[R-390] R-390A B+ Current?

[Randy and Sherry Guttery]

Hi Chuck!  Great to see you back!

On 3/15/2012 10:22 AM, chuck.rippel at cox.net wrote:
> While mitigating inrush is interesting, it brings little to the
> performance table but may help increase the life of R390A power
> transformers.
>
> The way to properly mitigate inrush in Linear Amplifiers is to have
> something like an 50 ohm, 40W wire-wound resistor and a 115VAC relay in
> series with the A/C power input...   ...relay contacts are wired to
> short the resistor, taking it out of the circuit.
> This whole process lasts something around 1 second.
Harris calls this "step-start" - and is found in many of 
their older transmitters.

I certainly agree that inrush isn't that big a deal with 
390s - as noted - they were designed to be forgiving of some 
pretty nasty power sources. The area that I think almost all 
vintage radios have in common - is that they were designed 
for mains that were somewhat lower than we see today - so 
their HV runs a bit high.  In the case of the R-390A - 115V. 
That coupled with replacing the rectifiers with solid state 
diodes - and the increased HV can stress some of the parts - 
and overall heat be increased due to higher dissipation. An 
easy solution - and one that requires no modification of the 
radio at all - is a buck-box.  A buck box contains nothing 
more than an adequate capacity (current)  filament 
transformer wired so that it's secondary is in series - but 
bucking the 120 - 125V mains common today. The "reduced" 
voltage is then passed on to the radio.  By choosing a 
secondary (filament) voltage appropriate to the nominal 
Mains "over-voltage" (but not considering the solid state 
issue) - then the radio is spared that stress / heat. Note 
that you don't want to also try to compensate for the extra 
HV of "solid state" here - that's because the tube filaments 
are going to also be effected by any "reduction" as well - 
and while it's good to run them closer to "design" than high 
- it's also desirable to not run them too cool either - as 
conductance of a weak tube falls off much faster under 
reduced filament conditions (which is how tube testers 
perform "life tests" - reducing the filament a "notch" will 
cause a weak tube to drop off quick - while a good/strong 
tube will remain near steady).

A "buck-box" can be made in a handy box - add a cord for the 
supply - and a standard outlet socket for the radio to plug 
into - and you're done.  No modification to the radio at 
all.  Since the HV is lower on "correct" input voltage - the 
small "increase" with solid state rectifiers becomes far 
less a problem.

just my .02


-- 
randy guttery

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