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Dual Self Split ECC99 amp stops humming when I measure the cathode Voltage

Discussion in 'Shock Brother's DIY Amps' started by reggiepe, Oct 14, 2020.

  1. reggiepe

    reggiepe TDPRI Member

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    I built a dual 12BH7 Self split amp...a little info troubleshooting the build is here:

    https://www.tdpri.com/threads/mojotone-studio-one-inspired-build.1050572/

    I know I can run ECC99's (with the cathode resistors changed and reflected impedance adjusted, of course) in this amp as well. But when I swap the 12BH7's for ECC99's I get a loud hum. That hum stops completely when I touch the positive probe to the shared cathode of V4 to get a voltage reading. Anyone ever experience this before? I don't mean with ECC99's, but with anything that was cathode biased.

    I have attached the schem for the Mojo Studio One inspired build
     

    Attached Files:

  2. andrewRneumann

    andrewRneumann Tele-Holic

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    Hello Again :cool:,

    Does the hum get better/worse when you measure V5?
     
  3. LetItGrowTone

    LetItGrowTone Tele-Meister

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    Just guessing that grid stoppers on the top half of each pair would stop this, maybe even on only one of them.
     
  4. dan40

    dan40 Friend of Leo's

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    Perhaps a small value ceramic snubber cap would help. They are not normally used for hum issues but it would be easy to try.
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    That is what I thought. Touching with the probe is probably introducing some capacitance. Also, it may not be hum as much as oscillation heard as hummm.

    Although, a grid stopper can solve issues as @LetItGrowTone opined.
     
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  6. sds1

    sds1 Tele-Afflicted

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    I've killed a particular hum in a particular amp with DMM leads before, I want to say on the PI grids...

    At the time I posted about it and nobody had a clue why the DMM would be able to do such a thing. Only speculation.

    Never seen a good answer for it.

    Anyhow, I would say ignore the part about the DMM. Red herring. You got a bad ECC99 that's my guess, I would recommend you start there. The ECC99 shouldn't hum in a circuit that wasn't humming otherwise. It needs no special configuration compared to the 12BH7. It should "just work" as you already knew.
     
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  7. reggiepe

    reggiepe TDPRI Member

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    Sorry, got behind on my forum posts

    Stays the same when I measure the other cathode.

    Where would the snubber cap go? across the cathode?

    I did assume that the ECC99 was bad. but I can run the each one individually and they seem fine.
     
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