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

[Dennis Monticelli]

The filament bootstrap circuit makes a lot of sense to me.  Heater to
cathode capacitance varies quite a bit from tube to tube and in response to
vibration and heat.  The heater is folded up and slipped into the cathode
cylinder. It's spacing is not as controlled as the other electrodes in the
envelope.  A calibrated TX may not longer be calibrated after a 1626 swap
out if the RF bootstrapping circuit was not used.  That effect plus
mitigating microphonics and thermal drift all point in favor of the
bootstrapping.  Bilar windings (or just interspersed windings) create a
high coupling coefficient and hence better bootstrapping.  The designers
probably tried it without the "bifilar" technique and then settled on
higher coupling as being superior.  Clever, those engineers at ARC.

Dennis AE6C

On Mon, Oct 6, 2014 at 12:07 PM, Mike Morrow <kk5f at earthlink.net> wrote:

> 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
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