Edmund H. Ramm wrote:

Those -50V you measured are the protective bias. When excited, the
resulting grid current will add to the protective bias.

ah that's why... It's working in C class with G1 current! I would never
expect an AM TX final amplifier to work in something else than A or B class.

Why, it doesn't need to be linear, unlike concepts where modulation is
applied at some earlier stage.

indeed this is correct, the modulation is applied to the final amplifier,
so it can work in class C, thanks again for explaining.


Thanks for the hint. Do you know if this protective bias should be present
also when the G.222 is operated in AM position?

Not off hand, would need to dig out the circuit diagram. It's ages
since I layed hands on a G.222TR. And I operate CW only. Maybe I'm
missing something when I see no advantage in using no protective bias
in AM mode.

here you can find the schematic:

http://www.qsl.net/i0jx/222.gif

Probably since in AM mode there's always present the VFO drive, they thought
it would be enough to just trim the drive voltage while monitoring the grid
current to remain in the safe range. If this isn't the case, there must be
a bad contact in the mode switch because indeed in AM mode I don't measure
any protective bias on 6146's G1.

Do you see any reason not to use a 6146B as final tube? I have a few of these
but no 6146/6146A left.


Thanks again and 73
Frank IZ8DWF