[AMPS] Another Stupid Question

[Tom Rauch]

>    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.

Not true. It sounds "nice", but it is not factual. In truth, the 
impedance barely changes.

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.

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:

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.

As I said, I learned my lesson many years ago when I laughed at 
someone for reading a blocking cap wrong by a factor of ten in a 
160 meter amp. After poking fun at him, I was amazed when we 
changed the capacitor to a "traditional" value of 1000pF and 
nothing we could see changed.

People waste far too much time worrying about enough blocking 
capacitance, and spend far too little time considering current 
ratings of the capacitors and the plate choke design.....which has a 
MUCH larger effect than the blocking capacitor on system Q and 
other problems.    

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.  
   
73, Tom W8JI
W8JI@contesting.com 

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