[ARC5] "Curing Chirp in Command Transmitters" MO Grid Bias, Cathode Modulation

[AKLDGUY .]

The reason for connecting filament pin 7 to cathode (pin 8) is to eliminate
the effect of capacitance change between the filament and cathode, caused
by vibration. This must have been a serious concern in an aircraft-mounted
transmitter and the repetitive vibration probably caused microphonic noise
to appear in the audio of the transmitted signal.

So far, so good for pin 7, but what about filament pin 2, the other end of
the filament? How could the designers also connect that to the cathode?
The answer is that it was possible only by way of an elegant solution. That
solution was as follows.

T-53 has two primary windings (A and B). The cathode (together with pin 7)
is tapped part way up winding A for configuration as a type of Hartley
oscillator. Filament pin 2 is connected via winding B.

Now, winding B is bifilar-wound with the tap-to-ground portion of winding A.
That means they are interwound very closely with each other and with the
same number of turns.

The nature of the bifilar windings means that pin 2 is now at the same RF
potential as pin 7 (and the cathode) at any instant. Thus this elegant
solution
means that pins 2,7, and 8 are all at the same RF potential and there can be
no ill effect due to capacitance change among any of them.

The tapping of the cathode part way up T-53A has nothing whatever to do
with biasing the 1626. Grid bias is developed across R72 and C59 in the grid
circuit by way of driving the grid hard and using the one-way (rectifying)
flow
of the grid current to develop the bias.

73 de Neil ZL1ANM