Peavey JSX flyback diode replacement

[booj]

diode for peavey JSX.doc

Peavey uses an SR2873, rated at 2KV 250ma. diode. Is there any reason why two 1N4007's in series wouldn't work?
I thought it would be fine to do this, but then I thought maybe I'd better check.Peavey JSX.pdf

[mozwell]

two 1N4007 will most likely be ok. I woudl add a 1M 1W resistor across each 1N4007 to ensure voltage sharing. There must be a reason why peavey woudl use a 2kV diode, when they are using 1N4007 elsewhere..... so i would keep the 2kV rating.

[booj]

Thanks Mozwell! I also found some info on the Theory and design forum in a thread entitled "Flyback Diodes" started by Wilder amplification and answered eloquently by R.G.
I have a question though about using resistors across the diodes. If their purpose is to clamp voltage, they would be conducting, at which point the resistors would do basically nothing. In the reverse mode, if one should break down earlier than the next, it wouldn't be "allowed" to conduct because the diode it would be in series with would not be conducting. A 1 Meg resistor would only serve to make the diodes more leaky, would it not? R.G. suggested 3 1N4007's, (per side) which is what I installed.

[jrfrond]

The reason that 2kV diodes are used is so that they don't short when hit by the very transients they are meant to dissipate. Fender has been using 3kV/200mA diodes (usually R3000) for years now.

I question as to whether these diodes are needed at all. In my mind, they are there to protect substandard tubes and transformers. You never saw them in vintage amps.

I've seen amps where people have installed three 1N4007's in series.

[Amp Kat]

I agree John and if all is running well why have them and IMO is just another part to fail when tubes go bad and they will go bad. OP should also make sure to change the tubes as that's what caused them to blow in the first place.

[defaced]

They're a safety device. RG's discussed this at length if someone wants to do the searching - I think it's even been discussed within the past week. In short, the flyback diodes are to dissipate the voltage transients if the amp is operated without load/something else goes wrong. It's an inexpensive way to save the OT.

[mozwell]

the 1M resistors are there to make sure the diodes share equally the reverse voltage. Without them, the reverse voltage is shared based on reverse leakage current of each diode. This is common practice is power electronics.
In practice, for 1N4007, and in your application it probably doesnt matter, and 3x 1N4007 in series shoudl work fine for you.

Glad you got it all working again

[Steve Conner]

I question as to whether these diodes are needed at all. In my mind, they are there to protect substandard tubes and transformers. You never saw them in vintage amps.

That's probably because high voltage diodes weren't 10 cents each back in "vintage" times.

And maybe because designers back then didn't expect their amps to be abused as badly as modern ones.