[ARC5] "Curing Chirp in Command Transmitters" T-53 Self Heating/ Manuals

[Mike Hanz]

On 10/6/2014 1:31 PM, Kenneth G. Gordon wrote:
> On 6 Oct 2014 at 13:01, Mike Morrow wrote:
>
>>> I wondered about that heating effect, too.
>> The total resistance of the windings of T-53A and T-53B through which
>> the filament current for the 1626 flows is, at most, 0.3 ohms.  If the
>> rated 1626 filament current of 0.25 amps is flowing in those windings,
>> a **very negligible** 22 milliwatts is generated against the very
>> substantial mass of T-53's open ceramic coil form.  Self-heating of T-53
>> is non-substantive.
> Neither of my two questions have yet been answered. Therefore, I will repeat
> them here and add a third:
>
> 1) What effect is there on operation of the 1626 VFO if T-53A/B is NOT
> used?

Fair question.  From The Radiotron Designer's Handbook Fourth Edition, 
page 957:

"(L)  When possible causes of frequency drift are being considered, it 
should not be overlooked that *valve interelectrode capacitances* will 
change during the initial warmup period.  A typical example occurs in 
the Hartley circuit of Figure 24.4(B), in which the heater-cathode 
capacitance is directly across part of Lo; in this case connecting one 
side of the heater directly to cathode and adding a suitable RF choke in 
series with the other heater lead will minimize the trouble.  This 
arrangement is also helpful in reducing microphonics caused by 
heater-cathode capacitance variations."

Pretty authoritative, it seems to me.  Microphonics in a piston engined 
aircraft has got to be a problem with all the vibration. Obviously the 
series DC filament string caused the ARC engineers to modify the 
solution slightly, but it would appear that it worked very well.

> 2) Does the fact that filament current flows through the cathode tap of the
> oscillator coil cause localized heating which thereby contributes to drift?
>
> (I would imagine that if there WAS localized heating, this would
> CONTRIBUTE to stability in the VFO in their original use at altitude and the
> accompanying low temperatures, but for OUR uses, that would have the
> opposite effect.)

You can probably argue this plausibly both ways, but looking at Mike's 
calculation, I would agree that whatever effect there was would be 
miniscule, especially in the short term.

> 3) Does applying AC to T-53 induce 60 Hz modulation into the VFO's output?

It would seem reasonable for it to do so to some extent.  The big 
question is how much.  Never having used an AC supply on any of my WWII 
equipment, I won't speculate whether it would be noticeable in the 
signal.  I know I have solved a hum problem a number of times by going 
to DC filaments, but these were low level audio amplifier circuits where 
I was pursuing the vaunted goal of audio perfection.

> Frankly, in my "uninformed opinion", the main, and possibly sole, cause of
> chrip in commmand transmitters is wimpy power supplies...

I would tend to agree with you...at least with "the main" cause part.  
There are too many other causes, if admittedly minor, for me to commit 
to "sole" cause. :-)

Vy 73,
Mike  KC4TOS