once again - fixing this one up for a friend.....

symptom was no/low signal and weird sound when turned on.

quick preliminary looked showed that the screen grid resistor on V5 (R42) was fried (low voltage there - resistor tested open and looked a bit burnt). ill change that.

BUT upon looking around the rest of the amp i also found that the PI voltages were bad. ~150V on pin 1 and ~330V on pin 6. now i remember having a similar issue on a deville i worked on. i also know more about long-tailed pair configurations now, enough to say that it looks like pin 6 is not conducting! both heaters are lit. i tried a different tube - same symptoms (a 12ax7, admittedly, not a 12au7 as it says on the schematic, but that shouldnt matter for this issue, correct?). ive resoldered all the pins and adjoining connections and checked the region and it all looks good. does NOT seem mechanical.......any other things i should check or am i just being sloppy with the mechanical stuff most likely?

this one is fine, right:

http://www.classictone.net/40-18000.html

?

when i test the center tapped primary, does it matter which two leads of it i test across?

also, where i live (in the san francisco bay area) there are a few too many amp and music 'enthusiasts' for any amp to survive in a junkbin long enough for me to find it - im fixing this for a friend and hes okay to buy a new OT -

any suggestions where to get one online? ebay seems like a bit of a crapshoot for something like this as far as getting one with appropriate specs that does in fact work (no more shorted turns, please!) - is triode electronics reliable?

sooo

i popped a neon out of some thing i had laying around and built the transformer tester -

i used a 9V and some diodes to get the voltage down to 6ish

the results were not encouraging but also a little confusing (trying to understand the dynamics of the transformer tester itself):

(all of these tests were done with all transformer windings open circuit - disconnected from anything)
a) the peavey transformer secondary did not light the bulb at all
b) the peavey transformer primary did not either (i wasnt sure how to test it (because its center tapped), but no configuration worked to light the neon
c) to make sure my tester was working, i tried it on the output transformer of a little champ i built last year - the secondary of this transformer also did not light the bulb! totally possible that output transformer has some issues because this was the first amp i had ever worked on and i probably turned it on open load who knows! the PRIMARY of this transformer did light the bulb however
d) for a final  test of my tester i tried it on an unused brand new transformer i bought for a mixing desk power supply im building - the primary windings are 125V windings and they DID light the bulb - the secondary windings are only 25V windings and they did NOT.

seems to indicate that the peavey secondary (and my poor little champ!) has a shorted winding. i guess a follow up question, though: should a healthy output transformer in a 50W amp be high enough inductance to give me 90V+ to light the bulb? isnt the amp only putting out about 40V p-p through the transformer secondary?

and: does it seem likely that the peavey PRIMARY also has a shorted winding? wouldnt this be fudging the voltages on the amp side of the output transformer?

and finally: its hard to tell because it flashes so quickly, but seems like only one of the little strips in the neon lights up when it does (if i just have the neon across line voltage from the wall - with a resister in-line, of course, both strips light up brightly).

wait - it turns on with 90V?

roly's diagram has it hooked up to a 6V...

i dont have a neon light handy -

could i juse a 9V, an LED, and a resistor in the same configuration? is there something special about the neon light (brightness, sensitivity, or ) that makes it ideal for this kind of test?

all im testing is if there is a shorted path around the light, so the light passes no current through it, right? if all is good, the LED will light brightly. if there is a short in the winding, it wont light at all or quite dimly. correct?

ah yeah i see both tubes that have turned white have a crack in a similar place alongside some of the pins - ill check the socket closely - might be bent in one side putting too much pressure on the glass

yeah screen voltage is more or less equal to plate voltage on both power tubes

roly,

are you saying its a physical issue with the valve itself? another valve in that same socket has done the same thing - what non-physical/mechanical things could cause it?

they are passing like 37 mA with about -50V on them - that seems normal, right?

v4 turned ghostly white!

since there is no reverb tank to speak of, if i just pull v4 out and play without a tube in that socket, will the amp be mad?

QuoteWhere 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:)

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)

Quoteyou 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?

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.