...add a third stand-off and put in a single fine wire from the braid of an RG/8U coax cable as a HV fuse. Space the pillars about 2" apart for this fuse so when it goes, the arc breaks quickly...
I also add a fine wire fuse in series with the diode stack before the filter caps. I've not lost a diode stack in years using it. Bear in mind that in this position, over time the fine wire fuse may fatigue from filter cap turn-on current surges and eventually open up for no reason.
Use two fine wires in parallel if the single one is not enuff.
That reminds me of the old Italian colonial houses in Eritrea as I recall from the late 1960s. You literally "mended" the a.c. mains fuses. The "fuse" consisted of two insulated blocks, and you purchased a roll of fuse wire that looked similar to a roll of small diameter solder, different gauges for different current carrying capacity. When a fuse blew, you removed the remnants of the old wire (carefully - the mains voltage was 240v @ 50~) and strung a new wire in its place to bridge the gap and presto! a new fuse. The whole thing mounted on the wall, completely in the open with all the wiring and uninsulated terminals fully exposed.
Standard cartridge fuses consist of a length of fine wire, but the voltage rating is limited, I suspect, by the the glass cylinder that encloses the fuse wire. When it blows, the vaporised metal leaves a thin deposit on the glass envelope, which continues to conduct if the voltage is high enough, until the metal deposit burns away and/or the glass shatters. Open-wire fuses would inherently make good HV fuses.
Many times I have used standard glass cartridge fuses JS wired in series with the plate caps of 866As and 872As when the main power supply fuse would sporadically blow. Use a current rating about the same as the actual DC pulled from the supply, increasing the size if the surge current immediately blows the fuses as the filter capacitor charges. Insert a 3rd fuse rated slightly higher than the actual plate current in the +HV line between the output capacitor and the load. If the 3rd fuse blows, that means the problem is in the amplifier or wiring, or the bleeder is shorting. If one of the fuses to the rectifier plates blows but the other remains intact, that indicates it is probably that rectifier tube flashing over internally. Replace the blown fuse and try again, and if the same one blows a second time, that pretty well isolates the problem. If it is intermittent (as are 99% of my radio malfunctions), just leave the fuses in place and operate the rig normally until the problem recurs and you can catch it in the act.
I have found that to be the quickest and least frustrating way to troubleshoot a flaky intermittent fuse blowing problem. Definitely worth the cost of a couple of packages of fuses.
Speaking of fuses, has anyone noticed the prices these days of old fashioned screw-in mains fuses? I still use them in the a.c. line to the transmitters, and the HF-300 rig uses them as HV power supply fuses, as do BC-610s. A box of 15, 20, 25 or 30 amp fuses used to cost couple of bucks at most, but now they cost about that much apiece!
