Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: velec on September 19, 2019, 12:02:17 am
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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.
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I would change that fifty year old filter can then re-evaluate.
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Well I took your advice, doing what I should have done before, and replaced the filter capacitors. No change :sad2:
I also completely disconnected the tremolo circuit from the signal path to eliminate that as a possibility. I am really stumped on this. :help:
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Replace rectifier; maybe bypass it temporarily with SS diodes. If that doesn't help then use a listening amp.
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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.
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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!
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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?
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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.