[ARC5] "Curing Chirp in Command Transmitters" MO T-53, Vibration, AC Heater Power

[Mike Morrow]

Ken, I hope you won't mind my reply below being made to the list.

Ken wrote:

> I have two questions concerning T-53B: 
>
> 1) what, exactly, is the EFFECT on the VFO stability when T-53B is NOT 
> used, and...

The circuit is designed to eliminate **vibration-induced MO frequency changes**. 

US patent 2,189,402 (applied for June 1938) was granted to RCA on February 6,
1940 for an oscillator circuit that is functionally similar to the one used
in the "command set" transmitters.  See

  http://pdfpiw.uspto.gov/.piw?docid=02189402&SectionNum=1
  http://pdfpiw.uspto.gov/.piw?docid=02189402&SectionNum=2

The patented circuit uses two bifilar RF chokes in the filament supply of a
Hartley oscillator and a large capacitance between one of the heater
connections to the indirectly-heated cathode/MO coil tap connection.  This
places the heater and the cathode at the same RF potential and eliminated
the effect of heater-to-cathode capacitance changes on the MO.

That is functionally the same as the command set MO, except that the command
set design is clever with its use of the lower tapped portion of the T-53A
main winding along with the T-53B winding to provide bifilar choke action.
That also allows direct connection of the heater to the cathode without use
of a capacitor.

The really interesting information is found in the explanation for the
advantage that comes of this arrangement, which eliminates the effect of
heater-to-cathode capacitance on the oscillator circuit.  It was found that
this capacitance VARIED WHEN MECHANICAL VIBRATION OF THE FILAMENT WITH
RESPECT TO THE CATHODE took place in indirectly-heated cathode vacuum tube
oscillators.  Such vibration-induced heater-to-cathode capacitance change
caused frequency change in the oscillator.

The patent claim illustrates this situation with the example of
loudspeaker-induced vibration in entertainment receivers.  It is much
more obvious how the high vibration aircraft environment and the high
frequency of transmitter oscillation justifies a complex MO circuit
that eliminates effects of heater-to-cathode capacitance.

If one is not operating his ham set in such a high-vibration environment,
there will be no adverse effects from separating the heater circuit from
the cathode, and eliminating T-53B from the circuit.  

> 2) if T-53B is left in the circuit when AC is applied to the filaments,
> will this cause an AC hum to be induced on the VFO output signal?

When I was still ham-hacking these units almost 50 years ago, I never
separated the heater circuits from the cathode, nor eliminated T-53B.
I used 12.6 vac on the heaters and never had any reports of AC hum.

This result in not surprising, and is fully predicted by basic circuit
theory.  In order for heater AC to become induced on the RF flowing in
T-53, the impedance AT 60 HZ of the T-53B winding and tapped-portion
of the T-53A winding would need to have some significant value.  But
these windings have essentially zero AC impedance at 60 Hz, so no
60 Hz AC voltage can develop across these windings.

Of course, it should be very easy to demonstrate this in practice,
one more time.

If one may have a favorite or "most-admired" component in a set like
the AN/ARC-5, I will readily indicate mine as being master oscillator
coil T-53.  It is a beautiful design mechanically and electrically.
I wonder how much it would cost to make today. :-)

Mike / KK5F