Pentode valve TC (fwd)

---------- Forwarded message ----------
Date: Tue, 7 Jul 1998 22:10:59 +0100
From: R M Craven <craven-at-globalnet.co.uk>
To: 'Tesla List' <tesla-at-pupman-dot-com>
Subject: Pentode valve TC

Here's a quick note about my current project, just for information.

Thanks to inspiration from Dave Sharpe, Rob Stephens, Pete Cassius and
others, I've finally got round to looking into building a tube TC. I had
access to a reasonably high power pentode, a 5CX1500A, so I decided to start
designing something.

I wanted to go for a valve powered TC mainly because I haven't built one
before: I like the idea of the ability to use reasonably closely-coupled
tank/resonator ccts, and I hope to play with using the finished unit as a
driver for magnifier type setups.

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.

Anyway, I've been struggling to achieve any significant spark output. It's
getting there now; 6 inches of flaming roaring brush at the weekend has
spurred me on a little. In between watching the World Cup, that is!

My tank is 9nF of induction heater capacitor, tuned by 16 turns of 8mm dia
Cu microbore on a 10" x 10" former. My secondary sits inside this, with k
variable from 0.08 to 0.38: the sec is a 6" dia x 25" tall closewound with
0.71mm magnet wire. The topload at the moment is around 10pF: the unloaded
sec resonates at 313kHz which is pulled down to 245kHz by the topload. My
tank L/C works out to 5000 or thereabouts: maybe a little high. I expect my
tank Q to be about 20 or so, loaded. My circulating RF current should be
something like 25 to 30A rms.

I have had problems establishing the grid leak bias, mainlty due to
ignorance on my part, but i now have it more or less sorted out. I expect to
derive the best part of 2kW average power (CW) coupled into the secondary;
this is for a plate diss of 700-odd watts (input PSU power will be 2.7kW).

One interesting thing that I've noticed is that the large em fields
generated interfere with my digital watch and my calculator, at ranges of
about 4 foot. This hasn't been something that I've experienced before with
spark-gap driven TCs: I am guessing that the high average power is the
culprit in this case. I have had spark gap TCs working upto about 3kW input
power, and haven't noticed this effect. The same goes for my outside PIR
security light: it keeps switching on and off with the valve TC but not with
impulse TCs.

Anyway, the sensitivity of the pentode suppressor and screen mean that when
i've got the full cct optimised, I should be able to drive those grids with
modulating waveforms: square waves at a few tens of Hz all the way through
to udio (singing brush etc. Just like Moses saw, I suppose).

Before I get to the final part of the project, ie setting up for maximum
output, I need to build asll of the rotection circuitry: plate overcurrent
trip, grid bias failure trip, screen overcurrent trip. Boring to do, but

One day I hope to making use of two high power triodes i have: one is a 60kW
device and the other is about 8kW. Both are watercooled so I might need more

A question that I have:

How do I calculate plate R for a pentode? For a triode it is u/gm but the
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.


Richard Craven, Malvern, England