Tubes to Watts Ratio

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.

By contrast, the classic Mullard "5-20" circuit used a pair of EL34s in ultralinear for "20 watts" . . . Which was more like 25 watts on the test bench, but this circuit also used a simple cathode-bias arrangement which tends to give up some steady-state power rating for a bit more peak power.

But in guitar amps, a quad of EL34s will easily put out 110 watts or so -- actually the most I've ever measured is about 215 watts at clipping. This was from a German Dynacord amp that was built in a 2-rack-unit chassis, with the tubes pc-mounted and horizontal (!), and an output transformer about the size of that found in a Dynaco ST70. IIRC, it was pentode operation with almost 800 volts on the plate, and maybe 500 on the screen. Needless to say, the thing chewed through power tubes like candy . . . But the particular guitarist was totally in love with it.

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.