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.
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.
The above would usually show a gradual increase in R.
Internal heating from VHF...parsitics or whatever, often show an
immediate increase of several times the original value and no
discoloration if it is a very fast and minor "zap".
Any serious parasitic will reward you with a very distinctive odor and
curls of smoke.
During development of a suitable suppressor for 6M QRO use I destroyed
many, many 2W carbons. In several instances I could watch the parasitic
on the plate and grid meters with no drive applied. Altho not strong
enough to show any external signs; they destroyed the
resistor.......unwind the coil and you could snap them in half.
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.
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.
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.
Instead they show up during tuning and/or operating and can be monitored
on the meters as instability in the readings.
BIG BANGS are caused by internal gas and sometimes by the breakdown of
the air gap due to extreme secondary resonances...not to be confused with
parasitics... in the tank circuit. Call it the lightning effect if you
wish.....Flash and then BANG.
73 Carl KM1H
then it
>suggests that the suppressors do get a large power input during the
>parasitic oscillation duration. If the change is due to long term
>heating merely because of the RF power dissipated from the plate
>current
>flow dividing between the resistor and the shunt inductor, then the
>change should be similar to that which would appear from heating the
>resistors with DC.
>
>Just to add to the debate, I haven't seen anyone come up with a
>boiling
>point temperature for gold in a vacuum - if it's lower than at
>atmospheric pressure, then the arguments about grid temperature take
>on
>a new dimension.
>
>73
>
>Peter G3RZP
>
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