[AMPS] 50MHz 8877 amp. plate circuit

[Chris Tran GM3WOJ]

Monday 8th May 2000

Hello all

I've finally finished a 6m amp. using an 8877, and the output
Pi network does not resonate, even with very low values of 
inductance, because of (I'm guessing) approx. 25-30 pF of
Cout + Cstray.    I'm using a 3-30 pF vacuum variable for C1 and
a wide-spaced air variable for C2.   Anode load = 2500 ohms approx.

(By the way, I use one of these Almost All Digital Electronics L/C
meters, and it's great - here in the UK they are available from
Mainline Surplus Sales +44 870 241 0810 for US $125)

Some advice please - should I modify the Pi tank to either the
'shorted-turn' or the 'adjustable-coil' design ?    Both seem a
bit tricky mechanically, with the 'shorted-turn' design probably
easier ?   
Are there any other designs which overcome these excess 
capacitance problems ?

I've not completely given up on the Pi-tank - I tried the idea
of a small inductor (0.2uH approx.) between the anode blocking
cap. and the tune capacitor (as in the 30S-1) but still not able
to resonate the circuit at 50MHz.

There's a lot of hot air in this shack at the moment !

73
Chris    GM3WOJ/GM7V
gm3woj@talk21.com       www.qsl.net/gm3woj/




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