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Chad: The 250mA rating is the one I use in a B/W sets' B+ Boost line. For horizontal output cathodes in color sets, the closest 5x20mm pigtail fuse I've found to 300mA so far is 315mA. I vaguely remember the 7A fuses from the RCA CTC22 and CTC36. No primary-circuit protection at all was included in any "roundie" RCA that I recall, only a PTR breaker in the secondary circuit to the doubler diodes. CTC27's and 31's IIRC had a "fuse link" in the primary circuit. I'd probably go with a 7A fast-acting 227-series pigtail to guard against a "hard" short like a bad rectifier, etc. and no larger than a 4A time-lag 228-series either pigtail or in a holder of some kind for "softer" overloads. In my experience, I've found that a time-lag fuse can be rated closer to the actual demand of the set without "false alarm" blowouts. Many guitar amps using a pair of 6L6 or EL34 and single 5U4 draw about 2A and have 2.5A slo-blo line fuses. That's only 25% over actual demand.
I knew a hobbyist/tech once who upped the fuse rating in a tube hi-fi amp because "It was blowing fuses every 1/2 hour but worked OK." Sure enough, about 2 days later, the real problem (a leaky 'lytic) overheated, blew its seal, then went to a dead short at which point the rectifiers and surge-limiting resistor smoked. I think that's called "finding the real problem the hard way" ... and cleaning up all that 'lytic goo couldn't have been much fun.
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04-17-2004, 05:25 PM
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