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
(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.
will give me an ultimate source of 3kV at 1A average. I wanted to go for
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
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

	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:
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
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.