MAC Autoformers?

I know of no tube amp designer who uses SPICE. can you name a few?

Nope. Maybe one? I know I don't! Victor Khomenko was trying to model our MA-1 in Spice many years ago before he became part of BAT. He called me up saying that the amp didn't run in Spice, but I had to point out to him that the amp actually worked despite what Spice came up with :)

As you can imagine, the ratings of the tubes tend to vary with load- and the amp will draw less power and run less heat if a higher impedance load is used. Also, as you've pointed out, the class of operation is affected. In our case the amp is biased to be class A2 on the proper load, but will be class AB2 if the load impedance gets low enough. We don't get crossover artifacts on that account though.

As far as high current spikes, at full power the tubes still have some dissipation left over, so they can handle spikes, however as the load impedance is reduced, eventually this will cause the cathode coating to fail.

Ralph,
Thanks for your answer. I think most of us develop amplifiers in real space verses cyber space. SPICE, by its name, was indeed designed to help people design ICs not tube power amps. 

The thing that concerns me about 6AS7s, it that they emit cathode sparks in my Tektronix 530 curver tracer at zero grid.That is consistant with your statement of cathode coating failure as it appears the sparks are bits of cathode coating. They actually look like sparks off of a 4th of July sparkler.

The thing that concerns me about 6AS7s, it that they emit cathode sparks in my Tektronix 530 curver tracer at zero grid.That is consistant with your statement of cathode coating failure as it appears the sparks are bits of cathode coating. They actually look like sparks off of a 4th of July sparkler.

The 6AS7G begins to develop grid current at about -15V or so; 0 volts on the grid is pushing them pretty hard (a bank of 12 power tubes at 0v on the grid can pop a 15 amp fuse rather quickly BTW). Despite that our driver circuit can push them to +15 volts on the grid while maintaining linearity.
Here are some tips for reducing arcing:
Precondition the tubes by putting filament current on them for at least 72 hours if you have the Russian variant (6H13C). If you have the Chinese (6N13) or American (6AS7G) tubes you will want to precondition for 96 hours. If B+ is applied before preconditioning or at any time during the process, you may consider the process ended. We built a jig for just this purpose. Preconditioning reduces premature arcing and can double the life of any power tube.
The Russian and Chinese tubes hold up much better in this regard. The 6AS7GA and its 6080 variant to the GA should be avoided for use in our amps- the grid heatsink is much smaller and tends to warp with grid current.

@atmasphere - Hi Ralph, I know we have had conversations about the 6AS7GA tubes and your comment above echoes the advice you gave me. However, Roger has a nice stock of those tubes and he asked me if I wanted to try them in the M-60s. So I took it over to the shop and he put in matched quads (2 matched per monoblock). Now I have no idea if the grid heat sink is warping and I wouldn't even begin to know where to look to check, but I have to say these have been the best tubes that I have had in the amp. The Chinese were not very reliable, even after preconditioning, the Russian were much better, and the RCA on par with the Russian. So far I think it has been a good 6 months since those tubes have been in the amp and no issues with arcing or lint shorts (Roger's term) or other type of failure. In addition, Roger didn't precondition the tubes. Now as to lasting 10,000 hours I guess we will see. Obviously still too early to tell.

I neglected to mention that the 6AS7GA tubes I am using are General Electric military spec. In addition, as Roger had performed measurements and had data on the tubes he was able to match them so they were drawing an equal amount of current. Matching the tubes also allowed the position of the DC offset adjustment screw to be set at the middle (near 12 o'clock) of its range to achieve the appropriate reading on the meter. My previous experience with other tubes was such that while you could still set the offset, you may not have had as much flexibility for future adjustments. In some cases I also had to swap tubes around a number of times to achieve the proper DC offset. Matching the tubes made the process a lot easier. So regardless of the type of tubes used, and even though Ralph doesn't require the tubes to be matched, based on my experience I would suggest matching them if you had a means to do so.