Problem in MAC MC240

Dear experts,

Recently my 240 has a big problem. The last power tube has lighted up as bright as a bulb. I have interchanged the tubes, still at that position the problem happened. Even I changee another set, still problem at there. Can anyone tell me some possible reasons?

Thanks a mil!!!
I am not an expert but would venture a guess that either the bias or heater voltage is way out of specification, otherwise the tube would not be glowing as you describe.

Any chance the bias is set wrong? Do you know how to check for this?

Depending on circuit design, it's also possible a resistor is burned and allowing the bias to run wild. Hopefully someone here at Audiogon with the technical expertise on this topic will advise you.

In the meantime, I would avoid putting new tubes in the offending socket, they may be damaged beyond repair.
MC240 is auto bias. Better have a tech to check out what's going on. You can try to measure the voltage ( be aware that is is deadly 450+ Volts )
Thanks to both. I'll have it check next week and in the meantime.......I can't enjoy the music from it :(.

To Chang. Finally I have all the tubes close to your recommendation, 4 RCA 7027a, 2 RCA blk plate 12BH7, 2 Telefunken 12AU7, and 3 Brimar 12AX7. I don't have 3 long plate Mullard 12AX7, but I have 4 idenitcal Brimar. When I put them on, sound good on my setup.

Thanks again.