>
>On Thu, 29 Nov 2001 19:12:35 -0500 "Tom Rauch" <w8ji@akorn.net> writes:
>>
>> > I think you'll find that if you do all the math that, for a
>typical
>> > loaded Q of 10-12 in the Pi-net, the coupling cap value should be 4X
>> > the value of the plate capacitor at the lowest frequency of
>operation.
>>
>> Well let's see...
>>
>> If the anode operating impedance is 3000 ohms and we use the
>> somewhat incorrect but common method of Rp/Xc = Q, then we
>> would have 300 ohms Xc with a Q of ten. That means the blocking
>> cap would be 75 ohms, which is without any doubt needlessly
>> large since it is in series with a 3000 ohm source impedance!
>>
>> A blocking cap of even 500 ohms reactance would have a minimal
>> affect on anode impedance seen by the tank, so why make it so
>> large?
>> 73, Tom W8JI
>> W8JI@contesting.com
>>
>
>Hi Tom,
> This was an interesting enough query to prompt the installation of the
>old Smith chart program on the new computer.
>
> If the blocking cap is made as small as you propose, it becomes part
>of the matching network rather than just a bypass and some interesting
>things happen depending on the frequency of operation.
>
> At higher frequencies, where given a typical tube the plate C
>represents a significant part of the plate tune cap, the effect is to
>significantly raise the plate impedance (as seen by the Pi-Net input) and
>the balance of the pi-net would need to be adjusted to maintain the
>desired Q & impedance ratio.
// Tanks are ordinarily tuned for max. outout. Typical anode-C is
<15pF, so even a 100PF DC blocking C would have minimal effect at HF.
Also, RF-rated titanium-dioxide capacitors do not consume noticicable
power - or they would melt their silver solder as was pointed out by Mr.
Rauch.
>
> At lower frequencies, where the typical plate choke begins to become a
>influential part of the overall network, a similar effect occurs.
>
// As long as the tank tunes, there is no problem.
> And at some median freq where the tube's plate C effectively cancels
>out the plate choke shunt L (i.e. the plate circuit is purely resistive
>at that point), the lowered value of the coupling cap looks less
>disruptive.
>
> I think I'll remain with the traditional size coupling cap and avoid
>the extra math. :-)
>
// If the tank tunes to max out at 1.8, there's nothing to berate.
cheers, Marv
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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