Power tube took out my amp

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Rick1114, Oct 6, 2008.

  1. Rick1114

    Rick1114 Member

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    555
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    Just for kicks I threw in an Electro Harmonix 6V6 I had lying around into my Vibrochamp. I was thinking it actually sounded pretty good (which usually seems to happens before an amp blows up) but when I turned the amp on again later that night, it didn't work and made some weird howling noises (like the type that sounds liks a bad tube usuallly). So I put the the old power tube back in and the amp worked, but it is still making those weird noises. I tried replacing all the other tubes to see if that resolved the issue but the noise is still there.

    Could the bad power tube have taken out something inside the amp?

    I have to open the amp up tonight and troubleshoot some more so looking for ideas....

    Thanks,

    Rick
     
  2. PRNDL

    PRNDL Member

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    I'd recommend checking the grid and screen resistors for the power tube, and the power supply electrolytics.
     
  3. Trout

    Trout Member

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    +1 on the electrolytics,

    If its an original blackface Vibro-Champ, they did not a have grid and screen resistor.

    I often add them one one comes across my bench, not exactly following the keep it mint philosophy, but a good idea none the less.

    I suspect a possible tube socket cleaning and re-tensioning might also be required

    Post big clear internal pictures if possible
     
  4. Rick1114

    Rick1114 Member

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    555
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    It seems like the power tub is getting REAL hot - it lights up like a glowstick. It doesn't matter what tube I stick in there. Does that signify anything?

    EDIT - also the noise is not affected by the knobs or having an instrument plugged in. It sounds like AM radio static coming in and out (bacon frying?).

    Rick
     
    Last edited: Oct 6, 2008
  5. Trout

    Trout Member

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    Sounds to me like the Bias resistor & or bypass cap might be shot.
    Get a picture of the board/guts posted.
    Also might be a bad connection on the tube socket.

    I would not run it any more until you nail it down, no sense potentially burning up the transformers and damaging more tubes.

    Edit, pay close attention to the cap and resistor on the left side of the board,

    [​IMG]
     
  6. Rick1114

    Rick1114 Member

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    555
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    I just masured the bypass resistor tonight and it was reading 250k Ohms. It should read 470k ohms, so that one is way out of spec. Would that be enough to cause the intermittent static sounds?

    The amp recently had a cap job only a few months before...

    Rick
     
  7. Trout

    Trout Member

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    I would re-check that measurement because**


    **The Resistor should read 470 ohms and that is actually the minimum, not 470,000 ohms.
    Did you measure it In circuit? or out of the circuit? If the bypass cap is bad it will give a false reading in circuit.

    Many guys increase the cathode resistor value to 560-680 ohms sometimes a bit more to improve tube life.

    Since the tube is re-plating, chances are better than average that it should have both the cathode resistor & bypass cap replaced.

    It is a pretty simple amp, and there are a very limited number of reasons that it is not working correctly.

    If it is way out on the bias(suspected) it most certainly will make a lot of weird noises as it ruins the power tube.
    Just hope that it did not damage the output transformer.
     
  8. Rick1114

    Rick1114 Member

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    555
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    Found the problem last night. I recently had a cap job but the 25uf/25V bypass cap failed. I had a feeling the parts he used were cheap - he used these crappy tiny nichion caps. I replaced with a sprague 50uf/50V I had lying around. Also replaced the cathode resistor with 1k Ohm metal film I purchased at radio shack - now the tubes don't run overly hot.
     
  9. Trout

    Trout Member

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    You might get by with that on a test only basis, but those resistors are only 1W.

    Yes the schematic says 1W, but that is only a borderline safe value. It is also the reason they fail so frequently.
    You should REALLY get a higher power rating resistor in there, It is widely accepted to use a 5W ceramic wirewound as shown in the picture above.
    I would never use anything under a 2W metal oxide in a champ.

    The constant cyclic heating/abuse is why they tend to drift off and fail so bad.

    YMMV
     
  10. Rick1114

    Rick1114 Member

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    555
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    Good to know about the resistor. I plan on order a carbon comp replacement soon anyway so I'll bump that up to 2W eventually. It just doesn't feel right to keep radio shack parts in a vintage amp! I just want to test the 1k for a while and see how I like that value as the cathode resistor. I read an old John Philips post that recommended 1K cathode resistior with a 50uf/50V bypass cap for champs after 1965. Mine is a '66 and the power tube definitely ran too hot - they were always slightly red-plating, and I think the sound suffered a little. So I will be testing the 1K/500uf combination for while before I order parts from AES.
     

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