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.

The M60 uses a self-bias circuit, which uses the current through the tube to develop the bias voltage. This type of circuit typically does not produce as much max power as a "fixed bias" one (the kind where the owner or an active circuit periodically adjusts the bias to a specific value), but they are inherently stable, and you can plug and play a variety of tube types, as long as they are matched.

Also, this amp idles at 160 watts per channel, and uses 280 watts at full power, so it's class AB, but conducting pretty well at idle.

Auxetophone, thanks for explaining the auto bias fact. I knew it was auto bias but not how this affected power.

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.