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Originally Posted by Tubejunke
The bad thing about the bulb method is that the voltage is so reduced that the DC never gets pumping enough (or at all?) to slow charge and reform the paper caps.
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Correct. When you "warm start" a piece of tube equipment by variac or series light bulbs, the transformer is operated at reduced voltages. This means the 5V supply to the 5U4 filament is reduced as well. At reduced voltages (especially at the low end of the variac sequence), the 5U4 filament isn't hot enough to "boil" any electrons, the electron cloud is not present, the plates do not conduct at all (no emission). You'll measure little or no B+ until a significant percentage of "full throttle" is reached. I believe it's just a threshhold where, when reached, the 5U4 suddenly fires and a sudden, near-full jolt goes to your vintage caps. This is why most in-circuit "reform" attempts fail.
Build a couple of these (see photo). It is a solid-state 5U4 that uses a pair of 1N5408 diodes as the rectification method. They work at all AC input voltages from 0 to 1000, incrementally and/or smoothly, to avoid this problem.
You'll want to have two of them in your toolbox, one I call left-handed and one I call right-handed. The unbanded side of the diodes go to pins 4 and 6 of the tube base (salvaged from bad metal radio tubes such as 12SA7). The banded side of the diodes, connect together, then connect the joint to pin 2 for a left-handed unit, or pin 8 for a right-handed unit.
It will be up to your specific application as to which unit to use. Either pin 8 or pin 2 will be connected to your B+ supply and power filters. The other will go thru the 5V windings of the power transformer. I don't know that it matters if the high(er) voltage runs thru the 5V windings or not, but I prefer not to do so. So use the unit that corresponds to the pin going to your power filters (i.e. a choke or the 1st electrolytic). 5U4 pins 2 and 8 are symmetrically interchangeable in practice, so don't necessarily trust the Sams schematic. Look at the wiring. I have at least one example where the sams is wrong (my 1949 Magnavox).
This all having been said, I agree that reforming 50-year-old can caps is a risky business. I just use the solid-state 5U4's for slow-starting to avoid smoke-and-flames until I'm sure everything's wired up correctly. After putting in new capacitors.