Re: [Amps] SB-220

[R.Measures <r@somis.org>]

On Nov 3, 2004, at 10:23 AM, Will Matney wrote:

> To all,
>
> A quick question here, and may open up a big can of worms, if there is 
> no such things as parasitics in amplifiers, why do they all come with 
> parasitic suppressors on the anode leads?

  For the same reason that some people knock on wood or toss salt over 
their shoulders?   Seriously, a VHF suppressor reduces the VHF gain of 
the tube by decreasing the parallel-equivalent R presented to the tube 
at the anode circuit's (unavoidable) VHF resonance freq.  The mechanism 
that allows a parasitic oscillation in a pair of 3-500Zs is the 0.3pF 
of C between the anodes and the cathodes.  0.3pF seems pretty trivial 
until one discovers that it amounts to a feedback path of c. 
-j5000-ohms between the output and the input.

> Now if I recall, a certain somebody used to work for Ameritron I do 
> think. So, don't Ameritron amps all have parasitic suppressors on all 
> their anode leads?

All except the AL-1500, which apparently holds the U. S.-record for 
gold-sputtering 8877s,

>  If I recall, and I'm not sure how far back, parasitic suppression has 
> been used in amps for 60+ years that I know of (1940's). ...

The first mention of what was essentially a low VHF-Q parasitic 
oscillation suppressor was in the 1926-edition of  "The Radio Amateur's 
Handbook", page 72:

“The combination of both resistance and inductance (i. e., 
resistance-wire) is very effective in limiting parasitic oscillations 
to a negligible value of current.”    --  F. E. Handy, W1BDI	
-------    researcher:  Dave Newkirk, WJ1Z.

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

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