[Diabolical Artificer]

Cute little devils, I've some QQV03-10 and other double beam tets lying around I've been meaning to to play with for a while. First off you'll need more than 300v to get 10w with g2 sitting at 175v, I really can't see you getting 16w+, that's based on gut feeling and a bit of experience.

i found an OPT with a Z pri of 10k worked best for for the QQV etc, I'd wire them up, fixed bias, use any old OPT and load it till you get the best results. Use grid stoppers, start off with 2k2 g1, 220r g2. G2 supplied from resistive divider, don't forget to bypass the lower R.

If you get oscillations bung a 4n7 cap and 1k ( 5w) from both anodes to ground. Also put three reverse biased 1n4007's from each anode to ground reversed biased in case of F up's.

You could try a 6AU6 triode strapped as the front end, they're cheap, with a cathodyne PS, something like an ECC82. Design things so you have extra gain to through away on NFB.

Some last thoughts, try and find some more TT15's as spares, Andy.