[Amps] Need Some Experienced Feedback Here...

[Robert B. Bonner]

The difference here I think is the ratios between the bread slicers and the
fixed caps.

If you have a 1000pf bread slicer and a 200pf fixed cap compared to a 200pf
bread slicer and a 1000pf fixed cap the currents would be swapped between
the components.

The variable cap can handle massive current and will change very little with
heating.  The tiny ceramic has limited current ability and will vary a lot.

>From my earlier post, if you need to pad 1000 pf you are better off with a
pair of 500's than 1 1000 or if you had a 100 and a 900 the 900 would carry
the majority of the current, heat easier and make the circuit unstable.

BOB DD

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Vic K2VCO
Sent: Wednesday, September 27, 2006 10:30 AM
To: Ed Swynar
Cc: amps at contesting.com
Subject: Re: [Amps] Need Some Experienced Feedback Here...

Ed Swynar wrote:
>
> I resolved to replace the entire padding network with fresh
> capacitors, avoiding the use of any of the older surplus rectangular
> "mica blocks" (which I've had fail on me in other applications in the
> past, as well) --- and yesterday I did just that, with another
> parallel / series combination of six door knobs, two of which were
> rated at 20 Kv! Anyway, as I bench-tested the new arrangement: the
> tuning changed again, ever so slowly, as if by some unseen hand ---
> once more I had to compensate with more capacitance from the air
> variable until it, again, ended-up with all its plates meshed, & the
> RF output down to some 200-watts...

I've had this experience with doorknobs in antenna tuners and traps as 
well as amplifiers.  The usual doorknobs with higher values of 
capacitance (> 100 pf) are N750 temperature coefficient, and they also 
tend to heat up.  So the value changes rapidly.  I've found that the big 
surplus micas seem to do a much better job at 160 meters, where they 
have high current ratings and I've been happy with them.

One interesting fact:  I had an Alpha 86 which used what appeared to be 
normal 5KV doorknobs for padding in the output networks, with absolutely 
no problems.  So either they used better doorknobs, or the circuit 
design is such that the current was not enough to cause them to heat up 
so much.
-- 
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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