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

[Mike Morrow]

> So, from what I gleaned from the notes, the circuit was designed to use a
> fixed DC bias otained from the LV line.

That is incorrect.  I have to wonder what circuit you are studying, for
there is no such circuitry in the MO circuit, nor any other place in the
the entire ATA, SCR-274-N, or AN/ARC-5 that uses the 28 vdc supply to
provide grid bias to any tube.  It's not even the correct polarity!
The 28 vdc power is used for carbon microphone excitation in the modulator,
but that's not the same thing as providing grid bias.

Look at the schematic of a typical ATA, SCR-274-N, or AN/ARC-5 transmitter.
The transmitter oscillator uses the very common technique of a grid-leak
capacitor/resistor combination (C-59 and R-72) to develop the 1626 grid
bias.  (Search for "vacuum tube grid leak" and "Hartley oscillator" for
more information.)  The manuals for these sets state this clearly, plus
the schematic clearly shows classic series grid leak bias circuitry, even
if there had been no discussion in the text.

> I believe that applying AC to the filliments will affect the MO circuit
> deleteriously .
> a) 60HZ modulation
> b) loss of stability due to lack of, or improper bias
> c) possible damage shortening the useable life?

All of these assertions are incorrect...they are built on the initial
interpretation (if any) of the circuit being wildly inaccurate.

SEPARATE NOTE:  There have been suggestions that AC filament power flowing
through MO T-53 will modulate the RF produced by the 1626.  But T-53 can NOT
act as a modulation transformer to impresses 60 Hz AC upon MO RF.  T-53
windings have ZERO impedance to 60 Hz AC, so they look like dead shorts to
60 Hz AC.  There won't be ANY transformer action at 60 Hz AC when the primary
and secondary windings look like shorts to 60 Hz AC.  That's why modulation
transformers need a lot of heavy iron and windings of hundreds or thousands
of turns, none of which are found on T-53.

The 1626 filament current flow of 0.25 amps AC through the 0.1 ohm DC resistance
of the tapped T-53A winding will introduce a 25 millivolt 60 Hz AC component at
the 1626 cathode.  The plate-to-cathode voltage on the 1626 is about 190 vdc,
so 25 millivolts of 60 Hz AC in the plate-to-cathode circuit is smaller by almost
40 dB.  Many will find it very difficult to achieve AC ripple voltage that is
as small on the HVDC supply that they are using.  No 60 Hz AC will be noticed
in the MO RF output.

Mike / KK5F