----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <amps@contesting.com>
Sent: Sunday, September 30, 2007 12:40 AM
Subject: [Amps] Blocking capacitor drift
>> Try and have it your way as usual. Ive seen plenty of
>> plate blocking caps heat up and change the output tuning
>> and still not be at the point of failure.
>
> You really ought to send the capacitor that drifts and
> doesn't burn up to a science museum!!!
>
> Let's assume the tube loadline is 3000 ohms and see if that
> claim makes any sense at all:
>
> A 1000 pF cap 44 ohms non-dissipative reactance on 80 meters
>
> A change from 1000pF to 100 pF would change the SWR seen at
> the tube on 80 meters by the following:
>
> 3000 ohm tube loadline
> 1000pF blocking
> 209.2 pF tuning cap
> 10.3uH tank coil
> 1375pF load cap
> tube sees SWR of 1.01
>
> Now lets make that capacitor drift to 100pF
>
> 3000 ohm tube loadline
> 100 pF blocking
> 209.2 pF tuning
> 10.3uH tank coil
> 1375 pF load
> tube now sees a roaring bad 1.136 : 1 SWR. Hardly a change
>
> So how far do we have to retune that capacitor that drifted
> from 1000 pF down to 100 pF???
>
> 3000 ohm tube loadline
> 100 pF blocking
> 207.2 pF tuning
> 10.3uH tank
> 1375 pF load
> tube is back to a comfortable 1.01 SWR
>
> The net change in tuning cap value to compensate for a
> change from 1000 pF blocking to 100pF blocking is 2 pF.
>
> If we have a blocking cap that drifts so bad it requires
> retuning the tank, it is a record holder.
>
> People get way too stressed out with the value of blocking
> caps. Unless the thing is cracking, arcing or smoking it has
> little effect on anything from efficiency to power output to
> tuning.
>
> 73 Tom
More wasted drivel and bandwidth when your last paragraph says it all in a
nutshell. A defective cap can be breaking down internally before it shows
any external sign of its impending destruction. The SB-220 cap still looks
fine during its initial failure stage but it is very hot to the touch (after
the DC is bled off of course). The early production molded 500pf/2KV cap
that is switched in on 80M also has a similar failure mode.
There is a big difference between tossing out a bunch of PC generated
numbers and actually diagnosing a real world problem using less than ideal
components.
Carl
KM1H
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