Thanks for the comments.
Here are a couple of scope screenshots with 600Hz signal at maximum power, kinda ugly, with 3rd harmonic being so high, but at lower levels the second harmonic takes over, and it sounds great to me. I plan to experiment with some negative feedback at some point. If this is ever "finished" I'll have to get some definitive measurements with a soundcard, as you suggest, or invest in a distortion analyzer.
I've tried with the two transformers as shown in the schematic; (Antek and Edcor) both are 6.6K secondary. They both seem to give very similar results.
Attachments
Take a note that the final results is also done from tube selection that can be made on circuit with a minimum of test set up.
I think the amp dseems to be working well; about feedback you have to refer one enf of the output of OT trafo to ground, then with a trimmer, p.e. 20kohm (configured at rehostat) , take a signal from the other hand and goes to the cathode of first tube. If you heard a sound like "motor boating" you must invert the connection on output ( phase).
You have to change the resistor on catohe of 6BH6, p.e. put 390 ohm and cap in parallel and a resistor of 47 ohm to ground, then the FB signal goes betwee nthe two resistor
Read the output without FB and then with FB so you can understand the weight; I think that 15-20 dB is reasonable
Until you have a symmetric clipping ( close to the max power) you are in the right way
It is normal that the 3° harmonics come up specially at high level.