peavey classic B coupling caps

[ilyaa]

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!

[g1]

  I have had new caps like that with some kind of white powder on them, it's not leakage.  Generally you only see issues like that with electrolytics.

[nashvillebill]

There are several electrolytics being used as coupling caps, have you replaced all the electrolytics?

If any caps look suspect--even film caps-- I'd replace them.  And getting that voltage back up a few more volts might help too.

[ilyaa]

NOTE: in the subject i classified this as a classic B - its actually a Classic A! who knew?! (schematic attached)

i swapped out the electrolytics in the preamp (some of the other ones looked newer...) and i think this amp is as good as its going to sound -

im fixing another one (identical amp), though, and am a bit puzzled -

it had the same phase inverted issue - i changed those transistors and that seemed to do it, except the PI signal swing was low
the JFET driving the phase inverter was giving me weird voltages so i changed that out and im getting about ~150V p-p when the bias is turned all the way down to -75V - so the PI is working good, i think!

BUT, the amp is only making like 12 watts RMS (measured at 4 ohm dummy load)

everything up to the power tubes is looking good, but they cant seem to deliver power. they are pretty much new, ive only used them a little bit in some other amps, so im really hesitant to blame the problem on them, but they are NOT brand new - could anything else be responsible? there is nothing between them and the load but the OPT and thats working so - any thoughts?

i was thinking maybe the big filter cap, although giving me good static voltages, maybe cant handle the voltages when the amp is driven? that cap is definitely old - could it be the culprit? i took some voltage measures when slamming a 1kHZ wave into the amp, and all the voltages are lower than their static values - is that to be expected? obviously im measuring DC when there is some activity (not quiescent), so maybe those values are useless.

[J M Fahey]

BUT, the amp is only making like 12 watts RMS (measured at 4 ohm dummy load)

everything up to the power tubes is looking good, but they cant seem to deliver power. they are pretty much new, ive only used them a little bit in some other amps, so im really hesitant to blame the problem on them, but they are NOT brand new - could anything else be responsible? there is nothing between them and the load but the OPT and thats working so - any thoughts?

you need some 50V RMS to drive those tubes, do you have them?
To be more precise, signal peak at the grid *must* be able to beat bias voltage and drive that grid a few mV  positive , that´s when the tube delivers maximum possible current.

i was thinking maybe the big filter cap, although giving me good static voltages, maybe cant handle the voltages when the amp is driven? that cap is definitely old - could it be the culprit? i took some voltage measures when slamming a 1kHZ wave into the amp, and all the voltages are lower than their static values - is that to be expected? obviously im measuring DC when there is some activity (not quiescent), so maybe those values are useless.

No, it´s normal to drop somewhat, say 10/15% under load.

One doubt: when you measure those 12W, do you scope the output and reach visible clipping?

[ilyaa]

you need some 50V RMS to drive those tubes, do you have them?

yeah ive got about 50V RMS at the grid - maybe a bit lower. and 12W at the output with visible clipping. if the PI was giving me a bit less than 50V RMS at the grid, would that really make the difference of 30+W at output, though?

[Roly]

What I would do.

Hook it up to my dummy load (and wattmeter  ).

Crank it up until I see clipping on the load, then see where that was coming from, i.e. is the PI clipping, or the output stage?

I hate to say it, but low output with everything else good (I assume the voltages and currents around the OP stage look reasonable?*) could be a shorted turn in the OPT.

So I'd set up some OPT tests, perhaps try to measure its turns ratio using heater power.

There's also the kickback test with the battery and neon.

(* are they?)

[J M Fahey]

if the PI was giving me a bit less than 50V RMS at the grid, would that really make the difference of 30+W at output, though?

Wrong question.
The sequence is this:
1) Class AB1 biased tubes reach maximum current, hence maximum power (whatever it is) with grids at 0V.
So peak driving voltage must be *at least* bias voltage.
2) Peak power fed into the OT will be peak current squared into plate impedance.
So if we do not get expected current, we won´t get expected power ... simple as that.
And since it´s a squared function, getting half the current means getting one fourth the power.

[ilyaa]

hmmm okay heres what im getting

with the bias voltage at about -50V, one side of the PI is giving me about 45V RMS and the other side only about 35V RMS - could this be the problem?

ON SECOND MEASURE: the figures above are when i measure at the collectors of the PI transistors - if i measure at the grids of the power tubes (after the grid stoppers) i get about 35V RMS on each grid.

thats reflected in the ouput, where one side clips way harder and sooner than the other...and there is pretty bad crossover distortion, too, when the PI isnt even clipping yet...

is a little imbalance a good thing or is that too much?

(the output clips way before the PI, also)

ALSO: the DC voltages around the PI dont look perfect. oddly enough, on the power supply end of the 82K 1W resistor, the voltage looks high (should be 460 and its at about 490), but on the PI end, its low (should be 300 and its only 260). the collectors of the PI transistors are a bit low at 130-140V, instead of 150. could the PI bias be off, causing the low output? (these measurements are with the power tubes installed - without the power tubes in, all the voltages look just like their schematic values)

[Jack1962]

if your using a light bulb limiter (I hope you are) the voltages will be lower than those on the schematic , from what I have read here check your PI tube or try a known good tube in that socket .

[J M Fahey]

Pretty bad crossover distortion translates to horrible biasing plus hard to drive (hence low power) tubes.
And you might simply have worn/bad/poor quality tubes.
Dont trust just the bias *voltage*  but its effect on the tubes.
Add a 1 ohm cathode resistor to each tube, measure voltage across it, 1mV means 1 mA current, and bias so they are around, say, 30 mA each.
Post results because I guess both tubes are not matched and then we continue.

[g1]

on the power supply end of the 82K 1W resistor, the voltage looks high (should be 460 and its at about 490), but on the PI end, its low (should be 300 and its only 260). the collectors of the PI transistors are a bit low at 130-140V, instead of 150.

Where are you getting those "should be" voltages from?  Are you using a different schematic than the one from your first post?

[Roly]

I'm still thinking shorted turn in the OPT (if the OP bottles aren't sick "they are pretty much new").

Note that the driver diff pair are within the NFB loop and may only be reflecting a downstream problem - such as a shorted turn in the OPT.

[ilyaa]

Where are you getting those "should be" voltages from?  Are you using a different schematic than the one from your first post?

its a classic A, not a classic B, i discovered

the new schematic is attached above (in my post that starts with NOTE:)

[J M Fahey]

Up to 20% difference in DC voltages is not usually a problem, so don't worry about 460 vs 490 or even 260 vs 300 .
I'd worry today about *real* voltage needed for each tube passing, say, 30 mA which is an indirect  indication of how worn they are.
As in: a tube with no emission left will need a very low bias voltage just to pass idle current.
And if they are confirmed good and reasonably matched and drive is there, yes, the dark possibility of a shorted OT raises its head.