45 Type Push/Pull

Generally, if you seek SET sound, it will be better to listen to a 300B SET amp than a 45 push-pull, though if both are executed equally well, the push-pull amp will yield more disciplined bass. Getting the deep bass equally controlled from 300B SET will usually require a higher level of execution, particular in the output transformers but also in power supply, at higher cost.

It may take some careful listening to discern it, but if you have similar power and execution push-pull vs. SET for time-adjacent comparison, you may discern the presence of subtle push-pull crossover notch grunge. If you don't notice it, no problem. If you do, once you hear the absence of it in SET it's hard to live with crossover notch haze and grunge in p-p topology ever again. It's least noticeable however with triodes.

Usually push-pull at same power and execution level as SET will give you better bass definition and more slam. Top end attack may be sharper too, but this will very much depend on SET voicing as well as on the choice of power tube brand, materials and plate type. In a 300B SET amp, there can be a substantial difference in voicing and dynamic character between a mesh plate Chinese tube and, for instance, a KR solid plate. There's a lot of variety in 300B tubes now.

The holistic tone and unity presentation of triodes in single-ended topology has an organic and harmonic completeness that is elusive if not absent in push-pull, despite the push-pull implementation almost certainly measuring better. Execution counts for a lot. A superior push-pull triode amp can easily beat a mediocre SET implementation, On all the qualities you'd assign to SET I the first place. Put another way, either topology has to be done right. Still, the opening postulate above applies.

Phil

>>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

>>...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

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