Fender Vibro Champ buzzy distortion problem

[velec]

Hi everyone! I'm new here, just found this place and signed up, looks like a great forum. Hoping that somebody can point me in the right direction here.

I am trying to repair a 1968 Vibro Champ for a friend. At all volume levels it has a really harsh buzzing distortion sound "underneath" the normal clean tone. Volume levels are normal, tremolo works fine.

The cathode bypass capacitor on the 6v6 was burned and letting a lot of current through when I got it. I replaced all the electrolytics on the board and verified good bias voltages all over. Tried known good tubes except for the rectifier as I don't have one on hand. Tested, and tried replacing, the bypass capacitor before the 6v6. Checked all resistor values and capacitor values. Tried a known good speaker. I'm going nuts here, nothing helps the problem!

That I can think of, the main things I have not been able to try swapping out due to not having parts on hand:
-Rectifier tube
-Filter capacitors
-Output transformer

In everyone's experience, what does the described symptom point to? I was initially thinking tube bias or bypass capacitor, but neither of those things helped the situation.

[sluckey]

I would change that fifty year old filter can then re-evaluate.

[velec]

Well I took your advice, doing what I should have done before, and replaced the filter capacitors. No change   

I also completely disconnected the tremolo circuit from the signal path to eliminate that as a possibility. I am really stumped on this. 

[jjasilli]

Replace rectifier; maybe bypass it temporarily with SS diodes.   If that doesn't help then use a listening amp.

[pdf64]

Could be a vibration induced bad connection, a bad speaker, or something vibrating in the cab. Try it through a different 4 ohm speaker cab and reassess.

[velec]

Thanks for both of your replies.

I temporarily put in a solid state rectifier that I had lying around, which operated the amp just fine but did not change the problem. I also went ahead and built up a listening amp adapter in a hammond box and poked around with that. Strangely, I could not reproduce the problem through a listening amp setup, even when probing the output. I had an 8 ohm cement resistor on the speaker output when I was testing.

Right now I have the speaker and metal amp chassis out of the cab and resting on my bench. I have tried two different known good speakers, one a wgs 8" intended for a champ, and one a 12" 8 ohm celestion. Both still yielded the same problem.

I'm going absolutely nuts!

[pdf64]

Old resistors can develop invisible cracks, such that the body in actually in 2 pieces that are being held together by pressure from their leads. They can measure ok but cause this type of symptom.
So I would go through the circuit, using a gentle pressure to check for that.

How is the issue affected if the negative feedback loop is broken, eg one leg of the 2k7 resistor lifted?

[velec]

That is a good idea to check the resistors. I will go through and give it a shot.

The 2k7 negative feedback resistor makes no difference if one leg is lifted - I initially suspected that and tried both lifting a leg and replacing it with another and saw no change in the symptoms.