Low Volume Princeton Reverb AA764

[jonathan_kea]

Recently had an issue with my blackface reverb where the amp began buzzing very loudly with very little output. I assumed the can capacitor was bad, so replaced with one of the CE mfg cans. I also replaced the bias capacitor with a 100uf/100v.

While the buzzing is gone, the output is still extremely low.

I tested all the tubes on my eico tester and they are all strong. Just in case, I swapped in known good tubes for everything but the 5AR4, which I don't have a spare for...

I took several measurements across the board and noted them on the attached schematic. Anything jump out at you, or advice on what to check next?

[sluckey]

Screen voltage cannot be more than OT C/T voltage. Other than that I don't get any clues from your voltage readings. Don't rule out the OT as a suspect.

[jonathan_kea]

I'm getting 388 and 389 at pin 4 of each 6v6, so I think that is ok.

I've been playing the amp with a 2X10 organ speaker cabinet through the cabinet extension jack, maybe that has killed the OT? They are cheap, so I will probably order one and see what happens.

[sluckey]

I'm getting 388 and 389 at pin 4 of each 6v6, so I think that is ok.

Not saying it's a problem, but you wrote 402 on the schematic and 390 for node A. That's impossible. Just pointing that out.

[jonathan_kea]

Aha. Thank you for clarifying - I'll check that again.

UPDATE: I was going to try plugging in a different speaker to the amp and noticed that I had PLUGGED THE SPEAKER into the EXT SPEAKER jack by accident.

I plugged it into the regular output jack and it WORKS at full volume! Much quieter, too.

Thank you for taking a look and checking my voltages - evidently the problem was between chair and monitor :)

[sluckey]

Back in the '60s a R&R friend gave me his PR practice amp because of low volume and he did not want to pay to have it fixed. All that was wrong was the speaker plugged into the EXT SPKR jack. I sold it to my brother in law for $50.

[dwinstonwood]

I sold it to my brother in law for $50.

You could maybe get a used Blues Jr. for that today.

[Platefire]

I was getting ready to say, have you checked your R&R hearing loss lately, but it was a simple fix.

[RadioComm]

UPDATE: I was going to try plugging in a different speaker to the amp and noticed that I had PLUGGED THE SPEAKER into the EXT SPEAKER jack by accident.
I plugged it into the regular output jack and it WORKS at full volume! Much quieter, too.

 

Thanks Jonathan. My story...

I bought a MusicMan well over 25 years ago for $50.00 USD.  Individual seller says "Don't know its background, I'll sell to you cheap". Aesthetically is was in great shape, obviously not neglected at all. Good deal. Brought it home, plugged it in, and *real* low volume. Like you, all voltages checked out, so started taking pot shots it, and finally decided it was the OT. I'll change it later... 25+ years later still collecting dust in the basement, and then I read your post. Uh? What if? I asked myself. Went downstairs to the basement this morning, plugged it in the other speaker jack, and what-do-you-know... volume!  Tickled pink, so funny. Need to get it back on the bench now, and tune it up after all these years.

Take care,

ttfn