[Boatanchors] [ARC5] BC-375: STABLE AM QSO on 7160 KC

[Mike Hanz]

On 2/2/2015 12:02 AM, Dennis DuVall wrote:
> Maybe time for a couple of reality checks here. First, has anyone 
> duplicated David's results and beyond that, checked the results with 
> different chokes as discussed by Arden.
> Trying different brands of oscillator tubes might give some 
> interesting data points as well.  I have a pair of Amprex 211s which 
> have considerably different internal construction
> than the usual VT-4C. Will plug one into the oscillator socket on my 
> -375 and report back.  Also, there should be some way to get cooling 
> air flowing around the suspect
> components in the TU and observing what results that might have.
>
> Like others, I had long assumed that the frequency shift observed in 
> these radios on key down, negligible on 160, slight nuisance on 75/80 
> and definitely a problem on 40,
> was a consequence of geometric changes that came with  heating up that 
> massive plate structure in the oscillator tube.  The time constant 
> seemed to be about right too.
> I have mused from time to time about schemes to keep the plate current 
> flowing through the oscillator tube continuously while, at the same 
> tome,  quenching the oscillations
> on key-up by some other means.   Never came up with an easy way to do 
> it though.....  :^(

Hi Mac,

As I recall, Walt Hutchens offered that diagnosis on the -375 in his 
Electric Radio series on military sets.  Seemed reasonable to me at the 
time, and like you, I took it as the likely culprit.  On the other hand, 
his subsequent VFO article that pursued perfection in the"not so drifty 
VFO" area did give me pause.  His comparisons of coil construction 
effects were groundbreaking, at least to me, and makes the oscillator 
coil construction of the 375 TU at least a suspect, despite its 
massiveness.  Its tight physical contact with the ceramic is one 
drawback, given Walt's experiments...his final solution for an almost 
zero drift oscillator suspended the coil in air from its end points, if 
I remember correctly.  In any case, agree with the desired replication 
of AB5S results - not because I doubt them but because there may be some 
yet unknown factors that have not considered.  Something about his 
changes has obviously settled something down...

I also have to applaud Dave for wading into an area that has been 
considered dead and buried for many years.  Out of the box thinking to 
be sure, especially considering easily reversible mods held as a guiding 
criterion!  Bravo!

73,
Mike  KC4TOS