I have written extensively on this subject--- the import of primary impedance and it's effects on loadlines and, hence, the behaviour of
the amplifier. If you do a search across the different audioasylum forums with my name and inductance--- you'll have plenty of material to read and absorb.
The article written by Voltsec (published on our magnequest.com site) is quite comprehensive--- and uses the 845 tube and circuit application as a the working example. What is shown is that the tube generated distortion itself goes down dramatically as the primary inductance increases. This is because the tube is seeing and working into a larger EFFECTIVE load impedance.
In essence what it required is that to have a stable (relatively non-reactive load for your tube to work into) you must have enough primary inductance to adequately support your nominal reflected primary impedance.
Years ago (and perhaps yet to some extent today) folks would take the nominal or advertised pri impedance of a transformer as it's effective impedance. Which can lead to a lot of mischief--- and misleading specsmanship.
How much inductance--- there is no hard and fast rule--- unless we were living in the Platonic world of forms--- in which case, of course, we would have transformers with infinite inductance and no resistances of any kind--- and no core losses of any kind----
but, here on earth--- we must juggle how much L we can aim at or achieve against a host of other design factors and etc. As a very rough rule of thumb--- the most excellent designs extant (that I know of) are ones that can produce approx 100 henries of L per thousand ohms of reflected pri impedance while keeping the leakage L at or below 1 millihenry per thousand ohms of the nominal reflected pri impedance.
The above level of performance is near rave.... the Peerless 20-20 Plus series will generally meet this spec... and there are perhaps a few other tranneys made worldwide to this high level of specification. But these units are tough to design and quite intricate to wind. They will have a ton of interleaving--- the insulation materials must be carefully chosen--- the lead positions, starts, and stops of the windings must be laid out near perfectly--- it is really the state of the art that goes into these guys.
Is this level absolutely required to enjoy music? Nope. It's an IDEAL. Quite, quite nice if you can acheive it design wise--- and even better if the customer can afford to buy and use it.
And, again--- my "rule of thumb" simplifies the shit out of the design process---- and how you must really optimize for a very wide range of factors in design--- stuff in addition to the amount of L that the primary has--- but the primary L is none-the-less one of the most critical design parameters.
hope this helped---
MSL
Builder of MagneQuest™ & Peerless™ transformers since 1989
