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Old 20th August 2011, 05:28 PM   #1
Bigun is offline Bigun  Canada
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Default Class AB PP - is it any good ?

I thought this was a question that would already have been asked but I didn't turn up much with a search. Seems to me that tubes = class A.

Coming from the SS side of life the Class AB amp is common. And has been researched to death. One is left feeling that for the most part, it's a topology that needs plenty of gnf. But it's a technology that has been refined and works well.

So what about tubes, they have different characteristics from transistors - how is the cross-over from class A to class B - is there an optimal bias ?

I get the perception Class AB tube amps are rare - why ?
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Old 20th August 2011, 05:46 PM   #2
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No, tubes don't mean class A. For transistor amps class A matter much more than for tube amps that work nice in class AB. Actually, class AB nonsense in transistor amps was the result of attempts to mimic class AB tube amps. First transistor amps even used output and interstage transformers. Today Teransformerless word is omitted and forgotten, but it was used when transformers for the first time were eliminated from transistor amps.

Yes, there are optimal biases, that depend on selection of limiting factors, such as power dissipation, real word transformers, geometries of tubes.

Plate curves are your friends dear Bigun!
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Last edited by Wavebourn; 20th August 2011 at 05:49 PM.
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Old 20th August 2011, 05:49 PM   #3
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In fact, most of the amplifiers produced in the heyday of the vacuum tube production used class AB1 output stages -- Virtually all Dynaco, Fisher, Heath, Eico, Scott, Pilot, etc. were all class AB1 amplifiers to name but just a few. Even most of the Mac stuff was class AB. More to the point, it is true class A amplifiers that are rather rare with vacuum tube amplifiers. The original Williamson design is an exception to this, as it was quite common, and was a true class A amplifier with a triode output stage.

There is always an optimum bias point, but depending on how the tubes are operated, this can be very sharply defined (i.e pentode operation), or more broadly defined (as is typical with UL operation). How well the crossover point behaves is primarily a product of the quality of output transformer used.

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Old 20th August 2011, 06:51 PM   #4
godfrey is online now godfrey  South Africa
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IMO, most transistor amps are "class AB" and most valve amps are "class AB", however the term "class AB" has a totally different meaning in those two worlds. An interesting metric is the ratio of quiescent current to peak current for the output devices.

For example in a solid-state 50W/8ohm amp, the output devices have to deliver a peak current of about 3.5A, yet the idling current is often only about 100mA - around 3% of peak. If the idling current was increased to 350mA, that would be considered as "very high bias", yet it is still only 10% of peak current.

I think if the idling current in the output valves of a tube amp was only 10% if the peak current, the amp would be considered hopelessly underbiased.

Maybe some tube-heads can give an idea of what's typical in a valve amp?
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Old 20th August 2011, 07:28 PM   #5
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Quote:
Originally Posted by godfrey View Post
IMO, most transistor amps are "class AB"
Nup: the most famous opamp that sold xx millions in hi end audio and studio consoles uses class B was the 5534....thd 0.002% and a noise floor well below -90dBu ?
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Old 20th August 2011, 07:40 PM   #6
godfrey is online now godfrey  South Africa
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Huh? I thought the question was about power amplifiers, not opamps. I'm pretty sure the NE5534 has a class AB output stage anyway.
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Old 20th August 2011, 07:55 PM   #7
Bigun is offline Bigun  Canada
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very interesting. I feel a need to start inhaling more information on this topic. My diy audio education seems to have a huge gap in it ! I had thought tube class ab was not a good idea because it was so much better to do it in SS where complementary n an p type devices are available.

Plate curves my friend - will have to look - It matters now how do tubes turn on-and-off compared with bipolar transistors - do they make better transition than typical SS ?

Perhaps the question I really need to ask is: What do you think I should expect in terms of 'sound' from tube class ab amplifier - is it possible to achieve good results without gnf without a complex design / tricks - do I look to these old designs Dave listed out or is there a modern 'standard design' approach (like the double EF for SS class AB) ?

p.s. yes, no opamps here !
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Old 20th August 2011, 08:14 PM   #8
tricomp is offline tricomp  Denmark
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Quote:
Originally Posted by Bigun View Post
Perhaps the question I really need to ask is: What do you think I should expect in terms of 'sound' from tube class ab amplifier - is it possible to achieve good results without gnf without a complex design / tricks - do I look to these old designs Dave listed out or is there a modern 'standard design' approach (like the double EF for SS class AB) ?
Even AB2 is possible (grid-1 current) with the right drive and suitable output tubes. Don't know if it can be called complex design or tricks but it does work. Judge yourself from schematics.
My 2C34 AB2 PP-amp's playing behind me right now.
Sounds wonderful.
Thread here: CF/CCS, can it be made simpler?

Newly re-drawn and corrected schematics attached.

rgds,

/tri-comp
Attached Files
File Type: pdf 2C34-PP_5687WA-CF_CCS_Final.pdf (15.8 KB, 46 views)
File Type: pdf 2C34 Power-Supply_V_Final.pdf (16.7 KB, 21 views)
File Type: pdf 6AN8A_Pre_Inv_Final.pdf (12.4 KB, 28 views)

Last edited by tricomp; 20th August 2011 at 08:18 PM.
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Old 20th August 2011, 08:37 PM   #9
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The "Big 3" of vintage tube amps, Marantz 8B, H/K Cit. 2, and McIntosh 375, are all Class "AB1". The class of operation for both tubes and SS is defined by the duty cycle. Class "A" entails a 100% duty cycle, Class "B" entails an exactly 50% duty cycle. Class "AB", naturally, falls between "A" and "B". With Class "AB", you really need to state whether the unit is operating near Class "A", near Class "B", or truly in betweeen.

IIRC, Krell offered Class "A" SS amps that ran quite hot, as is too be expected.
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Old 20th August 2011, 09:13 PM   #10
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Quote:
is it possible to achieve good results without gnf without a complex design / tricks - do I look to these old designs Dave listed out or is there a modern 'standard design' approach (like the double EF for SS class AB) ?
There are two new designs that come to mind that fit the requirements. Both can be built to power levels from a few WPC to hundreds of WPC. Both are very long threads revealing multiple build options.

A poster asked how to build an amplifier using 6L6GC's in AB2. He was looking for about 25WPC. Together we designed an amplifer that can run in class A , AB1 or AB2 in pentode, UL, or Triode mode. I ran several types of output tubes at power levels from 15 WPC to 100 WPC, but never made a finished amp out of it. None of my versions needed or used GNFB or any local feedback. Chris however did build a nice amp that looks and sounds good. I don't remember if he used GNFB.

6L6GC AB2 Amp

Pete Millett designed a mild mannered 18 WPC P-P AB1 amp using sweep tubes. I bought one of his boards and remarked that I would be disapointed If I didn't get at least 50WPC. I set my sights too low. I have seen the board make 250 WPC for a weekend of very loud music, but settled on a "conservative" and very stable 125 WPC build. Several people have built the 125 WPC version. I am not the only one who likes the sound. The board has the option of running GNFB. I tried it for about 2 minutes and unhooked it. It is much more dynamic without GNFB, but some local feedback around the output tubes is used.

Posted new P-P power amp design

You can't get 200+ WPC from a TRUE Class A amp using tubes, silicon, GaN, SiC, or nuclear fusion unless you have the cooling system from an old Cray super computer lying around. Sales and marketing people have been known to stretch the definition of "Class A" just a bit.
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