How can the Mini Silver Jubilee 2525C/H produce 20watts with 2x EL34 tubes?

[d3vCr0w]

Hi, I was looking at the Marshall Silver Jubilee 2525C (I'm looking to buy one) and noticed that it's a 20watt amplifier, I though that it had a pair of EL84 tubes but when I looked at the specs I noticed that it comes with a pair of EL34L tubes, which are normally used for 50watt amplifiers.

How do they use the EL34 tubes to produce less output power, do they feed less voltage to the tubes?
Would it be the same as running the tubes cold-biased?
And lastly, since the output tubes are being used for less power, should that increase the life-span?

Thanks beforehand.

 

[Kinkless Tetrode]

It's a type of power scaling, combined with cathode bias.

If I recall correctly the 20 watt mode ran about 300 volts on the plates. The 5 watt mode was a little less than 200 volts. The preamp tubes and the PI run the same voltages as the 50 watt and 100 watt versions.

BTW, I have both an 87 50 watt and the 2525H. I use the 20/5 watt version more often, because I can usually crank the amp up to where it sounds more lively.

 

[nortiks]

Yes, its all about plate voltages. It is not the same as cold biasing though, since bias is primarily about eliminating crossover distortion.
Also, running them at low plate voltage will increase lifespan.

 

[Pete Farrington]

Would it be the same as running the tubes cold-biased?

It depends what you mean by cold biased. The term is only generally used in regard of class A stages, where the design goal is more normally centre biased. An example of a cold biased stage would be the second stage of a 2203/4, with a 10k cathode resistor.
The class AB output stages used in valve guitar amps are inherently cold biased.

 

[nortiks]

It depends what you mean by cold biased. The term is only generally used in regard of class A stages, where the design goal is more normally centre biased. An example of a cold biased stage would be the second stage of a 2203/4, with a 10k cathode resistor.
The class AB output stages used in valve guitar amps are inherently cold biased.

Well, not in terms of common usage of the obviously relative term. People describe hot biasing class A/B amps here all the time. And with good reason since most A/B amps have bias adjustments that allow anywhere from "cold" with plenty of crossover distortion to "hot", as in red-plating. And virtually nobody talks about hot biasing a class A for the simple reason it is as you describe, normally center biased, and not readily adjustable. Now they'll often complain of how their tubes get cooked in a class A, lol. So it really is a semantics thing, but the common usage is to describe idle current settings in class A/B. jmo of course.