That phenomena certainly does affect 80-10M amps and National addressed
the problem in 1963 by connecting a 10pf 1kv silver mica directly across
the 20-40M switch contacts. Ive mentioned that on this and other forums
going back to the 90's.
It also works well on the SB-220, TL-922 and other amps known for eating
bandswitches.With the 160-10M amps the capacitor is more effective
between 80-40 but dont be afraid to experiment as the way amps handle
the coils varies.
The capacitor is added to the Tune Cmin so 10-15M tuning will change
slightly. In some amps the 10M coil may need a very small stretch.
With 10pf 1kv micas not all that available Ive used 10-12pf ceramic TX
caps rated at 5kv with no problems. A 2kv disc vaporized in a B&W
PT-2500A
Carl
KM1H
----- Original Message -----
From: "Phil Clements" <philc@texascellnet.com>
To: "'Amps reflector'" <amps@contesting.com>
Sent: Saturday, April 25, 2009 8:07 PM
Subject: Re: [Amps] Fw: High SWR = Baluns actually
>
>
>> >The other was where the output of said transmitter would arc in it's
>> >output tuned "tank" sections, due to self resonance of the unused
>> >coils.
>> >I designed a replacement without that problem, that also added a
>> >lo-pass
>> >filter for harmonics. We didn't think of where to get such a
>> >bandswitch
>> >that would short out all the un-used open coils, but my boss told me
>> >to
>> >tell them the whole story, and they found the bandswitch addition
>> >that
>> >shorted out the coils, and we swallowed my design.
>>
>> REPLY:
>>
>> FWIW - I had a similar unwanted resonance problem in the unused
>> coils,
>> but mine was caused when the unused coils WERE shorted out. I rewired
>> it so they were no longer shorted out and the problem went away.
>
>
>
> This problem reared its ugly face when 160 meters went QRO, and the
> WARC
> bands were added. An 80-40-20-15-10 meter design was very forgiving.
> You
> could use military/commercial band switches from WW II, and with even
> a not
> so perfect layout, you came out just fine.
>
> I built several QRO amps, attempting to go 160-10 meters, and the band
> switch always arced and melted down; usually on the 40-meter tap. The
> problem is the Tesla effect. It can happen on any section of the tank
> coil.
> I burned up several brand new RF Switch Co. band switches trying to
> follow
> instructions from articles and the ARRL Handbook before I caught on to
> what
> was going on. If you are trying to build a single or dual 8877 amp,
> look at
> how Alpha did it with the 77DX/SX tank circuit. This will give
> coverage from
> 1.8 to 30 MHz with no fires or meltdowns. It is a product of choosing
> the
> right band switch, and layout. You will not se this problem if you
> omit 160
> meters and/or the WARC bands.
>
> (((73)))
> Phil, K5PC
>
>
>
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> Amps@contesting.com
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