Modern HV EL34s?

[Chuck H]

You're right. The zener type number must be wrong.

You can almost count on it. And it's probably not accidental either. First, why bother with a mere 6.8V drop? Second, Mesa is notorious for misleading information on their schematics.

[Helmholtz]

Concerning linear reduction of grid drive: The first thing that comes to my mind would be decreasing the grid-to-ground resistors to 100k or lower. But of course there is a number of other possibilities as well.

[Jazz P Bass]

OKay but please put up with me as I ask another question. My confusion is that the drawing shows a screen voltage on pin 4 of the V10 6L6 of 454 volts. It also shows a screen voltage on pin 4 of the V12 EL34 of 383 volts. Granted there is a 1K7 more screen resistance and a 6.8 V zener in the feed to the V12 EL34 but that seems not to explain the extra 71 V lower voltage at the V12 EL34 Pin 4. Can you tell me what I am missing? is the sample schematic just bogus?

The Mesa schematic as shown does not include the power supply circuit.

It is entirely possible, that is where the screen node is dropped to 383 Vdc. (something like a 6 K/ 7W resistor)

[Chuck H]

IIRC I used 150k for the grid load. Which isn't a huge reduction from the common 220k value. I honestly don't remember what I did to reduce drive on the two tube AB1 amp. The other was single ended so I just reduced gain from the driving stage.

[Reader]

The Mesa schematic as shown does not include the power supply circuit.

It is entirely possible, that is where the screen node is dropped to 383 Vdc. (something like a 6 K/ 7W resistor)

I don't suspect that since both screen feeds are attached to the same node in the lower right of the schematic. I'm open to correction if I'm still misunderstanding something.
Mr Chuck H said that the figure was lifter from a Mesa schematic and that Mesa is often deliberately deceptive. A case of learner beware?

[Pedro Vecino]

OKay but please put up with me as I ask another question. My confusion is that the drawing shows a screen voltage on pin 4 of the V10 6L6 of 454 volts. It also shows a screen voltage on pin 4 of the V12 EL34 of 383 volts. Granted there is a 1K7 more screen resistance and a 6.8 V zener in the feed to the V12 EL34 but that seems not to explain the extra 71 V lower voltage at the V12 EL34 Pin 4. Can you tell me what I am missing? is the sample schematic just bogus?

That part of the scheme corresponds to the Road King model. I think the exact diode is 1N5372 (62V/5W). It must be a typo.

[Reader]

That part of the scheme corresponds to the Road King model. I think the exact diode is 1N5372 (62V/5W). It must be a typo.

Thanks for that input! Makes a lot more sense now. It's hard to figure this stuff out when people just host a keyhole View how's the whole circuit. but that seems to be common here for some reason. I figure it's because of some limitation in file size.

[Gregg]

It can't "regulate" the voltage to below a limit during spikes but it does reduce them by the amount of the zener voltage.

That's right. I never said it will regulate the voltage, just lower it at idle around and below 450V.

The zener number on the schematic (Roadking II) is obviously wrong. It's more like 1N5372 which is a 62V 5W zener.