Are the tubes running in triode or ultralinear ? That's most likely your answer.Triode is usually less watts.
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Mike in md is correct, but Mike in ct (that's me) can think of at least 2 other factors. One is how hard the designer "pushes" the tubes in terms of where within their operating envelope are the output tubes running? To get a higher power output, a different designer may operate the tubes in a less linear region and/or with parameters that will shorten the tubes useful life. Second is how conservatively the marketing people rate the product. I'm sure that the technophiles among us can name some others.
There are a variety of factors that govern the size of transformers. In power transformers, temperature rise can be a big factor- transformers that don't get as hot tend to be larger.
In output transformers, the amount of bandwidth that the transformer has on the low end has an effect on the size, as does the power that the transformer can deliver. If the transformer is set up in a class A amplifier, it might be a bit larger as well.
This is all general...YMMV
I think its the design choice also.Sometimes,tubes sound better being pushed hard,other times,the opposite.If I didn't need the extra power,I'd lean toward the cooler,lower wattage one,if it sounded the same.I think four EL34's would give abound 30 watts in triode.Cary has some amps that put a lot of watts out for a tube type in one amp,the opposite in one of the other amps,from the same company.
I am sure that Rodger Modjeski (sp?) of RAM tubes and RM amps fame can give you an idea. Some of his amps seem to produce very few watts per tube while other an astonishing number of watts per tube. I think there are some SS in the cascode of amplification in some of his amps. I think ARC uses SS as well beyond simple rectification duties, but I am not entirely sure.
I own a Jadis DA 60 with enormous heavy trannys it weighs 80 Lbs. It's rated power is 60 wpc from 4 KT-88s. It does run in class A I am told which may account for the relatively low nuumber. In practice I use it to drive speakers that are 87Db sensitive and I never run out of power. So I am convinced that it can make more power "peak" than 60 watts.
I also own monoblocks that have 4 X 6CA7s each. These are just as heavy @ 78lbs. These however with less "plate dissipation" per tube are rated at 75-80 watts per side. The big difference may be that It runs in Parallel PP.
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.
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.