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From: Rich Measures[SMTP:measures@vc.net]
Sent: Monday, December 08, 1997 12:13 PM
To: km1h @ juno.com; Peter Chadwick; amps@contesting.com
Subject: Re: [AMPS] parasitic suppressors
>
>On Mon, 8 Dec 1997 14:48:20 -0000 Peter Chadwick
><Peter.Chadwick@gpsemi.com> writes:
>>Rich has said on a number of occasions, and, I believe, without
>>contradiction, that it is well worthwhile measuring the value of
>>parasitic suppressor resistors after a flashover. My question is:-
>>
>>Do the values change because of long term over heating, which would
>>possibly show up in surface colour change?
>
>OVERHEATING would show an external change Peter and it would not take a
>long time to be evident. A 2W carbon will discolor the paint in short
>order even at 2x overload for 10 minutes or so.
>You can run the tests for yourself at DC.
Agreed.
>
>One source of discoloration, never mentioned by Rich, is the coil tightly
>wound around the resistor. IMO, in poor or marginal suppressor designs
>the coil is the primary heat source.
When #16 -gauge copper buswire carries 15 continuous amperes, it feels
barely warm to the touch.
With 15 amps @ 28 MHz #16 gets blistering hot in short order. Even #12 gets
hot. That's why we use 3/16" to 5/32" tubing at 28 MHz.
It's a sure bet that Rich would not mention
this is a possible heat source.
>
>The above would usually show a gradual increase in R.
>
This seems unlikely considering the apparent lack of heat.
...snip...
>I have switched to metal oxide primarily due to the cost of decent
>carbons. At 50 MHz and below there is no noticable effect from the very
>small Xl. Metal Oxides can also fail completely open with no external
>indicator so caution and testing is advised in their use.
>>OR
>>
>>Is there an enormous pulse overload causing the value change? If so,
>>why
>>is there no apparent visual colour change - or is there? Does it
>>depend on the resistor type?
>>
>>If we assume that Rich is correct with the parasitic theory,
>
>
>That is a major assumption Peter; I for one do not completely buy into it
>as the answer for all problems....it is only a part.
Agreed
>
>IMO, the instaneous BANG when the amp is even on standby has nothing at
>all to due with parasitics. I agree with Tom and others that this is
>strictly a gas problem.
Curiously, the proponents of the rauchian gas theorum never seem to
bother checking for tube gas with a high-pot. after they declare victory.
>ANY commercial amp that has been on the market long enough for a
>shakedown cruise does not exhibit instantaneous parasitics of such a
>nature to cause a BIG BANG.
The owner of many TL-922s and SB-220s have reported it. The Henry 3KA
and 2K-4 apparently do it on occasion.
>Instead they show up during tuning and/or operating and can be monitored
>on the meters as instability in the readings.
>
Intermittent vhf parasitics rarely, if ever, politely show up on cue by
one's adjusting the tank.
OTOH, it's pretty easy to come up with not-quite-adequate parasitic suppression
in a typical amp so that VHF parasitics can be made to
politely show up on cue by adjusting the plate tune cap (and maybe the grid cap
if used). With slightly better but still not totally adequate suppression one
may sometimes induce a polite intermittent VHF parastic to show up under
transient conditions such as keying.
Dick W0ID
Rich...
R. L. Measures, 805-386-3734, AG6K
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