On Thu, 7 Jan 1999 03:56:41 -0800 Rich Measures <measures@vc.net> writes:
>
>>
>>Re: SB-220 on 10 M.
>>
>>K4AB damaged the bandswitch
>
>? Did he damage the bandswitch through error?
Yeah, Heathkits....not voodoo.
>
>>before the 10M contest
>>so he bypassed the bandswitch, wiring the SB-220 tank
>>circuit directly for 10M. Output went from the usual 900W
>>to 1200 W.
>>
>>Wonder what could be done to improve efficiency on 10M
>>using the bandswitch?
Exactly the point I was trying to make yesterday. It is not the SB-220
tank coil that is at fault since I use the same 10M coil on 6M and it
works just fine at 60+ % efficiency.
Several times I have suggested that the SB-220 switch shorting scheme is
the culprit behind blown switches , arcing Tune caps and parasitics have
nothing to do with it IF the suppressors are not damaged.
The shorted turn effect of the switch coupled with stray C and L forms a
tuned circuit at some VHF frequency. When condx are just right things go
zap. Before they go zap this circuit is consuming power and reducing
efficiency. This power is normally shunted to ground thru the low
impededence of the capacitors, until the right combination causes a high
enough impedence to force a breakdown.
Proof? Simple, just put the amp on a spectrum analyzer sweeping 5-300
Mhz and Peak Hold on. There will be just enough VHF energy getting thru
the circuit at the antenna port to be seen on a sensitive analyzer, or
you could just use a sniffer probe. No need to run full power for the
test, 500W or so in the CW position will be fine and save the switch
while you vary the caps.
One way to detune/deQ that circuit is to place a 10-12pf 1KV silver mica
or other RF rated cap across the 80 and 20 meter positions right at the
switch. Improved efficiency and less switch problems unless severely
detuned. While you are at it remove L2 and replace with a 15 to 20 Ohm
20W real wirewound. It will add HV tube arc protection plus remove a
potential cause of instability since this choke is resonant very close to
the 3-500 parasitic frequency.
Ive done the above with several SB-220's using a 3500VDC supply and the
wide spaced ( SB-221 and Heath replacement) Tune cap and the owners have
yet to blow a switch.
Before someone gets on a high horse let me say this is not a fool proof
cure, just a marked improvement over the original. Murphy is always
watching.
Any meaningful, non-voodoo related discussion welcomed (- : .
73 Carl KM1H
>>
>? Tom: Could the SB-220's bandswitch have been dissipating 300w on
>10m?
>
>- later
>
>
>
>Rich...
>
>R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>
>
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>
>
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