[ARC5] Transmitter capacitors

[Brian]

Hello Roy and Neil,

The studs are rivetted and soldered to the variable capacitors' side cheeks. The studs are stepped and the add-in plates appear to be jig-bored to mate very accurately with the stepped portion of the studs. Them's heavy duty!
We will never know what was in Dr Drakes design notes, unless Gordon Eliot White captured them. So, whatever explanation we conceive will be mere hypothesis and conjecture.
However, there are differences between the tuning capacitors for the VFO and RF PA:
  a.. the VFO capacitor drive has captive ball thrust bearings at the front end of the worm drive shaft and a jeweller's bearing at the rear; these two need controlled spacing if there is to be neither binding nor bowing of the shaft-cum-cheek plate 
  b.. the PA tuning cap worm drive is tensioned only by a Belleville washer 
  c.. the VFO cap has a trimmable moving plate at the drive end – for tuning dial tracking? 
  d.. all moving plates of the PA cap are plain; during war, a 10% drop in power output through slightly misadjusted finals is far less important than being on frequency.
>From a logistics point of view, it would have been simpler to have all cheek plate drive shaft supports rivetted, soldered and then tin-plated in the same bath. Perhaps the bolt-on plates were there to provide alignment during rivetting and soldering?
Dr Drake was excellent on design and prototype production - and hob-knobbing with the brass to win contracts - but he wasn't as smart on volume manufacturing for production as the Western Electric engineers. As the war progressed, I doubt there was much time for experimentation – that would all have taken place in the 1930s when Drake was solving the US Post’s problem of losing mail planes, and taking a punt on Edwin Armstrong’s superheterodyne principles, stolen by Sarnoff at RCA. I have not seen any examples of Dr Drake's very early Tx variable caps. What differences were there?

73 de Brian, VK2GCE.

On Monday, December 28, 2015 2:34 PM, Neil said to Roy: 

Hi Roy

Yes, I understand the requirement for stability, especially in the VFO cap.
Something just occurred to me.
Maybe the plates are not there to add rigidity. Maybe they're designed to
expand and contract to the same extent and at the same rate as that side
of the capacitor frame, so that the shafts don't bind in the studs. If the
plates weren't there, extreme temperature changes from ground level to
20,000 feet would tend to result in bending of the capacitor frame and
binding of the shafts.

So my thinking is the opposite of rigidity. Deliberate expansion. If this is
right, the shape of the plates is not simply random, they were made with
great care, perhaps after lots of experimentation.

73 de Neil ZL1ANM
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