[ARC5] ARC-5 VFO question

[k0ng@inebraska.com]

Hello Neil:Well, I have pondered that some myself. I think it simply raises the
filament and cathode above ground for RF, much like a bifilar choke of today.
It makes for less capacity to ground than if the filament was not raised up.

Along those lines, I also ponder if the filament current through the Hartley 
Oscillator tap (the above) could just ever so slightly raise the temperature
of the oscillator coil and contribute to drift?? If you have not seen the size
of the wire on the coil that the filaments run through, you might not be
concerned. I have. I thought someday when I have nothing to do I will put
a heavy bifilar choke outside the coil and cap couple to the now grounded coil
and see if the drift is reduced, but it is easier to speculate. (??)

Oh!! If AC is used for the filaments, could it not modulate the oscillator some 
and cause HUMM?? Hmmm?

73 DE Charlie, K0NG.

Quoting Neil Barnett <neilba@clear.net.nz>:

> In the ARC-5 transmitter, why is the return side
> of the 1626 heater connected to the (cathode) tap
> on the oscillator tank coil?
> 
> Is it because:
> 
> a. The 1626 was not the original choice for VFO
>     tube (a directly-heated tube being the prior
>     choice) and when it was later decided to use
>     the 1626, a decision was made to simply
>     connect the cathode to the filament pin?
>     If this is the case, was this done so that a
>     substitute (directly-heated) tube would be a
>     plug-in replacement if the 1626 proved to be a
>     bad choice?
> 
> b. Heater current (noise) was deliberately
>     introduced into the VFO tank circuit in order
>     to ensure reliable start-up of the oscillator
>     each time its turned on by the relay?
> 
> c. Some other reason related to wear and tear of
>     the heater caused by DC operation of the
>     heater?
> 
> This q. has puzzled me for 35 years, and I never
> have seen a satisfactory answer.
> 
> 73 de Neil, ZL1ANM
> Auckland, New Zealand.
> 
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