Anyone have a fairly accurate figure on this? I read everything I can
find- have found everything from 20 ohms to 110 ohms. The Russian tube spec
sheets don't contain this information.
The one solid value I found on a particular webpage (90 ohms as I recall)
wasn't accurate, at least for the tube I am using. In that case I designed my
own tunable Pi input (6m monoband). It tuned perfectly (roughly half-meshed
each cap) using a resister/capacitor to ground in parallel to simulate the
cathode Z of 90 ohms, in situ it doesn't use near as much capacitance as I
expected, though it does work like fine & is fairly broad. I am not sure how
much drive the circuit is losing as a result however.
Asking because I built another one previously using the "T" input that
seems popular (yes, I have read that the "T" is really a "pi" when one takes
into account the tube capacitance). The circuit is generally ~4000pf silver
mica in series from the RF in relay to a 10 turn 5/8" coil, variable ~20-120pf
cap to ground and another 10 turn 5/8" coil to the cathode (fed in my case thru
a .01 RF rated ceramic cap in series for blocking). Maybe I haven't found the
"magic touch" with the T- this circuit seems extremely hi Q, I am having a
devil of a time now getting the first amp input tune to go below 2 to 1. Best
I could get in the past was 1.7 to 1.... had a problem with the fixed bias,
repaired it, and now for whatever reason can't get the tuned input circuit
under 2 to 1. Squeezed, expanded, removed turns... Am about ready to forget
using the T as the tuned input unless I can find the right cathode Z to
simulate. It is a PITA to repeatedly tweek the coils & completely as
semble & test the circuit in situ. 73 Dave N3DB
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