>
>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. 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.
Rich...
R. L. Measures, 805-386-3734, AG6K
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