>>I guess it happens to those who think that every bang is a parasitic
>>oscillation. For one, with proper suppressor resistors you shouldn't have
>>an oscillation.
>
>? The Challenger o-rings were supposed to be plenty ok in cold weather.
O-rings in solid rocket boosters has nothing to do with this. You've still not
explained how an amp biased in cut-off can have an oscillation. If you can
explain this to my satisfaction, then fine. If not, admit that it is possible
that other nasty things can happen in an amplifier that is NOT a parasitic
oscillation.
I agree that without a proper suppressor circuit, the amp can break into
oscillation while being keyed or keyed rapidly as you suggest. But all common
sense and logic tells me that when an active device is biased in the cut-off
region, it is just that: cut off.
>
>>I don't think the oscillation changes the value of the
>>resistors.
>
>? A big-bang event happens in a 2-holer.. The suppressor resistors are
>100 ohm +/- 5% units. The resistors appear to be in mint condition. .
>Each resistor is virtually shorted by a few turns of #16 copper wire.
>The measured resistance of the suppressor resistors was over 400 ohms.
>I was there. I made the measurements.
OK. You say the resistors "appear to be in mint condition." That tells me
nothing of their value prior to the oscillation.
Resistors in an anode line will get warm. Warm from some amount of RF flowing
thru them in the form or tube output and circulating current. With tubes with
high Cout, this can get large on the upper bands. The resistors also get warm
from thermal effects. Heat is conducted thru the anode line and convection
also warms the resistors due to the fact that the ambient temp in the PA
cabinet has raised. The power handling capability of resistors is directly
related to the temperature of the resistors. There will be a point when the
resistor has ZERO dissipation regardless of how many watts is printed on the
side. Resistors running for years at high temperature will also age faster and
have a shorter MTTF than a resistor held at 25 C. Tube amps would fit the high
temperature thing. I believe the resistor failure would be a gradual cascade
effect over a long period of time. Eventually, the resistor will get to the
point where it can't handle any power at all and it fails. It may not be
visually noticeable either.
The thing that backs my theory Rich, is that I destroyed all 4 of your "anode
fuse suppressor" resistors in my amp on 10M. Some of you may not know what
these are. They are 3 1 Ohm, 2 Watt resistors all in parallel in parallel with
a 1 Ohm 3 watt resistor. 0.25 Ohms at nearly 8 or 9 watts dissipation. Should
be able to handle plenty RF current right? Wrong. Pop went the weasle. Sure
they worked for a while but were eventually smoked. Parasitic? Hardly.
Rather the resistors got warm from all the RF currents and thermal effects and
as a result that 9 Watts dissipation rating dropped to zip. Bang, they ended.
My belief is that the same thing happens but on a much slower scale in nearly
all parasitic suppressor resistors. Perhaps the old carbon composite resistors
where more susceptible to a long term failure mode than modern metal films.
But damage is even less noticeable on a carbon composite. So the resistor has
failed and then the parasitic oscillation happens. IMHO, not the other way
around.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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