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Lots of good info. Thanks all.
My Capehart's restoration effort was mentored by the original owner's son, remotely over the internet from about 800 miles away. An interesting mix of frustration and enthusiasm. The set was put away, almost working, in an upstairs New Jersey home's closet in mint condition in 1970. We had a very strong desire to see his dad's set work without any aftermarket tampering whatsoever, which is one pretty good reason to at least try a re-form. In hindsight I could probably have left it alone and have donated it to a nearby Farnsworth museum as-is (which was sort of the original plan). After replacing all the caps, it ceased to be "pristine", and therefore not so much museumworthy. But it works now and I like it.
Our attempt to re-form may or may not have worked. We brought it up slow to 18VAC with the train transformer, then to 95VAC with the train transformer behind a step-up transformer (I bought a variac later, works ok but it buzzes and smokes uncomfortably, in my opinion). At around 95VAC, that's when a few resistors started smoking and cooking nearby components. At this point I lost my nerve and replaced all the wax capacitors. No longer museum perfect, I did the cans too, discarding the possibly reformed cans without ever testing them for good or not. So the re-form ended up an academic exercise only.
Ok, so I just did a simple test with my Sencore tube tester. I put in a good 5U4 testing 100%. I set the filament voltage knob to 1. The meter stayed at zero, no emission whatsoever. I turned the filament knob to 2V, the needle rose to almost the "?" area indicating a weak but good tube. At the 3V setting the tube reads almost normal emission, about 95% of max. The next setting is "4-5V" and at that point the emission is 100%.
1V at the 5U4 filament would already be (roughly) 20% of full throttle, or, say, 25VAC line voltage in our re-form experiment. At this point the HV windings might be putting out 70 or 80 volts but not showing up at the electrolytics at all.
We know by the tube-test experiment that the 5U4 is conducting very well at 40% (roughly) of full-throttle, so the only question is whether or not it's a smooth rise between 20% and 40%, or a relatively sudden ramp-up. Based on some reading on how Thyratrons and other more exotic tubes "fire", I simply surmised that the cathode ions begin to boil at a specific temperature, and are then pretty much boiling. A yes or no condition. This suggests to me a rapid rise between 20% and 40% could occur. At that point there might be 100V coming out of the high(er) Voltage windings. One could imagine a significant amount of that would hit the electrolytics -- a lot more than the fraction of a volt at which you'd want to start reforming a vintage cap. So my theory is that the commencement of ions boiling in a 5U4, on a TV variac warmup, might zap some fragile can electrolytics, that otherwise might have re-formed ok with a solid-state device. That was the theory presented to me by my mentoring authority, so it was the theory I posited here.
I've read in guitar-amp builder's blogs and forums that you don't want solid-state rectifiers in operational gear, unless the amp has a "standby" setting. Something about wanting the other tubes to warm up concurrently, a cold tube with full B+ gets some exotic damage that shortens its life. But that's hearsay info, so take it for what it's worth. I wouldn't run the SS devices in a good TV on a daily basis.
- kb
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