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A lot of what has been said is hooey ... except, of course, the bad reputation
of the 6x5.
There is nothing in a recapped radio that is going to be hurt by 2N1007 replacing
a 6X5 if the power supply caps are high enough voltage rated, as are the replaced
paper caps. I looked at Zenith schematics from 1941 using 6x5 rectifiers and they had
250 v B+ voltages. With 450 V electrolytics and 630 (or even 400)V film caps,
you don't even need additional series resistance, as the transformer resistance will be
safe for 1N4007s.
To be picky, I usually start with a 500 ohm resistor in series with the diodes,
get the radio running, set the line voltage so the heaters are at 6.3 volts,
and adjust that resistor to get the rated B+. I might also add a thermistor startup
slower-downer in the AC line, but only after that was done.
BUT ... at a 250 or even 375 volt B+, none of that matters except if your line voltage
is high enough to endanger the heaters. Now at 400 or 450 V B+ on the schematic
I do always worry about the resistor and thermistor.
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