plate diodes

[scole]

what is the purpose of the three diodes om each power tube?

[martin manning]

...to allow flyback voltages (from running without a speaker load connected, say) to drain to ground. If the reverse voltage gets high enough, the diodes are sacrificed to save the OT. See here for more discussion: https://tubeamparchive.com/viewtopic.ph ... highlight=

[scole]

why is tw the only amp with these?

[soma_hero]

its not. look around at other schematics.

I had issues with 300W bass amps that ran plates at 500V on 6550s, KT88s, arching tubes. Flyback voltages develop at I believe (-)2*B+ on the opposite side of the primary. Flyback diodes prevent this voltage by clamping to ground. Keeps the tubes safe. Look at old ampeg schematics.

Please correct me if the math or theory is wrong.

[jaysg]

He picked it up when he was working at Ampeg. Some of the V Series amps have that, though implemented with a more obscure diode.

[scole]

then should'nt we do this to all amp builds?

[jaysg]

then should'nt we do this to all amp builds?

I do.

[husky]

Anyone try the diode from the grid to cathode to keep the grid from turning into an anode? Like on the Fender Pro series amp Pro reverb from the early 2000's?

[markr14850]

Anyone try the diode from the grid to cathode to keep the grid from turning into an anode? Like on the Fender Pro series amp Pro reverb from the early 2000's?

Would this be like the "Paul Ruby Mod" with strategically chosen zeners from grid to ground?

[Zippy]

then should'nt we do this to all amp builds?

Agreed. That was the intent of my thread on "Bulletproofing an Amp":

https://tubeamparchive.com/viewtopic.ph ... t+proofing

[Clyde]

I don't think these diodes are necessary. I've never used them in any of my builds (many), no problems.

[husky]

I don't think these diodes are necessary. I've never used them in any of my builds (many), no problems.

Well the grid ones are for tone more than protection

[husky]

Anyone try the diode from the grid to cathode to keep the grid from turning into an anode? Like on the Fender Pro series amp Pro reverb from the early 2000's?

Would this be like the "Paul Ruby Mod" with strategically chosen zeners from grid to ground?

kind of yes but might be a better way to do it
At least with non cathode biased amps.

[Clyde]

Sorry, I should have clarified my posting. I meant the plate diodes. My understanding of the diode from grid to cathode implementation is that it occurs on dc-coupled stages and cathode followers.

[soma_hero]

The flyback diodes are only necessary normally when you're using high plate voltages. I said 500 earlier, i actually meant 700V. Since i started using flybacks, no more arced tubes.