[AMPS] Another Stupid Question

Tom Rauch W8JI@contesting.com
Mon, 3 Dec 2001 15:17:53 -0500

My point, which is sometime lost when threads are so long, is that 
the rule of thumb of using a blocking cap 25% of the reactance of 
the tuning cap is not a good rule of thumb.

The blocking cap reactance only need to be low compared to the 
operating plate impedance of the tube. 

> > Let's assume the blocking cap, at the lowest frequency, would be set
> > a reasonable value of 10% of the impedance looking into the tank, or
> > approximately equal to the value of the tuning cap with a Q of ten
> > (using the simple but not absolutely correct value of Rp/Xc.
>    You have inserted the phrase: "at the lowest frequency"  which was
>    not
> a feature of the original question.  The "10 % of the impedance" was
> formerly at the frequency of operation.

Respectfully, no person with any understanding of how this stuff 
works would ever size the capacitance of a blocking capacitor at a 
frequency ABOVE the lowest operating frequency.  

Some things are not said because they are assumed to be 
universally understood. We are sending quick e-mails, not writing 
detailed engineering descriptions. 
> > Using an Xc equal to 10% of the plate operating impedance, and
> > assuming a choke somewhat larger than the plate impedance, the
> > change in impedance looking into the tank is:
>     Wrong!  The plate choke may not be ignored as it is typically only
> "marginally larger" than the plate impedance in 160 meter ham amps.  I
> stated in my previous message that it is a worse offender, at this
> point, than the coupling cap in the real world.

Somewhat larger and "marginally larger" are both subjective terms 
meaning the same thing.
> > 300pF blocking (~300ohms 1.8 MHz) 2878ohms
> > 
> > 1200pF blocking (~75 ohms 1.8MHz) 2964ohms
> > 
> > The tank input impedance, with no other changes except a 
> > readjustment of the tuning cap by 2 pF to compensate for the 
> > reactance change, changes less than 100 ohms out of 3000 
> > ohms....a totally insignificant change.
> > 
> > And the small effect above is on 160 meters, where the problem is at
> > its worse point. On higher bands, the effect would be less because
> > Xc would decrease with increasing frequency.  
>    Today it does, but, look at the way the question was originally
>    posed,
> where for instance 50pf could be used on 20 meters, then the 10pF
> "typical" tube output C comes into play and you'll see what's
> troubling.

You are forgetting or ignoring in this case that the 10pF is a major 
player in the tube's output impedance. When that is considered, if 
you follow the rule that the blocking cap only needs to be a fraction 
of the plate impedance, everything is fine. 

>    Now, at the risk of prolinging this thread, I'll just add the
>    comment
> that the smaller value (<500pF) ceramic doorknobs are typically
> fabricated from lower loss material than their larger valued cousins. 
> If one chooses to use a lower valued part, thus making it part of the
> matching network calculations, these small guys are better suited to
> the task.

Indeed they are. I've read articles where someone claimed, 
mistakenly, that .01uF coupling caps on a 3CX800 amplifier was 
used to "insure high efficiency". 

Try to find a high-current high-voltage .01uF capacitor sometime.
73, Tom W8JI

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