Princeton AA964 clone low volume output and various other issues

[Les s of Paul]

Dug through all the voltages and only one doesnt check out right. i'll list that one in bold.
7025
pin 1 200v
after resistor 300v
pin 3 1.5v
pin 8 1.55v

12AX7
pin 1 273v (flucuates by 10 or so)
pin 3 1.75v (should be 2.4)
pin 6 234v

Power tubes
pin's 4 420v
pin's 3 417v

Rectifier line 420v. middle of resistors, connected to half dome side can cap, 416v

Intensity return -40v
Tube Bias, 17.5mv (read across 1 ohm resistor soldered direct to ground)

Tweaked the ground, theres still more to go, but the volume seems to be a bit greater, not much. next i'll read the signal through the amp.

MV signal, volume, tone, and guitar vol-tone maxed.
7025
60mv @ pin 2
2.5v @ pin 1
.4V after 250pf
100mv after .047
250mv after .1

touching pin 7 causes very large ammounts of hum, so much so i wasnt sure it was safe to keep the meter there long enough for a reading

250mv @ pin 6

12AX7
300mv @ pin 7
320mv @ pin 6 maybe 360

.5v @ pin 5 of power tubes
5v @ pin 3 (output transformer line)

All numbers are as close as i could read, used a steady guitar signal and checked multiple times. all voltages with respect to ground.

[nickb]

The 1.75V is probably OK. It's not the root of your volume loss anyway.

Good job on the measurements. You seem to have sensible level signals around the tone stack, plenty of gain from 7025 pin 7 as evidenced by the hum. You didn't give an AC mV figure for the wiper of the treble control but it seem s there could be a problem between the tone stack and the 7025 pin 7. Carefully check the wiring and values. Good luck.

[Les s of Paul]

Unfortunately i have not had time to rebuild the tone stack, however i did turn the amp on recently and discovered that unique putt putt sound is still around. i should note, the sound ONLY appears with the volume completely off, Eg signal routed straight to ground. hoping to rebuild things tomorrow, just thought the little piece of information might lead to a revelation.

i will also re-iterate, Thank you greatly for all the help Nick.

[Bruce / Mission Amps]

No... that front end triode is measuring wrong IMHO.
You should have something like 4vac to 6vac coming out of pin 1 with a 70vac to 100mvac input signal at the jack.
That stage runs flat out in most all BF or SF Fender style preamps.

Start your tests over again.
You have too many things looking wrong and messed up.
EX:
7025 Pin 1 measures 200v... after the resistor is 300v.. WTF, what resistor, the 100K plate load?
Pin 5 of the power tubes: .5v !??? ... should be like -35vdc to -45vdc .... or something like that
Pin 3 of the power tubes: 5v !??? ... should be like 420vdc or something like that
See my point?
I can see where your explanations are a little nebulous.

Find a similar style PR schematic... ignore what the original schematic says the voltages should be.
Measure each B+ rail node from the rectifier tube, lug 8 all the way to the last main filter cap.
Write those on the schematic at their appropriate places.
Go back and measure all those preamp tube test points again... lugs 1, 2 and 3 especially and write them on the schematic.
This way, at least we could see where the DC voltages are going.
These are super simple amps and should not be difficult to trouble shoot if you work your way through systematically... not hysterically.

[nickb]

Bruce,

Les switched to using ACV measurements for the bottom half of the results. The DC values are in the ballpark but the AC path in wrong.

[Bruce / Mission Amps]

Yes, the AC signal voltages are messed some how.
I just did a complete, ground up, refurb on a real 1966 Princeton Reverb this Wednesday afternoon so the whole thing is very fresh in my mind.
The preamp stage(s) in his build is not amplifying very much of anything.
I'm surprised to see 5vac, or 2-3 watts at the speaker jack.

[Les s of Paul]

minor problem there Bruce, your running a PR i have a straight up silverface AA964, not an AA1164 or otherwise, which in my understanding has less gain than a PR. plus the DC voltages are much lower. Some of the terminology i have used is a little off yes, but as i stated before, i'm new to this amp building. i'll attach a schematic with voltages listed shortly, just need to write it out, both signal and DC listed.
Still havn't figured out why this thing sounds like an idling motorboat when the volume pot is at 0. ah well...

Heres the schematic, there's a couple missing voltages highlighted in green, and the one in orange, what is that supposed to read? looks like 638V, but i remember reading something like 64V there.
Pardon the poor handwriting.

[JoeM]

I have both the PR and non reverb Princeton. Voltages in the non reverb are not lower, maybe even higher with fewer tubes. Gain is quite different, you can put the non reverb model all the way to '10' and it will hardly breakup (if then). The non reverb seemed gutless to me, so I used the 1/2 tube that was for tremolo, and built it closer to the PR version.

[nickb]

Les,

There is a decent signal at the the treble pot but you hear next to nothing (only 5V signal at the primary). You also said there is a load of hum when you probe grid 1 7025 pin 7. My best guess is there is something wired up wrong between this two points. Could it be you have the wiper of the volume connected to ground?

[Les s of Paul]

Solder bridge maybe, i've had to dissassemble and reassemble this amp several times. but if the volume wiper was grounded wouldn't all volume be gone? and yes, probing pin 7 to ground produced a bucketload of hum, more volume than the amp with a guitar plugged in even! go figure...

]according to the schematics i have, the PR has more Dc voltage than a standard Princeton, but that's neither here nor there.
Skip that statement, Whoops. i Opened mouth and inserted foot.

[nickb]

I was thinking that maybe the pot was turned so that you had a high impedance between the grid and ground and some stray capacitance coupling the signal into that path making it weak with lots of treble. Rather than speculate, go look.

[Les s of Paul]

i would, if i had my voltmeter... just examined a few of the conenctions quickly, is it possible a poor connection between the treble and volume pot could produce these symptoms?
the one thing i have not tried is shorting the treble connection side of the volume to pin 7 to see if anything improves.

[nickb]

It would account for the loss of volume.

[Les s of Paul]

well, i have FINALLY discovered the source of the poor volume, and its completely unexpected until you actually check.
Faulty Volume pot. for some reason the treble input side of the volume pott became disconnected from the track, the ground was still connected, so reading the ohm's appeared to be correct Until you try to read across the pot with the ground lifted. just goes to show, When in doubt, Check it out.

all thats left is to clean up the grounds, and re-install the pot i have on order.

Thanks for All the help guys, many extra thanks to you Nick. i should have some celebratory pics within the week. (still finishing parts of the cab)

Les.

[nickb]