[Amps] Re: SB-220 plate choke

[R.Measures]

On Apr 26, 2004, at 5:40 AM, Roy Koeppe wrote:

>
> On Apr 25, 2004, at 5:52 AM, Roy Koeppe wrote:
>
>> Discussed...
>>
>>
>  On Apr 23, 2004, at 3:20 PM, Jeff Weinberg W8CQ wrote:
>
>  Hi All,
>   I am trying to find a plate choke for the SB-220 for a friend in
>   Australia.  Anyone got a spare lying around?
>
>  ð  No, but they are a piece of cake to make.  See
>  http://www.somis.org/D-amplifiers4.html
>  -  cheers, Jeff.
>
>   Thanks,
>   Jeff Weinberg  W8CQ
>
>  Lots in that URL I don't agree with. First he seems not to distinguish
>  between series and parallel resonances at all.
>
>   Roy --  How do you tell the difference between a series-resonance
> and a parallel-resonance with a dipmeter?
>
> My recall is that you measure for parallel-resonance dips with the 
> choke
> open circuited, and preferably installed in the amp. (wired in place).
> For series dips, you place a shorting strap directly across the choke,
> and have it out in open space at the time.

  Sounds logical.  However, in the amplifier at:
http://www.somis.org/PBox_top_inside.jpeg
it was a resonance near 13.8MHz measured with the (not shorted) HV-RFC 
wired to the anode that apparently caused a choke fire during 20m 
operation.  It is my opinion that resonances with the choke wired in 
place and not shorted should not be ignored. //  notes -- the fire was 
near the top of the larger choke (on the left).  The fix was to remove 
enough turns to move the resonance to 15MHz,

cheers, Roy

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org