Bias supply and Rg1

[hylaphone]

Hi all, I'm having a hard time finding a definitive answer to this - does the bias supply resistance itself factor into the maximum Rg1 rating of a power tube? Or is this only referencing the grid leak + stopper value?
I'm working on something with KT120 which has a max Rg1 rating of 51K, but the PT I'm using has no bias tap. Deriving bias supply from the HT secondary will mean a big series resistance and I wonder if I'm better off adding a dedicated transformer for this.
Thanks--

[Helmholtz]

- does the bias supply resistance itself factor into the maximum Rg1 rating of a power tube? Or is this only referencing the grid leak + stopper value?

The way I understand it, the limit applies to the total DCR between grid and cathode (or ground).
So it is the sum of grid leak (aka bias feed resistor), grid stopper and bias supply shunt resistor.
51k means a heavy load for the PI, lowering PI gain and drive capability.
So make sure to use a strong PI (not a 12AX7 with a 100k plate resistor).
Best would be CF drive.

This said, most designs exceed the Rg1 limit of the power tubes.
Making Rg1 too large bears the risk of thermal runaway caused by the grid building up a positive charge.

[hylaphone]

The way I understand it, the limit applies to the total DCR between grid and cathode (or ground).
So it is the sum of grid leak (aka bias feed resistor), grid stopper and bias supply shunt resistor.
51k means a heavy load for the PI, lowering PI gain and drive capability.
So make sure to use a strong PI (not a 12AX7 with a 100k plate resistor).
Best would be CF drive.

This said, most designs exceed the Rg1 limit of the power tubes.
Making Rg1 too large bears the risk of thermal runaway caused by the grid building up a positive charge.

Great, I thought so. Any drawbacks, theoretical or practical, I should be aware of when getting bias voltage from HT secondary?

[Helmholtz]

Well if the bias supply shunt resistor (that's the resistor across the output of the bias supply) already needs to be around 50k, then you're having a problem.

[pdf64]

My understanding is that the g1-k resistance limit applies at full dissipation.
As dissipation reduces, the g1-k grid circuit resistance can be increased.
So provided your design isn't attempting to squeeze the KT120s hard, you may be ok to eg double the total effective grid circuit resistance.

[catalin gramada]

My understanding is that the g1-k resistance limit applies at full dissipation.
As dissipation reduces, the g1-k grid circuit resistance can be increased.
So provided your design isn't attempting to squeeze the KT120s hard, you may be ok to eg double the total effective grid circuit resistance.

Yes, bias till max 70% power dissipation and it may double it. Some datasheets published the ratings for different power dissipation.