Tom,
In the mid 70's I went to work at Heathkit as a Ham Tech. One of the first
rigs I had to work on was a SB220. I replaced burned chokes/caps on the
tube
sockets and sent the rig back to the customer. I then contacted Benton
Harbor
on my "technical assistance" line and asked why these parts were there. A
week
or so later I was told they "thought" they had something to do with
metering.
Not much of an answer and I never did get one that made sense. Bottom line
is,
there is no valid technical reason to have them in the circuit.
Personal experience with the 25 to 30 SB220s Ive restored/repaired/modified
over
the years indicates directly grounding the grids makes for a more stable
amplifier.
Sounds to me like the resistor and cap suggestions came from someone with
parts to
sell
73, Bob, W5AH
-----Original Message-----
Gentlemen:
The information I have read concerning improvements to the SB220 contain a
recommendation to replace the grid chokes-to-ground with resistors and to
replace the stock 200 pf mica capacitors from the grid to ground with 1800
pf of capacitance per tube socket. My Digikey catalog lists some ceramic
disc 680 pf @ 1KV for .31 each and some silver micas, same value @ 500V for
1.86. Since I have two amps to modify, the cost is an issue and I am
wondering if the ceramic discs will do the job. Also, is the 1800 pf a
critical value, as I will end up with 2040 pf per tube using the 680's. Any
thoughts would be appreciated.
Tom K0SN
Porterfield, WI
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|