>
>I bought an SB220 amp that wasn't putting any power out on 10 meters, but
>1200 to 1300 on 20 and 15 mtrs. It turned out to be a fried bandswitch on
>10 thru 20, but was hanging on with a hair on 15 and 20.
? Did the bad bandswitch resemble the toasted bandswitch shown on p.33
in the Oct. 1990 issue of *QST*?
> I replaced the
>bandswitch and while I was at it, i installed Harbach mods.
? If the parasitic suppressors are short on L-sup, they may not reduce
the gain of the tubes far enough at 110Mhz to prevent regeneration. I
would increase L-sup and see if arcing decreases. // Since Tune-C
arcing is much less destructive than bandswitch arcing, I would bend the
first rotor plate on the Tune-C off center so that the Tune-C breaks down
at a lower v than the bandswitch. If you have a high-potential breakdown
tester, a breakdown of 3700v is about right for the Tune-C in a 220.
Mo' volts is not mo' betta.
- Another method of reducing VHF gain is to put a synthetic VHF load
between the cathode/fil. and ground. Such a device consists of small
amount of C and R in series from the cathode/fil. to gnd. Typical C is
30 or so pF and R =s around 10-ohms (with low-L). The idea is to make
the series circuit, (including tube lead L) resonant near 110MHz.
>suppressor, soft key, soft start, and rectifier-metering board kits) I also
>installed new resistors across HV caps from Rich Measures yesterday. Now
>on 20 it will arc a bluish color on bandswitch when driving it up to abt
>300 watts out.
? Congratulations on having a pair of 3-500Zs with above average vhf
gain. .
- cheers, Bob
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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