I designed this:-
Bits came yesterday. This is just the trial fit for the parts.
It says lo-cost in the thread title, but that's comparative. The parts cost is at least twice what you can build a very good solid-state amplifier for, $220 - $250 including chassis.
I haven't finalised the design, although I've got a pretty good idea where I'm going with it. When I do I'll have a more accurate costing.
Here's an early simulation, although I've changed a few things since then...
That's the nice thing about simulations, you get the opportunity to do revisions and tweak the performance. I've since decided that maybe I'll do without the global negative feedback, I can get a milliwatt into 16 ohms (110dB SPL in my Klipsch Image S3's) with about 0.2% THD (almost all benign 2nd. harmonic) with no GNFB. The amplifier is intended to offer about 300mW and drive > 166mA or +/- 9V, although with increased distortion. Exactly what can be done in terms of power and distortion is yet to be determined. I don't think anybody will be arguing that RMAA results are insufficiently accurate though.
Nominal output impedance is switchable, 4-32 ohms.
Obviously it's single-ended, class A
As drawn the tubes are Russian 6N6P, $7 the pair. These are said to sound good when you get them over 8mA quiescent, both stages run 10mA. 6SN7's, which are widely held to have low distortion, could be substituted.
The output transformers are Hammond general-purpose 125SEB, rated 5W and 45mA DC bias, 100Hz - 15kHz, 1dB. They're considerably underrun at the power and DC bias stated, and the frequency response should be considerably better than nominal, they should easily achieve 20kHz and will probably extend down to 20Hz with 1dB flatness.
The PSU uses a silicon bridge rectifier to achieve ~280V with a capacitor-input CLC filter, I originally drew in active regulation, but the choke makes for a much simpler implementation. Tube rectification is an option at the cost of some HT voltage, but for the moment I've got a relay drawn in to delay the appearance of HT.
Unfortunately I incorrectly dimensioned the choke in my haste so a more suitable one won't get here until about Wednesday, along with some other components which I'm still missing.