On 25/03/2013 19:21, Vic K2VCO wrote:
> 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.
>
Personally, I'd put the higher HT on the '1000 - you'll get a bit more
power out with a bit less anode current and you'll less likely dip in to
screen current on he negative-going half of the RF cycle.
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