Deluxe Reverb oscillation

MrCoolGuy

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So this amp started life as a Hotrod Deluxe. Someone (wish I knew who)
converted it to an AB763. The power transformer blew up. That's when I got it.
We decided to rehouse the circuit board (a decent looking handwired turret board) into an actual BF Deluxe Reverb chassis and cabinet. I removed the mods that had been done and got the board down to a normal AB763 and wired it all up. Installed a new power transformer (Mercury Mag. FBFDR-P-FS), powered it up with the current limiter, no tubes, then with tubes... everything looked fine (except B+ was a little high, 450), so I tried it out. With any any amount of signal (or just turning the volume up without a signal), I just get loud oscillation.
I tried reversing the OT secondaries... it got worse. So I switched them back.
I've tried swapping tubes, I've tried checking for loose connections/Jack's, the reverb isn't even hooked up...

One thing I've noticed is... if I remove V6, I can still gear a signal through the normal channel, quiet but no oscillation. But the vibrato channel still oscillates if I turn up the volume at all.
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schmee

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It appears you are using two speaker jacks that are shunt jacks. Could that be allowing neg feedback to do something undesireable? Not so sure, but one is just a mono jack normally.

Did you put the PI input wires to the two 220k resistors under the board? They are usually not under there....
 

MrCoolGuy

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Your guess is as good as mine. I wonder if there might be a problem in the power supply somewhere which is allowing signals from the power amp to couple with signals from the pre amp. Electrolytics are fresh and power supply grounds all checked?
The filter caps are new, but the cathode bypass caps aren't. Though they were working fine before I pulled it out of the old chassis. I'm sure I have something wired wrong, but I can't find it. I triple checked everything on the bottom of the board before I put it down (since it is so hard to fix anything down there)... whoever made the board had several mods: both channel reverb, trem always on, and some weird choices that I didn't understand, but I'm fairly certain I removed all of that. I really thought I had everything right...
 

Wally

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I really thought I had everything right...
In Robert Pirsig’s “Zen and the Art of Motorcycle”, he describes this type of trap. I forget the name he gave it. When something you have done does not yield expected results, this type of thought…I cannot have made a mistake… or I thought I had everything right…..gets in the way of solving the problem. Embrace the attitude that indeed you may have made it an error. This will enable one to SEE the situation more accurately. Delusion gets in the way of solving the problem.
Great book, imho.
 

MrCoolGuy

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In Robert Pirsig’s “Zen and the Art of Motorcycle”, he describes this type of trap. I forget the name he gave it. When something you have done does not yield expected results, this type of thought…I cannot have made a mistake… or I thought I had everything right…..gets in the way of solving the problem. Embrace the attitude that indeed you may have made it an error. This will enable one to SEE the situation more accurately. Delusion gets in the way of solving the problem.
Great book, imho.
Oh, belive me. I'm SURE I made an error!
As I said.
 

andrewRneumann

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Thank you both. Good call @andrewRneumann .

Each of you have helped me multiple times! I wouldn't have thought of checking the filter cap ground for the symptoms it had. What made you think of that?

When you said you still got some signal even with the PI removed. I started thinking it could be sneaking in onto the screens of the power tubes. Really just a lucky guess. Glad it's resolved!
 

MrCoolGuy

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They were connected, but the ground wire that comes through the chassis was stuck where it was kinda hard to see.
 

Lowerleftcoast

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I started thinking it could be sneaking in onto the screens of the power tubes. Really just a lucky guess.
Good guess!
That one would have taken me a minute. It all makes sense looking at it from hindsight. I bet I would have stumbled through finally finding the lack of ground connection and then realizing how the signal made it to the power tubes.

Not too shabby Andrew.
 

MrCoolGuy

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I'm still not sure I understand.
In fact, I'm sure I don't.

There's another thing I don't understand...
These transformers sound fantastic, I'm really impressed. But why does everyone (Mercury magnetics, Heyboer, Hammond, etc...) Make their Deluxe Reverb power transformers 350-0-350 (or even higher)
when the originals were 330-0-330?
 




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