Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TurboGuitarMelton on November 28, 2018, 12:07:43 pm
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Hey everyone,
My Princeton Reverb clone was getting what sounded like some blocking distortion (cutting in and out) with the volume anywhere from 7-10 and the bass maxed out (the problem subsided mostly when i turned the bass down). Sounded fine at "normal" volume levels. I had ruled out all cold solder joints/etc.
I read from another post where someone was having the same issue it turns out it is the V2 12at7 reverb driver tube that was causing the issue. I pull it out and crank the amp - tonal bliss. Put the reverb driver back in,the problem has returned.
From what I gather, the issue is the tube oscillating and causing things to go sour down the line at high volume. However, I'm not so sure the tube is actually faulty. I purchased a new set of low noise Jan NOS tubes for the preamp and they should be here soon so i can verify the issue is my old 12at7.
Here is my question: On the B1270 PR schematic there is an extra 2000pf cap on the reverb return to the amp that isnt present on the original AA1164 schematic. Is this there to help stop this oscillation from happening? That cap is really just bleeding high frequencies to ground. Should I go ahead and add this as well? The PR Reissue also has 220pf caps on the in and out of the reverb tank (which i suspect are also doing the same thing)
Thanks!
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Not an exact answer to your question. I have read that the reason CBS added the little caps was because the lead dress was not so great in later amps and these caps prevented oscillation. Since we can't see your build then maybe you could try moving the wires about, or cleaning up the lead dress.. not saying it is not good but cannot see it to know. Additionally ... as an experiment try adding the .002uf cap and see if it helps. You can just clip it in with jumpers if you have them or lightly tack it in with solder.
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Thanks for the reply mresistor! My pic is too big to upload. Ill double check my leads. I tried to keep them all short as possible. Ill probably also just go ahead and add the cap for extra protection too!
Also, i didnt mention this but my step down resistors in the power suplly are different values from the fender ones. My B+ is 429 and i have a 1k2w resistor, and two 15k 2w resistors down to (D) on the schematic. According to the fender voltages i should be getting 240v or so there but im at 271v because of the 15k resistors. I also have some 18k resistors on the way to replace them.
Could the fact that the preamp is running hotter with slightly higher than normal voltages cause a problem like this? I know the 12at7 reverb driver is running around 400v on pins 6 and 1 like normal but maybe the tubes before or after it is giving it slightly more kick than its used to and is causing oscillations. Makes sense to me. Especially since when i turn the bass up it seems to overload and when i play quiter or turn the bass down the problem goes away.
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Disconnect one end of the 25µF cap from V2 pin 3/8. Any better?
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I don't think a little higher plate v on the small tubes is really a problem.. are there any mods in this amp like the Stokes or Paul C mods? Personally I took those two mods out of my '78 as I like it better
the way it is.
Let is know why you come up with.
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Hey guys, sorry for the delay. Sluckey, thanks for the suggestion! I will try that if all else fails. I know im being a bit ridiculous but id rather not go in unsoldering/clipping things just yet. The cap there is the standard 25uf value and is visibly okay.
Mresistor, yes i confirmed the voltages arent causing the issue. I had the stokes mod in it before but changed it back to stock. Totally agree with you. I think the princeton reverb is perfect the way it is.
So heres what I did:
I added a .001 cap to the reverb out. This didnt change the blocking distortion problem but I like what it did to the tone of the reverb so im going to leave it.
Changed the power resistors - there was no change in blocking distoriton here either. But All of the voltages are spot on now! :icon_biggrin: Peace of mind!
Finally, i took out the 12at7 and temporarily tried a cheap 12ax7 i had around. BAM, its fixed! It must be that tube doing something weird. Even with the extra gain of the 12ax7 the reverb worked great and i got no blocking distortion on 10. I still have to wait for my new set of tubes to come but once they do i can confirm once again.
Anyone know why that tube is making the amp behave that way? :dontknow:
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Hey guys,
Changed the reverb driver tube to a low noise NOS 12at7. Problem is gone. I am assuming that the issue was that the old tube was just going bad. I tapped the old tube before i changed them and heard ringing through the amp, microphonic?
Ive read a lot about this particular issue and it sounds common among fender amps. Hopefully thats the last of it for me!
However, I really would like to understand what was going on with that tube to make it do that. Anyone have some insight? :help:
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sounds common among fender amps.
425vdc on the plates just might add to tube failing effect :icon_biggrin:
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Uh oh... Its now doing the freakout thing again. Except this time its a high pitched kind of shrieky distortion. Same thing though... Amp on 7 or higher and the amp goes nuts.
If I remove the 12at7 or the output reverb cable FROM the amp it stops. Leaving the output cable and removing the return cable makes no difference.
Could be the reverb tank? I checked it and it seems fine. This is actually a different tank than the one I used when this problem first started.
Tube again???? That NOS 12at7 hasn't been in there too long... And Ive been gigging with my other amps.
Reverb transformer?
:dontknow:
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Here is a pic of the lead dress... I think its all pretty clean. Correct me if im wrong
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Disconnect one end of the 25µF cap from V2 pin 3/8. Any better?
I should listen to you every single time man... Haha you always know what to do. This fixed the issue perfectly.
Will I need to leave this disconnected or is this an indication that the cap is bad?
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Update: added a 1M grid stopper to grids of V2. Fixed the horrid overdrive tones. ✌
I read another post here on el34 world where a guy was having a similar issue with another blackface amp. Hmm. Makes me wonder if people complaining about the PR phase inverter distortion is really the reverb driver.
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Maybe grid stoppers on the PI, per Merlin & Robinette, will also help. You could also try using a smaller bypass cap on V2, like around 1uF, for less bass boost.
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Update: added a 1M grid stopper to grids of V2. Fixed the horrid overdrive tones. ✌
I read another post here on el34 world where a guy was having a similar issue with another blackface amp. Hmm. Makes me wonder if people complaining about the PR phase inverter distortion is really the reverb driver.
Thanks for posting this! I just picked up a '74 PR and it had a 12ax7 in V2 when it arrived, so I promptly changed it to an old cv4024 and I had the exact same symptoms as above. It was fine with a 12ax7, but with the correct tube in it, any digging in above 3-4 on the volume caused an ugly buzzy distortion tone that quickly decayed into the type of distortion/overdrive that is pleasing to the ear, the one that we all love. Notably, with the tank disconnected the amp was fine with either tube in it.
I tried messing with a bunch of things, tested every resistor and solder joint on the board, changed the cathode resistor value up to about 10K gradually (since Fender had a .47K in it anyway, which is not correct by the schematic), all which did nothing. Then I read this thread and put a 10K grid stopper on V2 and voila, sweet chimey glory! Buzzy nonsense gone.
Thanks again!