> From: Rich Measures <measures@vc.net>
> During a telephone conversation with Mr. Rauch in late-1990, he told me
> that he had seen plenty of parasitic damage in the 400 SB-220s that he
> had repaired. Mr. Rauch did not design the SB-220.
Nonsense Rich. You distort everything you hear to support your kit
sales.
I did tell you very clearly that the 220 had a problem with VLF
parasitics on occasion from the parallel tuned circuit in the grid.
If the load impedance is correct, it will "pass through" the low-pass
tank, and under the correct conditions form a TPTG oscillator.
> If your amplifier arced mysteriously, it might be interesting to measure
> the resistance of the VHF parasitic suppressor resistor.
Heat any carbon comp resistor to 150-200 degrees C for years and
measure it. Carbon composition resistors change value from long term
temperature exposure, anyone in electronics for a period of time
should know that much.
It's pretty darned hot up at the anode connectors. That's why the
resistors change. It matters not what the heat is from, nor can you
tell some time later what did it. All you know is it was heat.
Once the resistor fails, or goes far enough out of range, the PA
oscillates. Then people grab the wrong kind of resistor, thinking a
regular carbon is non-inductive, and never get it right again.
There is nothing sinister or mysterious about any of this.
73, Tom W8JI
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