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

[J Mcvey via ARC5]

No need to get so worked up!
You are right about the transformer not having any response in the 60HZ range and the DC will be very low, almost non-existent on the cathode side.However, can you explain the the purpose of the winding on the LV side of the filament? ( You left that part out in your diatribe.) The filament current is substantially greater than the RF current and an AC filament voltage possibly could have some influence, not to mention the ac across the filament itself imbedded in the middle of two RF windings.  
Besides, what is the effect of tying the cathode and heater together? This was before my time, so I'm not sure if it doesn't make the tube equivalent to a direct heated type. There is more to this arrangement than meets the eye. Any VT designers on the list ? Inquiring minds want to know...


Also, a positive cathode voltage relative to a zero voltage grid is seen as a negative voltage by  the grid.So, yes, you can bias a tube with a positive voltage. In fact, that is the purpose of cathode resistors in tube tech- to set the bias with a POSITIVE voltage at the cathode.
When an engineer designs a circuit, he starts with with part specifications that will meet the needs of the device.The tube spec sheet has to be where the ARC guys started and worked out from there.

 

     On Tuesday, October 7, 2014 10:44 PM, Mike Morrow <kk5f at earthlink.net> wrote:
   

 > 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
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