45 Type Push/Pull

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).

This is only correct if the amplifier is biased class B.

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.

If the amplifier employs enough feedback it can behave as a voltage source and no worries with the bass. But I expect the amp you're describing here is zero feedback, in which case it will behave as a power source. That means the speaker has to be designed to expect the amp to have a higher output impedance associated with this behavior. There are a good number of speakers for which this is so- any speaker that works with an SET for example. For more on this see

http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html

 

subtle push-pull crossover notch grunge

This is something that should not happen with a DHT p-p design. Crossover or notch distortion are artifacts of transistors and designs that are not class A, unless there is a serious design defect.

I have a pair of type-45 amps that I have been playing at home for several years. They started as 45 SETs, but on examination of the internal circuit it was obvious they were candidates for improvement in the wiring, parts and input circuitry.

With each update the amps sounded better- more detailed, smoother sound, etc. Ultimately I pushed them to a P-P design and the improvement over the SET version was dramatic! I think they can be improved further, but its been convincing enough that I just play them for the most part.

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.

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.

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.