[Amps] Tuned input board on SB-220

[Roger D Johnson]

Gary Schafer wrote:

> You can look in Bill Orr's handbook and he says you need 13 pf/meter of
> capacitance at the cathode for proper operation. That's 1040pf on 80 meters.
> 
> Then figure out what L network values are needed to match the particular
> tubes input impedance to your 50 ohm driver. In most cases you will find the
> capacitance will fall short of the 13 pf/ meter value.
> 
> Here is a handy calculator to do it.
> 
> http://bwrc.eecs.berkeley.edu/Research/RF/projects/60GHz/matching/ImpMatch.h
> tml
> 
> 73
> Gary  K4FMX

I just typed a msg to George, K6GT, and it occurred to me that an L net
will not match 50 ohms to 50 ohms. It would seem to me that, if at some
frequency/loading combination your amp looked like 50 ohms, the LDG
tuner would go into bypass mode and you would have nothing on the
cathode side.

Don't get me wrong, I think that this is a terrific idea but LDG needs
to go one step further and use the freq determining circuit to switch
in the proper input capacitors to give an appropriate Q.

73, Roger


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