Flyback protection diode question

[drewl]

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

Thanks.

[g1]

One string per OT half. Number in string dependent on voltage rating. Some newer amps just use a single higher voltage diode per half.

[drewl]

Cool thanks.

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

[Enzo]

The diodes are protecting the transformer, not the tubes themselves. So as g1 says, one per side of the transformer winding.

[pdf64]

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.

[Jon Snell]

I would use a pair of R3000 diodes, (3kv at 200mA) to stop back emf from arking over in the transformer.

[Bill Moore]

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.