On Tue, 25 Nov 1997 08:33:28 +0008 "George T. Daughters"
<gdaught6@pobox2.stanford.edu> writes:
>
>hi all,
>
>rich wrote...
>
>> If your amplifier arced mysteriously, it might be interesting to
>measure
>> the resistance of the VHF parasitic suppressor resistor.
>
>it is perhaps important to note that an increase in the resistance of
>a composition resistor (or a film resistor, too) can increase when
>the resistor gets too hot. (rich has said this, too.) it doesn't
>have to be so hot that it bakes the color bands to new colors. the
>point i want to convey is that it doesn't *have* to be a parasitic
>oscillation that causes this change. i've seen resistors in dc power
>supplies increase by a factor of 3 or 4 by chronic overheating, but no
>
>smoke was emitted.
>
>73,
>George T. Daughters, K6GT
George and others:
I have brought this point up many times here...I'll try AGAIN.
A 2W carbon resistor will show a gradual and steady R increase when run
above 1W or so....this is at DC.
At RF, the heating usually occurs in the 10-15M range, depending upon the
tubes used and the suppressor design.
At RF the 2W carbon can double, triple, etc. in value in a few minutes
or over several years...again it all depends on the design. While
initially experimenting with SB-200 and SB-220 6M conversions the
resistor stench in the shop was intense!
In the stock SB-220 the original 47 Ohms will become 80-300 Ohms by now
if the amp has spent any real time on 10M...time to change the oil.
My own rule of thumb is that 300-400W is the maximum RF on 10M per 2W
carbon for reasonable long term suppressor reliability. If you have more
RF then double or triple up the wattage or switch over to a modern metal
oxide.
Take a look at the catalogs....new 2W carbons are over $2 each. I prefer
the 5W metal oxides that have recently come on the market; Mouser is a
good source and they are cheap. With a pair of those 5W in parallel and
some decent L it is darn near impossible to smoke them.
I believe that Tom gave some very realistic R and L values here a few
months back that took various popular tubes into consideration.
IMO, the 2W carbon will quickly become obsolete.
73 Carl KM1H
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