Marshall Major plate voltage

[Doug]

680V? Really? I glanced at the bias calculator page and 600v is the highest listed. Are today's tubes rated to take that voltage? Will biasing way down work or can I knock the voltage down somehow?
BTW, I'm measuring voltages without power tubes installed. This one hasn't been up and running in a while, so I wanted to check it out. Should I expect a voltage drop with the tubes installed?

[Alex R]

With the four KT88s installed I'd expect it to drop to around 600v on the plates. Normal for big strong KTs.

A cautionary note: running an amp like this with the valves out can cause voltages to rise above filter cap ratings. The doubled-up filters in there are probably 450v - 900v total - but the 50u's in the preamp might be seeing a bit too much voltage especially if the preamp valves are out too.

[AdmiralB]

It's been a looong time since I've been inside a Major, but as I recall there are two 'viable' primary taps. One is around 110 V and the other about 125V, but I don't recall exactly. With 'normal' US-ian wall supplies the lower tap gives about 680VDC and the higher about 620VDC. The heaters are closer to 6.3 with the 620V tap, so that's how I set them up.

Most of today's KT88/6550 will survive in a Major but none of them will deliver the full 200W+.