[R-390] 26Z5W replacement

[Charles Steinmetz]

Roger wrote:

>What if we added a thermistor in with the diode?
>It would limit inrush current.

Unfortunately, it's not an inrush current problem -- it's the 
opposite.  With a choke-input power supply filter, the DC voltage 
when NO B+ current is being drawn is much higher than when normal B+ 
current is flowing (about 1.6--1.8 times more).  So, if the diodes 
are rectifying before the rest of the radio wakes up (which is the 
case if they are SS rectifiers in a tube radio), the B+ soars until 
the radio circuitry is drawing normal operating current.  There is no 
problem with a tube rectifier with an indirectly-heated cathode, 
because the rectifiers warm up slowly just like the rest of the 
circuitry (typically a little slower -- they are designed that way 
for just this reason).

Note that this is NOT just the normal B+ sag from resistive losses, 
which you also see with capacitor-input filters -- it is a 
fundamental characteristic of choke-input filters.

So, what we need is a dummy load on the power supply while the radio 
circuitry warms up, or some way not to have the SS diodes rectifying 
until the radio circuitry warms up.  Neither of these is a trivial 
design challenge.

Further, whatever solution we adopt for turn-on should also be 
activated when the radio is switched to "Standby," because the B+ 
current drain goes down and the voltage goes up in standby 
mode.  Note that the original design does not address this -- if you 
switch a stock 390A to standby, the B+ voltage goes up.

Best regards,

Charles