45 Type Push/Pull

>>...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

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.

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

Phil, if you can't see it on the scope, that does not always mean it does not exist. But it does mean that its pretty slight- if you have a really high bandwidth scope (200MHz and higher) that stuff usually shows up pretty well if its there.

However P-P is prone to harmonic distortion that SETs are not. A lot depends on the topology of the amp. For example if the amp is P-P at the output but combines single-ended circuits in the front end, the result will be that is has a 2nd harmonic that predominates, with a 3rd and a 5th also showing up. This is because the Single-ended distortion is compounded by the later stages and even though P-P can cancel even orders in the load, it can only do that with distortion that is also generated by the output section itself, not the driver or voltage amps.

If on the other hand if the circuit is fully balanced the only harmonic that shows up is the 3rd. This is because even orders are cancelled throughout the circuit. A lot also depends on the inherent linearity of any of the circuits; if things are bit off other harmonics and IM (inharmonic) distortions can show up pretty fast.

Its my opinion that P-P has gotten a bad rap in this regard, mostly due to some sloppy design and execution. To me this is very much a baby and the bath sort of thing.

Hi, I build pp45 amps, I’m not here to sell them, but have a few comments that I hope will be helpful.

first I would say that the power difference between 2a3 and 45 designs is a paper thing mostly, they tend to sound close-to equally powerful (or weedy depending on your speakers and tastes). They do sound subtly different but it’s more about the execution than the tubes per se.

next is the magnetics. I use the best vintage Japanese magnetics i can find, along with USA UTC parts when I can find them.  Recently I made a pair and the only difference was utc vs hashimoto driver/interstage transformers, and they do not sound the same.  I believe that the loading of the interstage secondary is a critical design point often glossed-over in schematics.  The loading network needs a lot of design and measurement to get right.  
 

i use exclusively Sowter 1475 phase splitter transformers and fully balanced from there on through the driver and power stages.  However, sorry to have to admit this to any zealous tube-only folk among y’all, I use a transistor discrete opamp (did he just say opamp?) to drive the single ended Sowter primary (sonic imagery or sound skulptor).  There is no global feedback.

of course I would say this, but these designs based on 1930’s schematics, 1940’s valves and 1960’s magnetics sound pretty damn amazing and have great clarity and sound-stage.  

there is something about pp that is overlooked: the output and interstage  transformers fall in and out of magnetisation at every zero-crossing of the signal (at every polarity change).  That is where the transformer is at it’s least linear transfer function.  Se amplifiers are different: the opt never falls out of magnetisation, never polarity reverses and always operates in its most linear region.  I am convinced this is a major reason they sound different, and some would say better, than pp types using the same tubes and supporting electronics, harmonic odd/even signature aside.

last comment about bass: this is tricky to comment on. Valve amps have feeble damping factors compared to SS, but is that a good or bad thing? It’s hard to give a definitive answer, and it’s more about the speakers and the room as to whether the ‘sloppy’ bass of valves sounds better than the ‘tight‘ Bass of SS, it can go either way, so keep minds open on that… my opinion is the low damping from tube amps works pretty well with closed box speakers, but that high damping factor is necessary to control reflex port behaviour.  Not sure about dipoles though.

 

 

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