>
>> Heathkit SB-200 Built circa 1977. Currently in need of a minor rebuild
>> for maximum life which I plan to undertake (Harbach power supply, new
>> antenna relay, replace some ancient caps, etc...). Currently has new
>> 572B's with < 5 hours operating time on them.
>>
>> Symptom:
>>
>> While transmitting a while back, I let the smoke out of a resistor that
>> is part of the RF choke on the anode clip (one of the 80 5w wirewound
>> guys with the coil wound around it). Obtained new resistor and soldered
>> it in.
>>
>> Proceeded to put amp back in service. Upon hooking it up, found a clear
>> frequency and got ready to transmit. As soon as the antenna relay
>> clicks shut, the plate and grid current peg and another resistor self
>> destructs. This one is off of lug 2 of V2 to ground (right most tube
>> socket when looking at the back of it).
>
>Chuck,
>
>Were you feeding power to the amp when it pegged or did you just throw it
>into transmit position but without the exciter putting any power into it?
>
>This sounds like a classic amp-taking-off parasitic problem. It would be
>interesting to note if it were happening by its lonesome or whether you fed
>100W into it when it happened. It would also be interesting to know what
>band it was being tuned to.
>
? With no RF drive, the abrupt change in DC anode current from 0 to
idling/ZSAC current rings the SB-200's anode-resonance at c. 100MHz.
This DC change rings this resonant circuit producing a damped wave, like
a spark transmitter. The tube has internal feedback between the
anode-output and the cathode-input plus it has gain. With enough gain,
regeneration is not an impossibility
>...
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
|