Basic Tube Question... PP EL34 at 20 watts

[turtle441]

As a true addict, I of course not only build my gear, but I still lust after the new shiny toys as well.  I've been looking at the releases at NAMM this week, and Marshall put out their Origin series.  The 20W appears as close to my "perfect amp" as I've ever seen on the market.  (Plexi-style tones, 20-ish watts, effects loop, and what sounds like VVR.)  So... how's 2 EL34's PP work for getting 20W's?  I'm used to that being closer to 50W.  I think you can starve the plate to get it down to that, but from what I'd read that's got a significant effect on tone.  Are there other tricks to get down to that?

[sluckey]

So... how's 2 EL34's PP work for getting 20W's?  I'm used to that being closer to 50W.  I think you can starve the plate to get it down to that, but from what I'd read that's got a significant effect on tone.  Are there other tricks to get down to that?

Some people might think that running EL34s at 50 watts is abusive. Not really tricks, but there are several ways to control the output power other than reducing plate voltage. You don't have to 'starve' the plate. Decreasing screen voltage is also a way to lower power. Or, you can simply increase the load impedance, or you could change the fixed bias point, or you could change to cathode bias, or just simply reduce the drive signal. These are things that will actually change the power level.

On the other hand, you could measure (and report) the output power under different conditions. For example, you might say this amp puts out 50 watts +3db from 20Hz to 12000Hz @ 10% distortion into a 4Ω speaker load. You might also say this very same amp puts out 20 watts +1db from 20HZ to 12000Hz @ 1% distortion into a 16Ω speaker load. Both statements accurately describe the same amp but the power was measured under different conditions.

[shooter]

I recently sold my interpretation of a Plexi.  It was 25W, totally bent maybe 30W.  I had 4 musicians argue with me that it was "way to loud" for 25W.  They kept insisting I, the builder, was wrong!  Like Sluckey says; there are many ways to report output power, I use Volts RMS @ the speaker @1Khz.
In order to "satisfy" my client, I finally relented, said I'd lie to him like the Big boys and say it's a 40Wpk-pk amp, they were ALL happy then

[sluckey]

That's how Sears rated their Silvertone amps back in the '60s. They had some powerful amps with itty-bitty transformers. 

[shooter]

itty-bitty transformers.

I was so blown away by truly gifted players swearing my amp HAD to be at least 40W
so I waved my wand, walla.  I'll now lie better NEXT time

[92Volts]

Low plate voltage isn't always bad for sound. Companies like Vox proudly advertise their "Class A" designs with low plate voltage, to enable higher bias current. The circuit needs to be properly designed for whichever voltage (and tubes) are used, of course.

You could operate EL34s in triode mode and you'd get about 20 watts, using the same voltages that could produce 50 in pentode mode. But I'd expect that to affect the sound even more, for good or bad.

Some people might think that running EL34s at 50 watts is abusive

I'll have to apologize to the pair of EL34s I used in an 80 watt amp for a while 

I eventually switched those out for KT90s because it was quite abusive, but tubes can be run harder than many guitar amp designs choose to