Reverse Diode protection for Power Tubes?

[leadfootdriver]

I seen it on a the Express layout. I wanna' put it on my single EL84 amp.

The Wreck has 3 diodes grounded from the plates on each EL34. How did he come up with 3 for each tube?

[JamesHealey]

If one blows the others are still doing there job. You need to change them with every set of tubes that fail.

[paulster]

It's about the reverse breakdown voltage, which is 1000V for a 1N4007.

Put 3 in series and you won't get breakdown until you've got 3000V on the primary, which means that you're not going to impact transients that you do want, but aren't going to let flyback voltages develop which will exceed the hi-pot rating of the transformer (or kill the tubes).

[leadfootdriver]

So do you think I should run 3-1N4004's in there for my little single ended?

[paulster]

I'd still use 1N4007s.

In a push-pull amp, the 3000V clamping voltage is on each side of the primary to the centre-tap, so you have effectively 6000V peak-to-peak before clamping, so with a similarly specified single-ended amp you'd still be wanting around 3000V.

The plate voltage isn't likely to be significantly lower than a push-pull and you'll still have the AC component of the audio riding on top of the plate voltage so it's still a sensible choice.

[leadfootdriver]

Alright then. The little amp has about 290 DC, and 600 AC coming off' the rectifier.

[guitarmike2107]

Correct me if I am wrong but in a SE amp you won't get much more than 2xHT voltage swing.

Not sure about transient spikes though?

[VacuumVoodoo]

It's not about protection against full signal swing but fly back when there is intermittent/abrupt open circuit on secondary side. These flyback transients can reach several kV depending on transformer leakage inductance and other parasitic impedances between primary and ground.

Diodes in series as protection is kind of a lottery, they don't always fail as short circuit, in which case you still have the remaining ones to rely on.
They can fail open circuit too - depending on how much energy was in the fly back transient - leaving you unprotected.
There are dedicated transient protection devices available like MOVs and Transorbs (Google: transorb, transient voltage suppressor).

[leadfootdriver]

Thanks for the info!