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As I remember, (going completely from memory, so take this with a grain of salt) the 5ar4 is similar in load capability to the 5u4. The 5ar4 was an improved tube for Hi-Fi, having two major differences. (1). The 5ar4 had cathodes instead of filaments, so it warmed up slower. the idea was that it is bad to have the plates hot on the tubes before the filaments come up. (2) The 5ar4 had less voltage drop, so you could in theory design a power supply with less sag, and possibly an amp with less distortion. If you substitute a 5ar4 for a 5u4 the plate voltage will be higher, probably too high. SInce you're running it on a variac, though, and you have schematic voltages, I'd just hang a meter on the highest b+ tap and use the variac to make sure it doesnt get out of hand.
If you go the other way, and put a 5u4 in where a 5ar4 belongs, you'll just have low plate voltage, more sag, and you wont have the benefit of the plate voltage delay on startup.
I like your idea of checking the DC voltages. Almost any bad part will screw them up (so some of them will be horribly wrong). I'll bet you find the problem this way.
John
Last edited by blue_lateral; 06-01-2005 at 10:32 PM.
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