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
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|