[AMPS] Grid bias supply?

[Wilbert Knol]

Thanks for reading this message.

I am in the process of reviving an old amplifier. It uses two ancient  
TB2.5-300 triodes in 'grounded grid' configuration.

The aim is to make this thing work in Class C (CW) and according to 
the specs the valves want -200VDC on the grids, for a total grid current 
of 2 x 40 mA = 80 mA.

My question concerns the grid supply. It appears to me (perhaps 
wrongly) that the grid supply needs to be able to hold the grids at -200 
VDC, and *sink* the 80 mA when the amp is provided with max drive.

Unless I have got this wrong, an unregulated supply using a bridge 
rectifier and an electrolytic cap wouldn't be able to sink the grid current 
(it would do a great job holding the grid at -200 VDC under no-drive 
conditions...it never really has to source any current)

Any grid current would cause the electrolytic to be charged up even 
further, resulting in a more negative grid.

My question is: if all this is correct, is it bad enough to worry about? 
Should I put a 210 V / 20 W zener diode across the grid supply to 
prevent the grid bias point from taking off past -210 V? Or a big bleeder 
with an 80 mA standing current?

Wilbert, ZL2BSJ



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