Pentode TC progress (fwd)
---------- Forwarded message ----------
Date: Mon, 3 Aug 1998 18:59:24 +0100
From: R M Craven <craven-at-globalnet.co.uk>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Pentode TC progress
Not much progress to report here: I burnt out my 8A variac brushes at the
weekend which curtailed some of the work i had planned to do.
I now get 12" brush discharges, and I have a fair amount of circulating
power in the tank. I haven't as yet measures the tank current that flows but
I expect it to be 20 to 25A rms at the resonant frequency of approx. 350kHz
(haven't tuned it up properly yet).
I amused myself by holding various ferrous items in the centre of the tank:
a stainless steel goblet got hot enough in a bout 2 minutes to cause the old
paint congealed in it to start smoking. A large screwdriver had its tip get
pleasantly warm in about 20 seconds.
My Tek scope probe (P6015) works nicely so I can actually see the waveforms
and dc levels as required. I am getting about -150V dc bias across my GL
resistor, and I am seeing around 150Vp RF on top of this (20% low but
getting there). By laying around with GL R//C values I can get staccato
modes set up ( as advised against in texts on class C oscillators!) which is
fun. I get approx 8" between pulses in this single shot mode (3 or 4 pps)
between points: maybe 200kV pulse.
However, I now have two problems:
The first is that my full-load conditions are stressing my PSU too much. The
PSU is a pair of MOTs, parallelled and phased into a FWR bridge and 100uF.
The MOTs are collapsing so that, just as i see the onset of bigger
streamers, the PSU V collapses and the cct self-regulates so that class C is
only just happening.
The second problem is the corollary to the first: the resonant load Z (tube
load R plus tank Z) is too low ie it draws too much PSU current.
So my solution will be to redesign my tank cct for the time being, so that I
am not over taxing the MOTs. Then I can hopefully get the main PSU sorted
out with a somewhat stiffer supply, replace the original tank components,
and then try again.
I could try knocking a couple of shunt plates out of the MOTs, if that is
mechanically feasible ( don't think it is on my particular two). Failing
that, I hope that the forthcoming radio rallies in England will turn up a
3kV 1A transformer or similar.
So, this weekend coming I will change my 47uH tank L into something like
67uH etc etc and see where that gets me. I think that should be the right
sort of % change in the tank that I need.
Richard Craven, Malvern, England