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

[chuck.rippel at cox.net]

Well said, John.  Way too much science and a lot of work toward no 
result.

In video #2, I demonstrated the difference between using the 2X 26Z5W's 
as rectifiers and 1.5A 1KV diodes.  The result was a 9V
increase in B+ voltage.  The demo on video was very graphic and well 
documented.  IMHO, its not worth worrying about.

I've see radios come in with a thermistor in series with the A/C line to 
lower the voltage/mitigate surge.  Those are not designed dissipate 300+ 
watts of
heat and I cut every one of them out and restore it to factory original. 
R390A's are not linear amplifiers who
require 40 amps at 6V AC to achieve proper filament voltage.  The 
receiver does not have 80-100 amps of cold filament inrush to protect 
against.
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.
The relay coil is tied between neutral and the radio side of the 
resistor.  The Resistor absorbs the inrush.  When the inrush is absorbed 
and the voltage drop agross the resistor
decreases the voltage on the radio side of the resistor will rise to 
115VAC, causing the relay to close.  The relay contacts are wired to 
short the resistor, taking it out of the circuit.
This whole process lasts something around 1 second.


Chuck

On Wed, Mar 14, 2012 at 5:42 PM, Roger Ruszkowski wrote:

> John,
>
> Way to much science. These are tube radios and need no where this much
> exactness. Remember these receivers were designed to run on generators 
> and very sorry power sources. At power on B+ does not surge
> to 1/4 amp and thus blow the B+ fuse.
>
> Rather than drop B+ as heat in a resistor, better to run a bit more 
> current / voltage / power in each tube
> and thus spread that 3 -4 watts across all the tubes.
>
> The idea of changing a filter cap can in the audio deck out for a 
> series limiting transistor voltage regulator comes to mind as 
> improving the B+ ripple. Some rewiring on the cap socket to
> get the transistor in series would be needed.
>
> Roger AI4NI