Edmund H. Ramm wrote:

I found in the technical bullettin n.83 the schematic and alignment
instructions. The first thing I noticed is the G1 of 6146 is supposed
to be at -85V in CW tx. How is that possible since the only negative
supply in this transmitter is -50V? I infact measured -50V when in CW mode,
but I didn't key the transmitter as the 8289 died before I could try.

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

Tnx,
73 Frank IZ8DWF

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

Edmund H. Ramm wrote:

Lots of "ifs": If the mode switch shown in the diagram only has
the three positions AM - VFO Beat - CW, and if the switch's position
shown in the diagram is the AM position, then there is no protective
bias in AM mode, indeed.

ok, that's what it seems to me too (the circuit drawing isn't also very
easy to follow sometimes), and that's what I can measure on the real
thing.


Do you know this urge to wring the designer's neck until he discloses
what drugs he was on when he designed this circuit?

Other transmitters use at least a kind of brute force circuit to pull
down the PA valve's screen grid voltage with no exitation present, in
order to keep the anode dissipation low enough.

I agree, if you don't monitor the G1 current in AM mode, the 6146 will
not survive for long time.

By the way, I ordered a new 6146, the PA has no neutralization circuit so
probably even the 6146B would work fine, but none of my used 6146B is in a
known state, so I would just add other unknowns if I'd try with them.

73
Frank IZ8DWF