? that's you bypassing the preamp altogether and plugging directly into the power amp - was that signal intermittent? if not, sounds like your problem is definitely in the preamp! a low/medium volume tone from the power amp sounds like a properly functioning power amp. (you split the amp in half by using the effects loop - a great place to start troubleshooting it because it tells you if your problem is BEFORE the effects loop - preamp - or after - power amp)

sounds to me like you have some bad/cold/intermittent joints somewhere in the preamp - open it up and take a look - turn the amp on and poke around with something non-conductive/wooden like a chopstick in the preamp (be careful of course if you open it up!!!!!) - see if you get any crackly/weird sounds - if you do, youve found the crappy joint!

crap -

new socket - same problem.....

dont know what other things to try - ill add one observation that i havent mentioned yet: when im measuring DC voltages to ground in that part of the amp, i feel a slight vibration through the meter. it feels hummy, like 60 cycle kind of stuff. as i move away from the section of the board with the PI it gets weaker and weaker and is totally absent when im measuring other voltages in different parts of the amp, even the ~500V on the power tube plates/screens.......??

oh, man!

doh!

you're right, of course - put the caps back, flipped the anode connections and we are all good! 49 watts into 4 ohms. and sounds good!

i learned something! i knew about transformer phasing but for whatever reason figured that because this was a new one and was made for exactly this kind of amp why should it not be in phase? but i guess its a crapshoot! should have tried that FIRST not last!

thanks, guys!

now on to the tremolo that isnt tremoloing......

woo!

1) pulled the death cap

2) i know bias is never positive - i meant as i made it more positive/less negative

3) im measuring across a homemade bias probe that interjects a 1ohm resistor between cathode to ground - but i think i figured it out!!

there did not seem to be any issues with the trimpot or any of that stuff. i scoped the power tube grids and turned on the amp and waited for the tubes to start pulling crazy current and lo and behold i got a huge AC signal on the grids with no input or anything! i opened the bias filter cap - no difference. i opened the caps coming from the PI and voila! once those were open the tubes biased up normally and current stayed where it should. i guess those were leaky! the high voltage on the PI collectors must have been leaking through those coupling caps and appearing as a large signal on the grids, pulling current through as if i was driving them!

so ill just some new caps for that spot pop them in and we'll see how that goes -

im curious why the tubes biased up properly with the old/bad OPT - any ideas? if those caps were leaky they must have been leaky before, too, but this problem arose only once i put the new OPT in....

its not called a long tail pair when they share the cathode resistor across two separate triodes? i thought thats what a long tailed pair was.....

its hard to mess with the socket contacts - im just going to wait until some new sockets come in the mail and just replace it and see what happens....

at this point ive also tried opening all the caps in the area to see if leakiness is the issue and that has made no difference, so im thinking it HAS to be a bad contact

alright got the rivets out

got the new OT - tested it with my tester and it looks good

plugged it in, wired it up, etc.

all voltages look good

BUT when i turned the amp on and tried to bias the tubes up, something bad happened - as soon as i turned the grid bias positive enough to let a tiny bit of current pass, seems like the floodgates opend and the tubes started passing like 140mA right away! and started to glow blue....i tried a couple different sets of tubes and they all did it - whats happening?? seems like there is some kind of path for current that there shouldnt be but i cant imagine what it is - i didnt change anything in the amp - just took the old OT out and out the new one in.....

....(only thing that looks weird in the amp is that one of the death caps (this amp has a 3 prong cord but still has the old death caps in it) looks it exploded inside - it looks all bloated!)

hmm i dono this ones baffling me -

all resistors in the area look good. i opened c11 and c13 on the PI plate side and that didnt make any difference. i took out the tube socket and gave it a good look and it looks fine! i might still change it out anyway and see if that makes a difference but i dont know......any other possibilities??

one other details: with no tube in the socket, both plates are even at about 400VDC...so the imbalance occurs only when the tube starts pulling current....

you check if the speaker(s) are blown?

if you have a multimeter you can just put it across the speaker tabs and see that it gives you somewhere in the vicinity of speakers' impedance (8 ohms/4 ohms/whatever they are).

if you dont have a multimeter you can get a 9V battery and use a couple of clip leads. attach one clip lead to the positive battery terminal and one to the negative battery terminal. then briefly touch those clip leads to the speaker tabs - one to each. the speaker should make a little sound as if its pushing a tiny bit of air out (low-ish sounding rustle). if it doesnt make any sound at all you can be sure its blown.

voltages at pin 3 and 8 are around 6.8V

pin 2 to pin 3 is about 6.3 V
pin 7 to pin 8 is about 6.1 V

they are not the same but only 0.2V off or so....i know a small grid voltage/current controls a large anode voltage/current, but this difference seems to small to explain the imbalance, right?

all the resistors in the area (R27-R33) measure spot on.

no obvious signs that c11 or c13 are leaky - both grids on the power tube side of the capacitors are at about -60V....how could i tell if they were leaky otherwise?

i do have to say that compared to the other tube sockets on the board, this one does seem just a bit wonky. but the tube feels snug when its in there. just not AS snug as the other tubes on the board. only other strange thing is that all the tube connectors on the tube side of the socket are silver-ish looking except the pin 1 connector looks gold for some reason....or yellow.....

okay as promised heres a SS amp -

ampeg 350H that was salvaged from a flooded basement. smells a bit musty in there and some of the chalk power resistors look like theyve absorbed a bit of water (the chalk looks darkish greenish kind of).

the good news is it powers up - my limiter tells me no shorts - and things actually look okay in there. the power resistors measure fine (should i be concerned about them) and im getting +/-60 and +/-16 on the main power rails.

edit: i had my scope set wrong. its giving me close to 20V p-p when cranked (into 8 ohms).

im not quite sure where to start - mosfet output stage but its hard to get to them because of an enormous heat sink they are glued to.

anyone give me some good advice for beginning to diagnose this kind of thing? (ive been in tubeland for a while and i have to be honest - my solid state theory needs some fleshing out)

i guess splitting the amp in half power/pre would be a good first step - ill do that and report back!

J M - ive got a solid state ampeg SVT next in line!!

oh, and pin 8/3 is giving me 6.8V....a little low - but seeing as both plates share the same cathode current, how could the voltage there explain one conducting and the other not?

yeah definitely rivets.....