Hi Jim,
-
Yep, a lot of 4-250a, 4-400a plug and play has been done
through the years. For a new project, I would separate
the grids and provide control bias to G1. The series
zener in the filament xmfr return works very well, but
another option is a very simple negative bias supply
(obtained via the heater windings) and a resistive
voltage divider.
Concerns about the supply regulation are noted but
do not appear to be a problem with modest but
respectable supply divider currents in AB2
applications I've built. The addition of a bias
supply shunt regulator is very easy.
While not appearing as simple as the zener bias
method, the supply method has a measure of
variable control bias using tap/slide resistors or
an adjustable regulator.
The advantage would be an easily adjusted range
of available control grid bias potentials for same
family tube swaps. Once built, the supply adjustment
does not require additional parts (diode) changes.
But, for a one time build, set and play... its hard
to be the simplicity of the diode set bias method.
cheers
skipp
--------- Forwarded message ----------
From: Jan Erik Holm <sm2ekm@telia.com>
Could be but the best amp I ever built was with 3 x 4-400A in GG,
no parasitics no mickey mouse stuff it just runs and runs, could I
have been lucky perhaps.
Has around 4 kV on it and pinns the 1 Amp plate meter, puts
out around 2.5 - 2.7 or so with 200 W drive, sure big difference
compared to my 4-1000A amp which is nothing but head ache,
why oh why was I so stupid that I did build sutch an amplifier.
73 Jim SM2EKM
PS: Remember that I once put 4-250A?s in an SB-220, I just
shorthed out the zener and it worked quite ok.
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