[R-390] B+ current

[2002tii]

One thing to note about the 390A is that it uses a choke-input B+ 
filter.  Although they are not often used these days, choke-input 
filters are in many ways far superior to capacitor-input filters: 
they reduce the peak current through the rectifiers enormously, 
improve power supply regulation, and reduce ripple.

However -- there is one thing to note about choke-input filters and 
solid-state diodes, when used in tube radios:

The choke drops a bunch of voltage because it impedes the flow of AC 
current and, therefore, averages the pulsating DC at the rectifier 
cathodes (in a capacitor-input filter, the input cap charges to the 
peak transformer voltage minus the drop in the diodes).  Thus, for a 
given DC voltage, you use a transformer with considerably higher 
voltage for a choke-input filter than for a capacitor-input filter 
(the factor is ~1.6, the peak-to-average ratio of a sine wave).  This 
is why the B+ in a 390A is in the 240 V range, instead of 400 V as it 
would be with a capacitor-input filter and the radio's 570 V 
center-tapped B+ winding.

But when the tubes in the radio have not warmed up, there is no (or 
very low) load current, and thus nothing for the choke to 
impede.  The inductor cannot filter the AC components of the raw 
rectifier output to average the rectified DC voltage, so the filter 
acts as a capacitor-input filter when you turn the radio on, and the 
B+ rises to ~400 V until the tubes warm up and start drawing B+ 
current.  This is true even if you add a series dropping resistor, 
because zero (or low) load current drops zero (or low) voltage across 
the resistor (E = I x R).

Cathodic vacuum rectifiers like the 26Z5 are designed to warm up a 
little more slowly than signal tubes, for just this reason -- by the 
time the rectifiers are working, there is a load and the choke can do 
its job.  [Note that filamentary rectifiers -- 5U4, etc. -- warm up 
much faster than the indirectly-heated cathodes in signal tubes, so 
they act more like SS diodes with respect to choke-input filters in 
tube equipment.]

The traditional fix is to add a bleeder resistor across the first 
filter capacitor -- but you need to draw enough current to get the 
input choke into its happy zone, which dissipates considerable power 
and puts more strain on the transformer and choke.  The input choke 
in an R390A is a "swinging" choke -- its inductance increases at low 
currents (to 12 H) from its full-load inductance (2H) -- so you can 
calculate the required bleeder using the 12H value.  But it will 
still dissipate a lot of power.  You can put the bleeder on a 
time-delay relay, so it is out of the circuit during normal 
operation, but now things are getting very complicated....

Or, you can just install the silicon diodes (and dropping resistor, 
if you choose) and let the B+ voltage rise to ~400 V at turn-on, 
provided that your filter capacitors can handle the ~400 V.  [This is 
one argument against the need for the dropping resistor -- if the 
filter caps (the most vulnerable parts from an overvoltage 
perspective) tolerate the turn-on voltage, they will be fine with the 
much lower running voltage, even if the running voltage is higher 
than it would have been with tube rectifiers.]

I am NOT suggesting that SS diode replacements are a bad thing -- I 
do it all the time.  Just know that there will be a period of high 
voltage at turn-on, even if you include a dropping resistor, and plan 
accordingly.

Best regards,

Don


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