Stage classification and efficiency
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 12th December 2004, 08:35 AM #1 eeka chu   diyAudio Member   Join Date: Nov 2003 Location: England Stage classification and efficiency A purely theoretical question here. I was a bit confused by something I read in 'Valve Amplifiers' earlier this morning. I understand classification on a stage, but something the Author mentioned about efficiency left me thinking, mainly because he didn't make any attempt to explain the statement he'd made. He was discussing the maximum theoretical efficiency of different stage classifications. He was measuring this in terms of sine wave at the output. He mentioned 50% for class A, and 78.5% I think for class B pp. At first I thought this would be, efficiency of sine wave reproduction at the output for a given sine wave input. But that's not right. Because otherwise the class B stage would have to be 50% per valve. And the maximum efficiency would depend greatly on how much distortion the stage was producing anyway. Then I thought that perhaps he was discussing it as a process relative measurement. For example, a valve only conducts in one direction. So one that is expected to produce a negative form as well as a positive form is against the nature of the processor. If that bassis was right, a class B valve would be 100% efficient, so that can't be right either. So I'm still kind of confused about the bassis of his efficiency measurement and, in consequence, what the measurements are relative to. I've also begun wondering how pp amplifiers got the name push PULL, since neither valve does any pulling, they just push against each other less. Is there an output stage arrangement which genuinely has one valve pushing current whilst the other pulls?
 12th December 2004, 09:28 AM #2 Geek diyAudio Member     Join Date: Sep 2004 Efficiency of a tube just means what percentage is of its DC input is converted into useful output power, the rest burned off as heat. So, a 50% efficient tube just means if let's say 250V @ 100mA is sent to the plate (25 watts), 12.5 watts is turned into useful output, weather this is audio signal, RF, or whatever and the other 12.5 watts is dissipated by the anode and (if applicable) screen grid.
 12th December 2004, 11:17 AM #3 eeka chu   diyAudio Member   Join Date: Nov 2003 Location: England Thanks Geek, But why would that change with stage classification? For instance, say you have a valve in Class A dissipating 25W at the anode, why would it's efficiency be any less than a valve in Class B dissipating the same wattage at it's anode?
 12th December 2004, 11:33 AM #4 pedroskova RIP     Join Date: Dec 2002 Location: C'ville VA, USA In class B, the tube is in whole (or partial) cut off through half of the cycle, and is not conducting (as much) current.
 12th December 2004, 12:21 PM #5 eeka chu   diyAudio Member   Join Date: Nov 2003 Location: England I've got that, but he defined these as specific limits on the efficiency of a particular classification. Rather than general limits. He's refering to some sort of process limitation as opposed to real world limitation. Also, if he was refering to the efficiency of reproducing the stage's input, a class B stage would always be 50%. Any more and it would be an AB stage. Any less, and it'd be moving into C Instead, he says that the maximum efficiency of a class A stage is 50%, and a B stage 78.5%. So he can't mean the efficiency of input reproduction. This's what annoys me. He made the statement and then made no effort to explain where it was coming from. You write a 600 pages plus book on electronics and you better expect the guys reading it to be the kind who won't willing accept such statements without an explaination!
diyAudio Member

Join Date: Jul 2004
Location: Melbourne, Victoria
Quote:
 Originally posted by eeka chu This's what annoys me. He made the statement and then made no effort to explain where it was coming from.
I think Morgan has to. The book is thick enough already, and if he explained everything in detail, many people would not buy it (because it looks 'too hard'), or they would get lost reading it.

You're referring to page 384 right?

I don't know how to arrive at the 78.5% figure myself (it's actually pi/4 or something), but you might find this useful http://sound.westhost.com/efficiency.htm#classb

 12th December 2004, 01:10 PM #7 pedroskova RIP     Join Date: Dec 2002 Location: C'ville VA, USA Am I missing something here? Class B is pushpull only, while Class A can be either SE or PP. Isn't maximum efficiency for class A SE 50%, and for class A PP 25%?
eeka chu
diyAudio Member

Join Date: Nov 2003
Location: England
Quote:
 Originally posted by audiousername I think Morgan has to. The book is thick enough already, and if he explained everything in detail, many people would not buy it (because it looks 'too hard'), or they would get lost reading it. You're referring to page 384 right? I don't know how to arrive at the 78.5% figure myself (it's actually pi/4 or something), but you might find this useful http://sound.westhost.com/efficiency.htm#classb

Really nice to speak to someone with the same book. I'd be interested to hear if you've read any other valve books you could recommend that are similar in quality.

Good night Australia!

 12th December 2004, 02:53 PM #9 bear expert in tautology diyAudio Member     Join Date: Apr 2002 Location: New York State USA The efficiency is a percentage, independent of actual parameters. So, the idea is simply this: when you go from class A --> AB --> B the relative quiescent power goes *down* while the relative output power goes *up*. Thus the apparent increase in % efficiency. Efficiency here is taken as the ratio of all power being dissipated vs. the actual output power. The 78.3% figure is best case optimistic, theoretical max. It's simply the heat vs output ratio in practice. Class A is high heat. Class B, low heat. _-_-bear PS. someone got the efficiencies backwards on PP vs. SE class A, PP is higher than SE. __________________ _-_-bear http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]
pedroskova
RIP

Join Date: Dec 2002
Location: C'ville VA, USA
Quote:
 Originally posted by bear T PS. someone got the efficiencies backwards on PP vs. SE class A, PP is higher than SE.
oops....that would be me ...

...so I'll just muddy the waters -

according to Mr. Broskie :

"The point that needs clearing up is the part about the relative efficiencies of output stage topologies. The efficiency difference between a single-ended and a push-pull Class-A amplifier is zero, both are 50% efficient if inductively loaded. (If constant current source load, both are 25% efficient; resistive loaded, 12.5%.) In other words, 8 output tubes in a totem-pole push-pull Class-A amplifier equal 8 output tubes in a single-ended Class-A amplifier. In the push-pull version, the output tubes must see an idle current equal to half the peak output current; for example, if the peak output current swing is 2 amperes, then the idle current must equal 1 ampere and as totem-pole amplifier has four output tubes in parallel per bank, each tube must draw 250 mA at idle.

On the other hand, in the single-ended amplifier the output tubes must see an idle current equal to the peak output current; for example, if the peak output current swing is 2 amperes, then the idle current must equal 2 amperes, with eight output tubes in parallel, each tube must draw 250 mA at idle. "

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