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Re: Pentode valve TC (fwd)
---------- Forwarded message ----------
Date: Wed, 08 Jul 1998 16:57:38 -0700
From: Ed Phillips <evp-at-pacbell-dot-net>
Reply-To: ed-at-alumni.caltech.edu
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Pentode valve TC (fwd)
"The PSU requiremnts for a pentode are somewhat more stringent than a
triode
(which would be the normal choice for a self-excited class C
oscillator), so
I have gone for a pair of MOTs which are FWR and smoothed into 100uF.
This
will give me an ultimate source of 3kV at 1A average. I wanted to go for
a
voltage doubling LSAC approach (as recommended by Dave Sharpe), but the
ripple in the output would mean that the screen on the pentode would end
up
acting as a virtual anode - too much risk of screen damage due to
dissipation. I might build a full doubler cct and then smooth the
output."
Feed the screen grid through a dropping resistor from the plate voltage
and all will be well.
"figures for my pentode don't add up. My u (SG1-SG2) is 5.5 and my
transconductance is 24e-3 mho. That would imply a plate R of 230 ohms:
that
is very very low and is not borne out by the slope of delta(Ip/Vp) which
would suggest twice this value: also extremely low! I thought that
dynamic
plate r would be several tens of k for a pentode."
The correct mu to use is the control grid to plate mu, which should be
quite high and give you a much larger Rp. Don't have curves for that
tube so can't check for you.
Ed