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45 Type Push/Pull

How many here use or have heard one of these type amps? What are the important factors here in this being able to produce quality sound of this tube with out the noise???
Can this amp achieve the sound quality of an SET amp?
jsman
213cobra
885 posts
 
>>This is something that should not happen with a DHT p-p design.<<

Yup. Assuming push-pull class A. But what about Class AB p-p designs using triode power tubes, or for that matter p-p voltage amplification sections using small signal triodes. A push pull design *can* forego notch grunge but not every designer elects to.

DeHavilland made a 75w triode push-pull amp built around a pair of 572 tubes. It was confusingly described as "100% pure Class A A/B." We see today an increasing number of push-pull triode amps with power ratings well above what could be expected from Class A push-pull and one of the effects is that they sound like Class AB amps and sometimes like Class AB amps that use tetrodes or pentodes. What's good for guitar amps (biasing AB toward B for sparkle) doesn't always benefit hifi.

Phil
atmasphere's avatar
atmasphere
12,100 posts
 
Phil, even if you run class AB1, you still don't run the risk of any crossover artifact, since at lower power levels where the crossover artifact might occur, the tubes are in the A region.

What I have found is this is more a problem relating to solid state than tubes, on account of how the output transistors are biased. In tubes, in only seems to show up if there is a malfunction or design defect.

IMO this is a very common myth! However, that is not to say that an SET does not have certain desirable qualities. Chief amoungst them is the ability to have distortion linearly drop to zero as power is decreased. This is where SETs get their 'inner detail' they are known so well for.

Most phase inverters in P-P designs introduce their own kind of harmonic distortion (having nothing to do though with crossover distortions). It is this additional harmonic distortion that causes P-P amps to often have increasing distortion below a certain low power level. It can be designed out by avoiding the use of stand-alone phase splitter circuits- by instead integrating the phase splitter function right into the voltage amplifier stage. This eliminates the need for a separate circuit and the less signal process the better (IMO). Traditionally the best way to do this is with a differential amplifier for the voltage amp.
213cobra
885 posts
 
>>...even if you run class AB1, you still don't run the risk of any crossover artifact, since at lower power levels where the crossover artifact might occur, the tubes are in the A region.<<

Agreed, but you seem to be presuming that notch artifacts aren't audible at higher power levels. I disagree. I once had a conversation with Julius Futterman about this back in the '70s. Yes, it's a long-running topic! There was an EE with me who also designed and built his own amps. We were listening to Futterman OTL vs. an old rehabbed Altec SET vs a Marantz 8B. Julius was listening to the two of us arguing about the audibility of notch distortion in push-pull vs SET. The EE was calling nonsense. Julius was smiling and interrupted. "Of course you can hear it, especially at power above the Class A bias point. It's just that most people either aren't sensitive to it, don't care, or they hear it as more texture." He finished saying something like "I'm an old man and I can hear it, but who cares? People want good bass and power so that's what we give them!"

He liked that Altec, btw.

>>What I have found is this is more a problem relating to solid state than tubes, on account of how the output transistors are biased. In tubes, in only seems to show up if there is a malfunction or design defect. <<

I agree with you on the transistor observation. Where I agree less is the implication that notch is not apparent in tubes used P-P. P-P pentode and tetrode make it more obvious to me than P-P triode, generally. And among tetrode amps it is least objectionable to me in the very simple Quad II.

You're the professional amp designer, Ralph, so you don't need me to confirm that you're absolutely correct about your phase inverter observations, but nevertheless I agree unequivocally.

Phil
atmasphere's avatar
atmasphere
12,100 posts
 
Thanks Phil for your reasoned response. I am basing my comments on the simple observation of the amps themselves on the bench. I've seen crossover and notch distortion in transistor amps (which in some cases it was possible to correct by adjusting the bias) but in a good number of tube amps its just not something that shows up on the 'scope like it does with transistors.

I *have* seen it at higher power levels in amps that were malfunctioning. But never in a functional amp- an example is the HK Citation 2, which I have always held in high esteem. You can run it to full output and no sign of crossover/notch artifacts.

I have to admit to having far less experience with class AB2 amplifiers. They seem to be uncommon in home audio. However in AB2 we have the problem of the driver circuit which has to be able to maintain linearity when the tubes are in the grid current region. If this circuit is not up to the task you will indeed see artifacts- but in my comments earlier I was careful to use the caveats of proper design and proper functionality.
213cobra
885 posts
 
It's also possible that something about the wave hand-off in push-pull is audible, that may not be measurable notch distortion and being something else doesn't show up on a scope. I've discussed that possibility with some amp designers and EEs as well, but for lack of another way to reference it the phenomenon inherits the notch handle.

Phil
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