SETs

Don't stress it--he'll/she'll be 18 in no time! (just kidding)

 

Well... In electronics tech school your taught that Class A (tube or sand) are about 25% efficient max, so if you have a sand amps thats giving you 25 watts of audio power out (and the spec sheet isn't lying to you) than its giving off 75 watts of heat. I've seen textbooks from the 40's as wells as textbooks from the 90's that say that limitation hasn't yet been overcome. Class AB amps are more efficient, but of course you have cross over distortions, etc.

Of course with tube amps you have filament heat generated as well, so thats another variable thats difficult to quantify.

Actually, thinking about it... if a tube (like the 2A3) has a 2.5V filament and takes 2.5 amps of current, then you have 6.25 Watts of power consumed in the filament.

Power consumed (in watts) = Voltage x Current

Of course some of that power is given of in the form of light (lumens), and some of it is giving off in the form of heat, just as an incandescant lamp at turning power into light (lumens) and gives off a lot of heat (compared to a flourescent light), I'd say the majority of that power in a tube filament is turned into heat. But even if all 6.25 watts is given off in the form of heat I wouldn't say thats enough to burn my skin. The rest of that heat I believe comes from the inefficiency of a class A amp. Of course putting more EL34's in a case will increase the temp because even if that PP Class AB amp is 50% efficient there's still more amplification devices generating heat to begin with.

If my reasoning is flawed, someone feel free to enlighten me.

Guess I'm going to have to pull a 2A3 out of my amp and connect 2.5V to it with no signal on the grid and that plate left unconnected and see how hot it gets.


Nelson Pass has DIY transistor amp project over on www.passdiy.com, the "Son of Zen" thats only 4% efficient. 500 watts of power consumption give you only 20 watts of power output. The other 480 watts replaces your furnace.

 

Drew,

I think your followup covered the points adequately, but there is another item to consider.

Most folk using SET are running efficient speakers (say greater than 92db/watt/metre) and in my case, I often listen well below 92dB, but for grins I'd say a watt would cover it. So 3 watts wasted in heat, by the 25% rule. Now the filament does look like a major contributor, especially in my case, as in my 300B it's a 5 volt filament. Assuming I also have 2.5 amps, there's 12.5 watts.

I was concerned in your initial post that you stressed the class A mode of the SET (compared to the class AB mode of the push-pull amp in the original question) and pointed out the envelope temperature. I felt that I had to say something - I did not mean to offend.

Edit: sorry, one more point to make. A great deal of power that is wasted in a SET is actually dissipated in the output transformer.

Regards,
Geoff

 

My initial post was rather vague, so I'm not offended. My output transformer gets a little warm to the touch, but not "sear your skin" hot like the glass envelope of the 2A3. I'd definately want to keep the whole thing away from the hands of children (if I had any), which was a concern of Indy Mike.