Tubes to Watts Ratio

Bojack, just to be clear, the proper term is 'self bias'. Self bias gets the bias for the power tube by dropping a voltage across the cathode resistor of the power tube.

An autobias circuit does not do that. All autobias circuits generally fall into the category of 'fixed bias' which is to say that something other than the current through the power tube is fixing the bias. It might be manual, and it might be autobias, but either way its considered 'fixed bias'.

IOW, the Quicksilver is *not* an autobias amplifier.

Kirkus, "60 watts from a quad of EL34s in Ultralinear is a bit conservative, but not by a huge margin . . . I'd say that a "70-watt" rating of the Marantz 9 is probably the most typical of what's found in a high-quality hi-fi amp with this output stage."

Kirkus, "in guitar amps, a quad of EL34s will easily put out 110 watts or so"

Guitar amplifiers typically feature pentode operation, as opposed to ultralinear. That explains a lot, if not most of that gap. Also, as has been mentioned by Auxetophone, cathode (self) bias versus fixed bias factors into the equation to at least a moderate degree. While there certainly are some famous examples of cathode bias guitar amplifiers such as the Vox AC30, fixed bias (Ampeg, Fender, Marshall, Mesa Boogie, etc.) represents the far more ubiquitous topology.

Also of note is the fact that the EL34 is a true pentode, rather than a "beam" or "kinkless" tetrode like the 6550, KT66, KT88, 6L6, etc.

The EL34 thus has advantages over the 6550/KT88 pertaining specifically to pentode mode . . . including higher plate voltage capability, resulting in significantly higher power output at the cost of somewhat higher plate resistance and higher distortion. They also have higher gain and can usually be kept in Class AB1, which keeps the driver stage simple.

The beam tetrodes tend to start drawing grid current (Class AB2) when used at higher power and bias levels, frequently requiring a follower to keep the drive signal from becoming non-linear. This is by far the biggest flaw in the Dynaco MKIII - trying to run KT88s directly from a split-load phase inverter. The MKIV/ST70 driver stage works much better . . . simply because it's running EL34s instead.