variable regulated negitive grid supply

[Svetlana]

I want to parell output tubes, in a PP amp.
All the tubes draw different ammounts of plate current for the same given negitive grid voltage.
So should I devise a variable regulated negitive grid supply for each tube, then adjust to set all of the tubes to draw the same plate current?

 

[fdegrove]

Hi,

All the tubes draw different ammounts of plate current for the same given negitive grid voltage.

If you put in matched tubes the currentdraw should be shared more or less equally among all of them.
A few mA difference won't matter much for as long as they stay within that range after burn-in.

Going for individual bias on every single one is a possibility but certainly not economical and in almost all cases absolutely unnecessary.

Cheers,

 

[Svetlana]

Hi Frank,

I wish they were matched.

The current range is from 60ma to 415ma at the same given negitive grid supply.

 

[fdegrove]

Hi,

The current range is from 60ma to 415ma at the same given negitive grid supply.

?? Are you sure about this?

Cheers,

What are these tubes BTW?

 

[Svetlana]

old 6v6's

 

[fdegrove]

Hi,

old 6v6's

Under no circumstances should you have more than 45mA idling current per tube.

You must be doing something wrong there...

Cheers,

 

[dhaen]

If the +B is unregulated, it's best IMO to leave the grid bias supply unregulated.
The reasoning is thus:
If the valves are running anywhere near max dissapation, ann increase in mains voltage could cause them to overrun. However if the grid bias is unregulated, this will tend to compensate, and the valves will be safe.
Of course you still need the supply to be low impedance.