[ARC5] "Curing Chirp in Command Transmitters" MO T-53A and B

[Mike Morrow]

> The VFO tube (1626) has two heater pins, 2 and 7.
> Pin 7 is connected direct to cathode (pin 8) and the reason is given in the
> manual:
>
> *"...so that variations of cathode-to-heater capacitance within the tube*
> *will not affect the frequency of operation."*
>
> Pins 7 and 8 are connected to a tap on the VFO coil (it's a type of Hartley
> oscillator).  Now, obviously the cathode and heater currents flow through
> part of the VFO coil, from the tap to ground.
>
> Since DC *was* used for the heaters, the only voltage that could be induced
> by the heater current into the secondary would be that due to a noisy 28V line.
>
> I believe that to counter the effect of the 28V line going noisy, the
> designers incorporated another winding, winding B, through which the
> other heater pin, (pin 2), is fed.

No...that conclusion is incorrect.

The applicable FULL T-53 circuit description found in the manual is:

"T-53B is a bifilar winding wound with the master oscillator coil, from the
 ground to the cathode tap, in order that variations of cathode-to-heater
 capacitance within the tube will not affect the frequency of operation."

The tapped main master oscillator coil winding T-53A connects to the 1626
cathode pin 8 and to heater pin 7 (in most transmitters).  Winding T-53B
connects to heater pin 2.  The purpose of these windings and connections
is to place BOTH SIDES OF THE HEATER AT THE SAME RF POTENTIAL AS THE CATHODE
and thus, as the manual clearly explains, eliminate ALL effects of differing
1626 cathode-to-heater capacitance on master oscillator operation.

This configuration has NO design intent nor effect on "the 28V line going
noisy".

> The two heater wires flow through two different windings that oppose each
> other, so the noise cancels.

No.  These few air-wound turns found on T-53A and B windings have absolutely
no effect on noise cancellation unless it was at high RF frequencies.

> Now, using AC for the heaters is simply using a form of noise on a grand
> scale, so there should be no hum or any other noise in the secondary.
>
> If I'm right about the phasing of the windings to minimize noise...

That's not the purpose nor even a side-effect of the T-53B winding.

> If the heater pins are rewired some other way, the benefits of
> noise reduction will be lost, *even for DC heater operation*.

The circuit configuration is NOT for noise reduction, so rewiring this
circuit does not reduce a "benefit" that was NEVER present.

FWIW, unlike the MF/HF T-18 to T-22/ARC-5, the ATA, and the SCR-274-N
transmitters, the T-15/ARC-5 500 to 800 kHz and T-16/ARC-5 800 to 1300 kHz 
transmitters do NOT use a winding T-53B.  The T-17/ARC-5 1300 to 2100 kHz
transmitter has a T-53B winding to maintain the RF potential identical
on both sides of the heater, but also has a 390 ohm resistor between
pin 8 and 7.  Thus the T-17 1626 cathode will be at slightly different RF
potential than the heater...but not as much as the T-15 and T-16 without
T-53B. For all three MF transmitters T-15 through T-17, the 1626 heater-
o-cathode capacitance effects are minimal at the operating frequencies
of these transmitters, so these units can get away with this.

Mike / KK5F