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Since my DC filament tests, I've been thinking a lot about the 'mixed fixed' bias setting. I think this is a compromise in a good way.

For instance, a 50% cathode/50% fixed scheme would allow much of the cathode bias benefits (ie soft clipping) while still allowing total control over bias current in each channel, even allowing swapping of KT88s and EL34s which could never be swapped in a cathode biased amp (unless you get out the soldering iron).

Of course, some still appreciate the safety of cathode biasing. I would just suggest fusing the HV secondary CT. That bit of insurance would be good for every amp.

Comments?

Shannon

In my first DYI35, I fused the B+ at the tranny CT, by placing a 3/8 A fuse and holder in series between the CT and the standby switch. A good simple and cheap safety precaution, in case one of the electrolytics should ever go bad, or a tube go short.

I haven't tried mixed bias, no reason it wouldn't work, as long as the G1 voltage relative to ground, and the cathode current are appropriate.

One "mixed" scheme I have seen, in the VTL amps, which have triode/tetrode mode switch selectable, is that in some of theor monoblocks, each pair (or group of 4 in an 8-tube circuit) is individually selectable, and yes, one can run it in a half-triode and half-tetrode mode if so desired. THeir one precaution is to not switch modes "on the fly"
while the amp is powered up. THis will shorten tube life considerably and make for some nasty ugly spikes to the speakers. Some of these VTLs have truly impressive power levels, up to 1250 watts in tetrode, 650 in triode mode. At a truly impressive price, I might add...
Way beyond my means, I'll stick with DIYtube projects and resuscitating old "retired" Dynaco and other tubeamps for now.

/ed B in NH