>
>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.
The suppressor resistor in my SB-220 was damaged on the 40m band. 10m
worked normally. I observed a double-dip in plate/anode current. A
big-bang was heard. The grid current meter shunt blew.
> At 28 MHz, the capacitance of the tube and
>the capacitance of the tune Cap are on the same order of magnitude.
5pF is the same as 20pF?.
>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.
>
... "phase margins"?
>
>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."
>
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
>
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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