Exactly!
I would bet that the majority of cases of smoke and other 10 meter problems
with SB-200s and SB-220s are from the resistor values changing due to
heating by circulating currents. At 28 MHz, the capacitance of the tube and
the capacitance of the tune Cap are on the same order of magnitude.
Therefore, a significant amount of RF circulating current flows through the
tube and hence the LR suppressor network. The resistors heat up and begin
to change in value over time. Also the hotter a resistor gets, the less it
can dissipate. The ambient heat of operation combined with the heating due
to the RF current greatly reduces their ability to handle power.
Sure, oscillations and smoke can and do occur, but it's due to the fact that
the resistance in the LR network has changed from what it should be and that
is what changes the gain and phase margins in the circuit.
73,
Jon
NA9D
on 3/22/01 10:51 AM, Phil Clements at philk5pc@tyler.net wrote:
>
>> Therefore, it's worthwhile to make sure the resistors are in good shape, and
>> replace them if needed. Measures recommends metal-oxide resistors which deal
>> with the heat much more gracefully.
>>
>> 73,
>> Vic, K2VCO
>> Fresno CA
>
> Sound advice! I recently measured the resistors on a SB-200 I had in for
> repair,
> and they had increases to nearly 60 ohms from 47 ohms over the years. There
> were no signs of heating or abuse. It is a pain, but you have to remove one of
> the leads of the resistor to check it.
-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)
Member: ARRL, AMSAT, DXCC, NRA
http://www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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