1966 Ampeg Reverberocket renovation help needed

[boodle]

All,
I am working on a friend's Reverberocket and I am getting some very strange sounds.  It was worked on by 2 other techs with very poor results and my friend essentially gave up on it.  I want to give it a shot for him as it was his fav.

Not sure of all of the details of what was changed, but either 1 or both of the transformers were changed.  I have traced the circuit and found a schematic that basically fits the amp with a few changes of component value (schematic below).  I have replaced all of the filter caps and have done some testing.  However, I am stumped by this problem.  It seems like it is caused by a power supply resonance issue.  It sounds like motorboating, but I believe that should not be happening with replaced filter caps.


This link in an mp4 file of the noise:

https://drive.google.com/file/d/1gE9vAn2DnTijiQioHf1PdDlUxhZov6-3/view?usp=sharing

The OT secondary did not have a ground reference.  The slower cycling sound is with the OT secondary with no ground reference, the faster and louder cycling is when the OT secondary is given a ground reference.  I go back and forth with the ground twice in the clip.

Would love to hear your thoughts!!

[Latole]

Do you have a 'scope ?

Try to remove V1 did you hear this sound ?
Yes ; remove V2
Do with all preamp tubes

[boodle]

I do have a scope.  I will try removing each pretube 1-by-1 and post results.

[jojokeo]

I can't hear what you're talking about because I won't d/l anything related to google just to hear what you're talking about. However, have you reversed the OT plate wires? Tried disconnecting the feedback loop? These should be your first order of business especially since the OT was replaced.

[PRR]

> sounds like motorboating, but I believe that should not be happening with replaced filter caps.

Bad caps is only the most popular motorboat, NOT the only possible cause. My first case was push-pull resistors drifted far out of match.

I don't see where you say what the several controls do to the motoboat. (If "none", that is a clue.)

The OT secondary MUST be grounded. A probable reason it wasn't (besides carelessness) is the new OT had opposite phase and the NFB loop howled. Breaking OT Sec ground broke NFB and howl.

[boodle]

First off,
Thank you all for your suggestions.  Please, keep them coming after you have a chance to digest what is below.  I appreciate it.

Jojokeo,
I'll give those a try.  Thx.  Feedback was my next investigation point.

PRR,
In case I don't find issue with NFB or OT phase, which resistors in the schematic are you referring to when you say "push-pull resistors"?
Also, as can be heard in the sound clip, when the OT isn't grounded, there is still a ~1Hz pulse. Pup Pup Pup Pup... When the OT is grounded, it jumps up to ~100 Hz range. (Using my ear as frequency analyzer)

None of the controls had any impact on the motorboating.  In fact...

Latole,
Results of pre-amp tube removal:
Remove                         Result
V1                                 Same sound
V2                                 Same sound
V3                                 Same sound
V1+V2                           Same sound (as predicted)
V1+V2+V3                     Same sound  (as predicted)

When I removed V4, the PI, no sound as one would likely guess.

One other thing I tried to see if I could change the time constant of the sound was to put a 22uF 450V cap in parallel with the 80uF (first filter cap).  No discernible difference in frequency of sound.
I will try some of those ideas and look forward to any other suggestions.

Also, the new OT is a Thordarson 24s55.  It is a Universal Output type with 5 secondary taps and a common.  I have looked around the web for info on it, but to no avail.  Anyone have a lead on where I could find info?

[Latole]

1- Try to reverse wires from OT primary, wrong wires to wrong tubes make noise.

2- Look your noise come from DC supply
3- With scope read DC supply at filter caps
    You should read straight line, not saw tooth .

Report

[DummyLoad]

pull V1-3 and leave V4 installed, with a clip lead, ground pin 1 of V4 - if it still motorboats, then problem is V4 onward. if it ceases to motorboat issue is between V1-3.


OT secondary WANTS a ground. disconnect R33 (5.6K NFB R). if that kills the motorboating, then swap OT primary leads.

[tubeswell]

So the problem stops when you remove the PI tube? That PI has a whole lot of extra parts. Try lifting C27 (plate bypass cap for the first triode)

[boodle]

In case anyone was waiting with baited breath...I have been distracted by a deck remodel project.  What joy.

Thanks for the help, thus far.  I believe some progress has been made.

All I have had the time and energy to do thus far is pull the negative feedback off.  The high frequency oscillation (~100Hz) stopped, but the ~1Hz oscillation is still there.  So, I will swap the OT leads, put the NFB back on and see if only the ~1Hz remains.

I will get to the other recommendations as time and energy permits and report back.

[boodle]

EL34,
I have lifted the C27 cap on pin 2 of V4 in the past.  It had no effect on either oscillation (~1 and ~100 Hz). 
FYI, the schematic that i posted in the OP does not have something equivalent to C27, but it matches the rest of the amp with minor noted differences, so I noticed that cap and pulled up one end to see what impact it had.  It remains lifted at this time.