which is a big part of why I don't go for that old stuff.
Good point Gadget, seems I go back and forth on this issue myself. You have the group that swears by an old ugly output transformer, while others wouldn't use it even if it was potted and painted. I suppose I fall somewhere in the middle, except for tubes, where I almost always prefer vintage over new.
I've been thinking about what Max said, and while that would certainly be a safe bet, I don't fully understand. Saying we are talking about "most" older designs, wouldn't the highest voltage you could expect be whatever your plate voltage to final driver or splitter is. The coupling cap goes from that plate to the grid of the output tube, with a resistor sharing the output of the CC with the grid of the output tube. So, knowing that resistors fail open most of the time, we will figure ours failed closed and took that side of the cap to ground. This, in my mind would be the highest potential for voltage, or am I missing something? So, say you're using the 12AU7 for your splitter. The max plate voltage is 300, but we have at least 2 resistors, usually 3 or maybe more between that and our B+. Wouldn't whatever voltage that plate is running at be the highest voltage we could expect to see? Again that's with a slow turn on device, especially if SS rectified. So even if they were running the 12AU7 at the maximum plate voltage allowed by law, and we had that one in a few hundred resistors that failed shorted, our 200v cap, that is really tested to 1.5 times rated value would surely survive a long time. This brings up another voltage issue that's always bugged me. Why did all the EEs disregard the max plate voltage ratings on the EL84 especially, or so it seems. How many vintage amps run these tubes at, or under 300 volts? I can't think of any right off, but that doesn't seem to bother anyone, or else we would have all kinds of threads about how to get it down then re-calculate the cathode R. I agree totally with playing it safe, and I sure wouldn't skimp without a dang good reason, just playing devil's advocate for the sake of discussion. Let me sneak this in too, as this looks to be a good learning thread for me. What do they really consider when coming up with a max plate voltage. I would guess it would be things like proximity of the plate to other components to prevent an arcing condition. I remember reading something about Mullard being very excited about the super high voltage rating they achieved while developing the EL34, something like 8 or 900 volts IIRC. Seems like it was the way things were laid out and how it allowed the internal wiring to be physically distant so that arcing wasn't an issue there?