Donnie2112
New Member
Hey guys, newb question here. Do you have any common examples of a Class A biased Push/Pull amp?
Examples of Class A push/pull
- Thread starter Donnie2112
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A real problem is that manufacturers today are extremely loose with the definition of "Class A". For example, in the Audiomat product, if it truly does produce 30 watts RMS per channel from two EL34 tubes, then it is impossible for it to be a Class A amplifier, as classically defined by both tubes always drawing current at all times throughout the dynamic waveform, up to maximum power output. It may be a design that extends Class A operation, but ultimately, to produce 30 watts of power, it must shift to Class B at some point. EL34 tubes are only capable of about 14-15 watts RMS power output when operating in Class A -- any attempt to produce more power will exceed the plate dissipation rating of the tube.
The only amplifier I know of that truly operates in Class A throughout its full power output range, and is quite common (or used to be), is the original Williamson amplifier -- of which the Craftsman 500 is the best working copy of imo. It is truly an honest Class A push-pull (triode) design, which is also why it only produces about 10 watts of power output. As I say, manufactures today are extremely loose with the definition of Class A operation -- no doubt for marketing purposes. But if you want one that is truly Class A by design versus a wannabe Class A designed by a marketing department, then the Williamson amplifier is the go-to standard.
I hope this helps!
Dave
The only amplifier I know of that truly operates in Class A throughout its full power output range, and is quite common (or used to be), is the original Williamson amplifier -- of which the Craftsman 500 is the best working copy of imo. It is truly an honest Class A push-pull (triode) design, which is also why it only produces about 10 watts of power output. As I say, manufactures today are extremely loose with the definition of Class A operation -- no doubt for marketing purposes. But if you want one that is truly Class A by design versus a wannabe Class A designed by a marketing department, then the Williamson amplifier is the go-to standard.
I hope this helps!
Dave
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There was a Musician's version -- could this be what you're referring to? It was simply the American version of the British design -- and other than using triode connected 807 output tubes and 6SN7 small signal tubes, followed the original circuit quite closely.
Max -- spot on with your easy way to tell whether a unit is really a Class A design or not. The Quad may also have been a Class A design -- I haven't really done any real analysis of that unit to know -- but being a Brit design, I'd be surprised if it were not. They really championed that mode of operation.
Dave
Max -- spot on with your easy way to tell whether a unit is really a Class A design or not. The Quad may also have been a Class A design -- I haven't really done any real analysis of that unit to know -- but being a Brit design, I'd be surprised if it were not. They really championed that mode of operation.
Dave
From Pete Millet's description of the Engineer's Amp (DCPP):
The output stage is biased such that it remains in class A1 operation until somewhere over 10 watts, where it enters AB1. Onset of clipping (where the power stage grids start hitting 0V) is at about 18 watts.
I get almost exactly 18W from mine per side before clipping.
The output stage is biased such that it remains in class A1 operation until somewhere over 10 watts, where it enters AB1. Onset of clipping (where the power stage grids start hitting 0V) is at about 18 watts.
I get almost exactly 18W from mine per side before clipping.
Sounds like an excellent description of Extended Class A operation with over 50% of the power produced in that mode (most Class AB designs don't produce nearly this much of the total power output in Class A before switching to B), but still, ultimately shifts to Class B at some point. Extended Class A operation is certainly an excellent design feature, but ultimately, the amplifier is still a Class AB design.
Dave
Dave
PakProtector
AK member
First of all...an AB amp will have both conducting up to some limit, and then cut one off and deliver further increases in power. It is AB all the way. A class A amp has both still conducting *AT FULL POWER*...anything else is AB, or if light enough, B( which still has a short period of both conducting ). Please stick to the original definition, not the one taken advantage of by marketing departments.
With any circuit, the possibility of Class A bias is directed by load, B+ and plate dissipation. The load is half the a-a load in the case of Class A. More dissipation allows longer conduction angles at given B+ and load. Increase the load, holding the other two constant and the conduction angle increases. Cut B+, and to the limits of constant cathode current, idle current can be increased and along with it conduction angle.
When you have a full 360 degree conduction angle *AT FULL POWER*, then you can call the amp Class A.
cheers,
Douglas
With any circuit, the possibility of Class A bias is directed by load, B+ and plate dissipation. The load is half the a-a load in the case of Class A. More dissipation allows longer conduction angles at given B+ and load. Increase the load, holding the other two constant and the conduction angle increases. Cut B+, and to the limits of constant cathode current, idle current can be increased and along with it conduction angle.
When you have a full 360 degree conduction angle *AT FULL POWER*, then you can call the amp Class A.
cheers,
Douglas
One needs to take some care with the use of the term 'cut-off', as if it is a well-defined point; not so for triodes. The Va-Ia lines are not straight, particularly at low Ia values. 'Cut-off' is therefore not well-defined, although the difference is not dramatic. The Quad-II final stage does work in class A. Although the topology is UL, the Va-Ia lines are of semi-pentode nature, thus with a better defined cut-off point. Although the maximum power output is given as 12W, the power transformer voltages are mostly on the high side, and with a somewhat high mains, the maximum output can be as high as 18W before clipping.
Thanks Pak -- Between your post and my original post in #3 above, hopefully we've hammered home the real abuse that manufacturers and chatter on the web today have given to the term "Class A" operation. Besides the original Williamson and the Quad II examples, real Class A push-pull amplifiers are few and far between.
Audio -- Thanks for verifying the operating conditions of the Quad II design.
Between the two, the Williamson (Musician's) will be by far the most common for the op to search out.
Dave
Audio -- Thanks for verifying the operating conditions of the Quad II design.
Between the two, the Williamson (Musician's) will be by far the most common for the op to search out.
Dave
I’ve never seen marketing literature tout class AB anything. I suspect most lay consumers do not know this terminology let alone percentages of conduction cycle. Most literature for class AB amps simply tout output power rather than class name.
I’ve only seen marketing tout the class if their product is truly or supposedly class A.
I’ve only seen marketing tout the class if their product is truly or supposedly class A.
All of Ralph Karsten's designs at Atma Sphere are class A.
maxhifi
Addicted Member
I just thought of another avenue - In the 50s, 10W 6V6 amps were pretty common, and lots of those are running in class A. RCA doesn't even give Class AB ratings for the 6V6GT in the datasheet. I think in those amplifiers, Class A was common because it's more tolerant of badly regulated power supplies than AB is.
This seems to be the cheapest way I can think of, to try Class A Push-Pull.
This seems to be the cheapest way I can think of, to try Class A Push-Pull.
Examples?
Down under here, way down in one of the southern states, is Earle Weston, he hand crafts valve amps and winds his own transformers, most of his amps are Class A, and a fair few are push pull. Basically the quoted specs are accurate for every one
Not really viable for someone on the other side of the world, but seeing as the OP asked for examples.
http://www.westonacoustics.com/default.htm
No affiliation, my amp is built on a sheet of MDF with front and back made from old bed head slats, and the cage is a bit like a chicken coop
But it doesn't stop me drooling over the Weston amps 
Down under here, way down in one of the southern states, is Earle Weston, he hand crafts valve amps and winds his own transformers, most of his amps are Class A, and a fair few are push pull. Basically the quoted specs are accurate for every one
Not really viable for someone on the other side of the world, but seeing as the OP asked for examples.
http://www.westonacoustics.com/default.htm
No affiliation, my amp is built on a sheet of MDF with front and back made from old bed head slats, and the cage is a bit like a chicken coop
But it doesn't stop me drooling over the Weston amps
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