[TenTec] another amp question

[Dr. Gerald N. Johnson]

On Thu, 2008-04-03 at 18:57 -0400, Gary Hoffman wrote:
> To me this whole parasitic issue is very nerve wracking.
> 
> This issue seems to be that absolutely no one can say what triggers it with
> certainty, nor do people agree on the fix.  Obviously ham radio
> manufacturers build amps that they feel are appropriate and well behaved.
> They work for years.  Then Blam !
> 
> Some people claim it is not even real - they just say that component
> failures happen, and trigger noisy failure modes.
> 
> But there is a very credible body of evidence to the contrary also.
> 
> There are many folks advocating relatively advanced modifications...and
> another group who claim these are not worth the paper upon which they are
> written.
> 
> I'm not convinced pro or con....but I have blown up two amplifiers, not due
> to operator error either.
> 
> I guess I better just sign off on the subject yet again, since I'm not
> really able to contribute much and no one has yet convinced me with
> certainty that they have THE answer either.
> 
> 73 de Gary, AA2IZ
> 
> 
I have one possible answer. In most ham sized amplifiers, parasitics are
suppressed by small coil (typically 5 turns of wire) wound around a 2
watt carbon composition resistor. That's usually the plate lead,
sometimes there's another in the grid or cathode. The parallel resistor
inductor combination provides circuit loading at the higher frequencies.
There is ALWAYS a trade off between VHF loading and power loss in the
resistor on the highest HF frequency expected to be amplified. There
would be a better stability margin with a lower value resistor or higher
inductance coil but it comes at the cost of lower 30 MHz efficiency and
hotter resistor operation.

Now a carbon composition resistor is known to change value with time.
Virtually always up due to age, heat, and humidity. Its always been
their nature. And the change is never twice the same. Anyway when the
parasitic suppression resistor value rises high enough (might double in
20 years running hot), it now longer will keep the amplifier from
oscillating at some frequency not coupled to the antenna, hence a
parasitic oscillation that builds (rapidly) up plate voltage that
flashes over in the tube, tuning capacitor, or circuitry. Which is the
bang.

Its a defect you won't ever detect with a DC digital meter without
unhooking the coil from the resistor. Check it out, then build a new
parasitic suppressor and expect more quiet operation. I like the 3 watt
tin oxide metal film resistors. I can't detect more inductance from
spiraling the film than a solid rod would have, and they are far more
tolerant of overloads than carbon composition resistors. Say up to ten
times rated load where they will glow and may ignite neighboring
material but they will still have the same resistance. The parasitic
suppression coil should probably be mounted adjacent to the resistor,
not wound over the metal film resistor.

73, Jerry, K0CQ