[Amps] AB1 input circuits

[Vic K2VCO]

I'm proceeding with my AB1 4CX1000A amplifier. I've taken the advice of several reflector 
participants and used a pi-section lowpass filter design for the input circuit. It is very 
simple: a 120 pf to ground input capacitor, followed by a 0.03 pf blocking capacitor (3 x 
0.01), then a coil of about 300 nH. The output capacitance is provided by the 80 or so pF 
input capacity of the tube. The grid is connected to a 50 ohm Caddock thick-film 100w 
resistor (in a heat sink which is in the air flow to the tube base). The bottom of this 
resistor is bypassed to ground with another 0.03 pF.

The SWR is shown as no greater than 1.2:1 from 1.8 to 30 mHz. Of course this is with 
everything cold, but I don't see why it should change in operation as long as there is no 
grid current drawn.

What is interesting is that I originally used 0.01 uf blocking and bypass capacitors, but 
the SWR rose below about 3 mHz to about 1.6:1 at 1.8. Increasing the size of these 
capacitors brought it down to almost 1:1.

So the power supply and input sections are done. Plate circuit next!

Incidentally, the transformer I have can be strapped to provide either 2600 or 3200 volts 
under load. I am thinking that it might be pushing things to try to get a clean 1500 watts 
output at 2600V. 3200 is technically too high for this tube and it will rise to 3450 under 
no-signal conditions.  Is this anything to worry about? Screen voltage will be the normal 325.

-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/