Using zeners for G2 regulation

[Lingwendil]

Throw the reference voltage created by the zeners to the gate of a mosfet. Use a suitably sized heatsink and you will have a much more flexible supply for the screens. I throw a potentiometer and resistor stack across the zeners to have a bit of adjustability in setting the screen voltage, something like a 100k pot on top of a 330k resistor can give you a bit of range to play with.

Something like what Pete does would be a swell solution here-

Choose zeners as needed to set the voltage you want.

 

[kodabmx]

Or use one of these and run it from the heater supply.
DC-DC 8-32V to 45V-390V Step-up Module ZVS High Voltage Capacitor Charge Board | eBay

 

[DAK808]

Thanx for the schematic. Unfortunately, the chassis for this amp is quite compact with no room for adding a heatsink and extra circuitry.

 

[Brice]

+2

Would it be for a reason allowing G2 to swing along with PS voltage? If one wants to investigate... just put a hard coupling between those nodes with a couple of BJTs.

I have an PPP 6L6 amp with OC3 regulator for G2 supply.
The way they did it is the V regulator removes 150V from the B+, and not clamping the G2 to ground.

So essentially they wanted the tubes to be driven in the same condition relative to the B+ swing.

That tells me there is a good reason but I don't know the theory behind.

 

[kodabmx]

I've done that with 0A3 to drop 400V plate to 325V screens... Worked well enough for me. One tube per pair.

 

[Brice]

1 regulator for pair? Interesting. Were you concern with current consumption? OA3 = 30mA. I do have OC3 = 40 mA but for 4 6L6.

 

[kodabmx]

Kind of. When one tube goes up, the other goes down in push pull, but G2 max current can be over 20mA depending on the tubes. Now that I mainly run triode connection, I don't need to regulate G2 anymore

 

[MarcelvdG]

Sounds about right. It's within spec if the zeners are %10 tolerance.

Do you know any Zener diode types which such a large tolerance? I'm more used to seeing 5 % to 7 % tolerances (for the standard models, less when they have been selected for better accuracy).

91 V Zener (or actually avalanche) diodes have very steep (dynamic) characteristics, so the fact that the current probably differs from the test current also doesn't explain it. I think SemperFi is probably right with his/her remark about self-heating and temperature coefficient.

 

[kodabmx]

Maybe I just use cheap ones, but 1N53** I find are always higher voltage than stated.

 

[mondogenerator]

I have experienced that too with those devices, although whether it's a trend if the device or heat stabilisation I couldn't say.

 

[disco]

Perhaps you observe the loading of a bypass cap at G2? For best regulation current excursions need to be as low as possible (against heating up), that's where the BJT (or FET) is of aid.
When you observe the dynamic properties of a sole 1N5377B, its impedance is 760 ohm at 1 mA, dropping to 75 ohm at 15 mA, while the maximum regulator current is 52,5 mA. A bypass of 1uF will ride along with G2 voltage at any signal level.