[MRCA] Looking for BC-AL-430 Transmittter Info and Related Items

[Jack Antonio]

Here is what I found with my BC-430 and trying
6AQ5s as substitutes.

The adapters I refer to are simply a 4 pin tube base
with a 7 pin miniature socket. Grid of 6AQ5 (1 and 7)
go to pin 3 of the tube base, Plate (pin 5) and Screen
(pin 6) tie together and go to pin 2, Cathode and Filament
  (2 and 3) go to pin 4, and Filament (4) to pin 1.

Made up some adapters, and initial tests showed 1/4 watt out
while drawing almost 200 mA from power supply.  Found that the
bathtub cap 112/113 was breaking down between sections, and feeding
osc B+ to the PA grid line. Fixed that, and was still getting 1/4 watt
out, but at very low B+ current drain.

I have a BC-230 with the correct tubes, and made some measurements.
RF voltage at PA Grid 180V, bias voltage -60. With the 6AQ5s, I
obtained 140V p-p and -50V.

Thinking that the bias generated, about -50V, was too much for the
6AQ5, shunted resistor 104 with 4.7K, dragging bias down to
-25V, and am now getting 3 watts output, with reasonable current
drain.

But no modulation to speak of. Now thinking that bias was too high
on the modulator stages, after some experimentation, put 27K from
the modulator side of resistor 100 to ground, which lowered bias on
the modulator stages, now modulating, but all negative, no positive.

Looking at my adapters, and looking at the schematic and wiring diagram,
the cathodes of the two modulator stages were about 12 volts apart. Made
up some different adapters, that tied both modulator cathodes to the 12 
V point of the filament string, and now get +40%, and -70%. Changing 
modulator bias voltage doesn't increase overall modulation any, but 
moves the envelope up into flat topping at about 50% or shifts
the entire envelope down.

I'm thinking that the RF tubes are happy, in that they are class C,
but the linear modulator stages just aren't gutsy enough.

This is not a final definitive work, more experimentation  needs to
be done, but I hope it helps as a starting point.

Jack Antonio WA7DIA/4