Hello John and Hi Tom...
According to a conversation I recall several years ago with Eimac's
(now-retired) former chief engineer for power grid tubes, this internal-cavity
type parasitic oscillation showed up in a new BC cmtr design using the
3CX15,000B7 tube, which it happens ETO uses in its current version 20-30 kW MRI
amps (principal operating ranges 21-42-64-85 MHz). At that time Eimac hadn't
made any internal suppression mods.
I won't try to explain it, but for whatever reason, a big Carborundum-type bulk
resistor mounted vertically near the anode external to the tube with one end to
chassis avoided the problem.
73, Dick
-------
From:
w8jitom@postoffice.worldnet.att.net[SMTP:w8jitom@postoffice.worldnet.att.net]
Sent: Wednesday, November 12, 1997 2:25 AM
To: amps@contesting.com
Subject: Re: [AMPS] Yet more parasitics
> To: amps@contesting.com
> From: John Lyles <jtml@lanl.gov>
> Subject: [AMPS] Yet more parasitics
To: <amps@contesting.com>
> Date: Tue, 11 Nov 97 18:21:43 +0000
Hi John,
> In the present amplifier I am developing using 150-250 Kw tetrodes, the
> bias is pulsed up from cutoff only 5% of the time, so the average
> dissipation is 30 KW or so. It is CLASS A during the on-time. This thing is
> for 2-6 MHz tunable. When I told one tube supplier that I was planning
> Class A, they gulped and said I would be lucky if it doesn't excite UHF to
> L band oscillation. The class A gain of big tetrodes is high, over 20 dB
> possible from the curves. The parasitic oscillation is mainly from the
> large radial geometry of these tubes in the plate/screen to grid region. A
> TE11 or TE21 circular mode can be excited. It is commonly seen when trying
> to make characteristic curves in the test of big tubes. So one version of
> my tube socket will have with a ring of lossy ferrite around the screen
> grid (out of HV harms way of course).
That is correct data, and exactly what Eimac and others tell me. This
is the data that Rich Measures mistakenly applied to small tubes.
Eimac had this problem with a 100 kW TV transmitter tube, the
way I understand it the vacuum head above the cathode was large
enough to sustain a cavity mode oscillation at or near microwave.
Since the oscillation was contained inside the tube, and the tube
drew so much current at the fundamental, the cavity mode oscillation
INSIDE the tube slowly cooked the upper area of the grid. that's all
that happened, the tube apopeared to run fine...no pops bangs arcs or
sparks. The grid just slowly deteriorated.
The problem was solved by adding some suppression INSIDE the tube.
Unfortunately, Rich received a spin-off of the correct data
you posted via an undocumented verbal conversation with an
applications engineer. No one at Eimac knows if Mr. Foote gave Rich
the correct data and Rich twisted it around to small power grid
tubes, or if Foote goofed the data up.
But the fact remains the problems Rich attributes to VHF parasitics
were only found in ONE high power TV broadcast ultra-linear
video PA tube, which had a near micro-wave cavitry mode
oscillation INSIDE the tube.
Unfortunately, QST published several articles and gave the incorrect
message a "Foote" hold, and unfairly failed to tell the whole story
to the amateur community.
But what you say is correct, at least according to Varian and a
Russian tube manufacturer. These oscillations have been found in very
large (multi-kilowatt) high power linear amplifier tubes in Russia,
and they are named after a Russian.
> the RF. Lucky that the glass and smaller ceramic-metal tubes (without
> handles!) have small interelectrode spacing, and these radial field modes
> are above a GHz.
That's right, and all the nichrome, carbon, steel, or ferrite you
like in the outside-the tube- world won't cure a radial field
oscillation INSIDE the tube.
Unfortunately for hams, they were sold a false story based on a true
fact that was totally misinterpreted. Unfortunately the wrong
information will go on and on like the Energizer bunny beating a
drum.
73, Tom W8JI
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