So when you're taking impedance I assume you are referring to the primary OPT impedance required for the tube.
Correct. Here's why, immensely simplified.
The output transformer in a tube amplifier is a step-down device which converts voltage into current. When the output device is a high-voltage tube (like 2,000 VDC instead of 600 VDC) the turns ration is significantly higher to scale the voltage.
The low-frequency (bass) frequency response (F1) is controlled by the primary's impedance (a function of wire) and inductance. The higher the primary impedance the greater the number of turns, so the higher the required inductance. L = Z1 / (2Pi x F1).
The high-frequency response (F2) is controlled by the primary impedance (again wire) (ZT) and parasitic inductance (LP), as this forms a filter. F2 = ZT / (2Pi x LP) But ZT is more than just the sum of the impedance of the primary (Z1) and secondary (Z2), because the secondary impedance (again, a function of wire) must scaled by the square of the turns ratio: ZT = Z1 + (N1 / N2)^2 x Z2)
Increasing primary inductance requires a high number of turns, but that would increase the impedance so thicker wire must be used, and the core material must also change to a more expensive type, and the laminations must be altered. Reducing the parasitic inductance in a high-turns transformer, furthermore, requires complex interleaving techniques. The parasitic capacitance is also of concern.
It may not be possible to balance the two goals, even with increased copper thickness, a different core material, a different lamination type, and different winding/layering techniques. Not to neglect linearity across the range, and at the high end and low end.
The transformer gods giveth and the transformer gods taketh away.
It sounds like this project is more of a pain in the butt than it's worth. I just look at those giant tubes and think WOW I'd bet they'd make a great amp! ... I know I could cobble something together on my own using conventional parts and these old 803 tubes, but as you said it probably wouldn't sound very clean.
You, me, and a host of others. We all stand in awe of the cathode, grid, and plate structures, drooling and saying, ooooooh, shiny!
Then reality sets in and we realize why such tubes only appear in boutique amplifiers costing nearly the price of a nice car.
The problem, in short, is that anything you build using a stock output transformer will not have the correct output impedance, won't have the load line, and certainly won't deliver the power you stated as a goal. Beyond that, the distortion will be very high. Not to neglect the issue of building a regulated supply running at 2,000 VDC.
I take it that your opinion is abandon this idea and go with something more conventional?
Pretty much. Sorry to be the one to dash cold water all over the transmitting-tube amplifier project.
If it's any consolation before I ruined your project I ruined my own.
It just seems like anything to do with tubes is so damned expensive these days. You can't even find a decent rig to rebuild for under $700.
The big brand names are expensive, but some of the console pulls will run you a lot less and you can have great results with those. Magnavox made some nice iron, as did Stromberg-Carlson. Neither has the brand-name appeal of McIntosh, Marantz, Fisher, Dynaco, EICO, etc.
I even looked the possibility of buying a kit from Korea, but these too are insanely expensive and I there are no US distributors (probably due to liability involved).
Yeaaaah..... While I do not have direct experience with the Korean kits, I suggest reading the complaints and reviews, which suggest the quality is reportedly quite poor. I would not purchase one of those kits as a result. You can make your own with better quality components.
Any suggestions? Back in the day, I would have gone to Lynn, MA to a place called John Meshna surplus and supply. I could get bags of parts, tubes, and chassis for almost no money. Those days are sure gone.
Dude!!! Back in the day I used to buy from John Mesha, Jr. by mail order! Do you remember when he was selling the rotating magnetic drum storage units, surplus from aircraft? I still have a few of those catalogs! I bet you also remember PolyPacks!
As far as a suggestion, try looking at the Pete Millet amplifier projects, as well as the other projects at diyAudio. Lots of easily assembled tube amplifier kits are available, with lots of high-quality tech support at an unbeatable price! (Free!!!)
See:
http://www.pmillett.com/
Plus diyAudio.com where the boards and detailed commentary and reviews are available.