----- Original Message -----
From: "John T. M. Lyles" <jtml@lanl.gov>
To: <amps@contesting.com>
Sent: Thursday, September 13, 2001 4:54 PM
Subject: [AMPS] amazing doubletalk...
>
> I am amazed at the lack of thought (I mean science) behind
> statements which frequently are repeated here. In the past years,
> this has been discussed and validated on this forum, and in
> professional RF power practice its not even a question to
> regurgitate, and yet, here it is again.
>
> As a professional involved in RF amplifier designs, I completely
> support and agree with Ian White's reply and references listed at the
> bottom of this note, and have also presented physical evidence in the
> way of photographs of the insides of tubes which have arced (2 weeks
> ago here). They were not due to parasitics. Energy is energy. Whether
> we have 3kV with 80 uF or 30 kV with 80 uF of capacitance. I pointed
> out that in the latter case, one MUST use an active shunt crowbar
> device in addition to series resistance. In ham sized tubes, series
> resistance alone may work. But the cause of an unexplained discharge
> within the envelope of a vacuum tube is NOT always because of an
> undesirable oscillation. Gas pressures inside a tube may change from
> effects from high voltage conditioning, changing RF level, DC
> current, load VSWR. We should ask other EE's and RF practicioners who
> design amplifiers to speak here; unfortunately, most will shun the
> unprofessional approach of the 'scientific' method as practiced on
> many internet discussions and won't bother.
>
> I have attempted to give my own experiences to this forum. I
> certainly haven't built as many amateur amplifiers as many here. Or
> troubleshooted them. It is extremely frustrating to read people
> preaching stuff which they believe in like religious zealots. We have
> enough of them in the news in the USA right now.
>
> On 8/30, Richard Measures <2> responded this way:
> JL first stated:
> "I recommend that hams consider other methodologies which can cause
> arcs besides an 'all inclusive' parasitic theory....
>
> <2> replied to this:
> "// The all inclusive parasitic theory originated with ....."
>
> JL stated:
> "in industry we don't subscribe to that theory."
>
> <2> replied:
> "file://nor do I"
>
>
> OK, we finally hear that there are other possibilities in mind?
> 73
> John
> K5PRO
>
Surely there are a range of causes for arcs in tube-based power amplifiers,
for example when I was younger (much younger, like 20 years ago) my first
W1SL 2 x 4CX250B amplifer for 144MHz had a poorly designed power
supply, the screen volts were switched from +330VDC to ground between
transmit and receive - the common contact of each relay was connected to
the tube's screen grid.
In this circuit during T/R change-over the screen grid would float for
10-20mS
(the physical change-over time for the relay) - this would allow the screen
to
rise in potential (towards the anode) and cause a flash-over on occasions -
maybe only once in 6 months but it used to happen. A simple fix was a 100K
resistor from the screen grid to ground.
Very poor, or no loading - for example the antenna falling off or becoming
disconnected could result is an instantenuous 30:1 VSWR with resultant high
RF voltage on the anode(s) and again... flash bang... this has happened to
us
during a contest and probably many others.
There are yet other causes for flash-overs, like out-gassing and some would
suggest cosmic rays and/or radio-active decay of heavy elements used in, or
accidently introduced into the tube construction. It is alledged that these
can
release enegetic nuceli which can strike other gas atoms and cause a break-
down which in turn can result in a flash-over.
All of the above are good reasons for a "glitch resistor" - which reminds me
that I must fit one in my 8877 on 6m.
I used 5 x 220ohm 14W Vitreous Enamel (glass) wire-wound types in parallel
in my last amp (3500V at about 1.2A) - this gives about 44ohms at 70W. Now
the specs say they are rated for 750V AC but in testing they didn't appear
to
mind a direct short to ground (ultimate glitch) but then again I also have a
16A magnetic circuit breaker (MCB) in the transformer primary which drops
out and saves the resistors from continued stress...
Mike G8TIC/M0VHF
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