[Amps] Cathode Z GS35b

David Craig davidhcraig at verizon.net
Mon Jan 26 13:51:45 EST 2009


    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 assemble & test the circuit in situ.  73 Dave N3DB 


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