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Flyback protection diode question

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  • Flyback protection diode question

    Hey, that rhymes!

    Anyway, do I need a diode string on all four output tubes or just one per side?

    Got a Marshall JCM800 1959 beast that customer uses with an attenuator and I'm trying to bullet proof it as much as possible.

    Installed a post phase master to also help


  • #2
    One string per OT half. Number in string dependent on voltage rating. Some newer amps just use a single higher voltage diode per half.
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


    • #3
      Cool thanks.

      I just used 3 4007's per string since I have a bunch.


      • #4
        The diodes are protecting the transformer, not the tubes themselves. So as g1 says, one per side of the transformer winding.
        Education is what you're left with after you have forgotten what you have learned.


        • #5
          As it’s getting attenuated, it’s not like the max clean output power is actually needed. So I suggest to increase the value of the screen grid resistors a little, eg 1k5 or 2k2, as that will ease off the stress on the output valves and everything else
          Yes it will change the tone a tiny bit, but so do those diodes.
          Last edited by pdf64; 01-12-2022, 04:21 PM.
          My band:-


          • #6
            I would use a pair of R3000 diodes, (3kv at 200mA) to stop back emf from arking over in the transformer.
            Support for Fender, Marshall, Mesa, VOX and many more.


            • #7
              When I did maintenance on my MusicMan Rd100, one of the diodes was shorted. The originals were 5KV, it has been working fine with 3KV.