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

Kenneth G. Gordon kgordon2006 at frontier.com
Mon Oct 6 16:19:44 EDT 2014


On 6 Oct 2014 at 14:24, Mike Hanz wrote:

> "(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.

OK, but doesn't this pretty much only address the calibration effects when 
one changes the tubes, OR if the circuit is not particularly stable to begin 
with, when this effect could become pronounced?

>  This arrangement is also helpful in reducing microphonics caused by
> heater-cathode capacitance variations." 

OK. That makes sense also when the VFO is operated in the environment 
for which it was originally designed: a very loud, cold, vibro-massage chair.

For our present uses, I believe this is not necessary.

HOWEVER, and again only IMHO, the circuit should be left strictly alone and 
NOT changed. I see no reason, so far, to do so.

> 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.

Agreed.

> > 2) Does the fact that filament current flows through the cathode tap of the
> > oscillator coil cause localized heating which thereby contributes to drift?
>
> 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.

But Mike's calculation referred to heating ONLY within T-53A/B itself.

I was asking about the effects of even such "miniscule" heating within the 
VFO main coil, the main frequency-determining component, via the cathode 
tap, which also would carry the filament current. Perhaps his calculation 
would also apply to that. However, in my limited experience, one does 
everything one possibly can to eliminate any source of unneeded heat in any 
VFO.

> 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.

Yes, but that is different and far more common.

> > Frankly, in my "uninformed opinion", the main, and possibly sole, cause of
> > chirp 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. :-)

Yes. Well, I admit to exaggerating that part of it a bit. ;-)

Thank you.

It would appear to me that in order for me to resolve my questions to the 
extent I would like to see, I will have to run some experiments and take some 
data.

As soon as I have time to dig out of my stash a couple of my terribly hacked 
transmitters, I'll run some tests on the VFO sections and will report here.

Ken W7EKB


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