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Twin Reverb Plate Voltage issues

Discussion in 'Shock Brother's DIY Amps' started by JKube, Jan 3, 2021.

  1. JKube

    JKube Tele-Meister

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    Hello all. Quick question for the gurus. I have an intermittent voltage issue that’s been plaguing me on and off for a while now on my Blackface Twin build. I see a plate voltage of 5-10VDC on start up with power tubes in, which jumps to the 100V or so range with a hard strum, which sounds like a farty, distorted sound. Zero-volt plate reading with power tubes out. If it sits long enough, boom, plate voltage shoots to 480V and the sound comes rushing back. Biased to 38ma (which obviously reads zero when voltage issues are present), but goes right back to 38ma when the problem goes away. I remember having a similar issue when dealing with bias resistors on the build. Does that sound like a possible culprit? Would a shorted bias resistor send the entire rectified voltage to gnd and kill the power at the plate?

    Full disclosure, it just got to the point that I couldn’t take it last night, so I have yet to pull the chassis at check B+ around the board, check the transformer output, etc. I’ll break her open this afternoon, but wanted to give some time for the question to simmer before I did.

    Cheers!
     
  2. sds1

    sds1 Tele-Afflicted

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    Trace that weird voltage backwards towards the power supply. Do you see your full B+ at OT CT for example?

    Seems like a bad connection type of problem? Any shorts of B+ to ground would presumably blow the fuse.
     
  3. JKube

    JKube Tele-Meister

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    What makes me think it’s a part and not a connection, once the voltage is there, it stays indefinitely, until I turn the amp off and let it sit a few days. Then it’s gone again until it sits running for long enough to come back. Sometimes 5 minutes, sometimes 2 hours. It doesn’t go in and out on a minute or second basis like a cold solder joint.
     
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  4. sds1

    sds1 Tele-Afflicted

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    Roger that, I know this is exactly not what I said... but we are on the same page here in that the B+ isn't shorting to ground but is simply not getting to the plates.

    I would pull the power tubes for remaining troubleshooting, this will rule out all connections from B+ thru the tubes, ie to grids (bias circuit) and cathode. If no B+ at plates with no tubes, you can work backwards towards the power supply and ignore everything else.
     
  5. JKube

    JKube Tele-Meister

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    Well, I think I found the culprit. Giving the terminals on the bias pot a little pry with a drumstick while running duplicated the issue. Looks like my bias pot has a loose terminal on the interior. Tried to wick and resolder the connections, it’s gonna need a new pot. Now I just have to figure out the values.
     
  6. dan40

    dan40 Friend of Leo's

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    A bad connection on the bias pot would not cause your B+ voltage to disappear like it does. Using your meter, you should be able to work your way back through the power supply when the amp is not functioning and find the point that your voltage disappears.
     
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