Your were fortunate, but I doubt every tube of the same
family and type would default idle at 60mA. Cutoff bias
is made with the neg supply method by simply lifting the
"grounding side" of the resistive divider.
-
3K has not reliably provided "enough" cutoff potential
in some of my projects. I had to "up the value..."
-
cheers
skipp
-
-
From: DF3KV@aol.com
I wonder what you want to do with grid bias in that configuration.
You will just get 60mA with OV bias at 4KV per tube.
In my L-4B with QB4/1100 I use a 3K resistor from the
heater transformer centre-tap to ground which I short out on transmit.
--------- Forwarded message ----------
From: skipp isaham <nospam4me@juno.com>
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
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