WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Purely as a rough guess, I'd think you'd be able to get something on the order of 7 watts out of a pair of 6BQ5s at 175V. Perhaps even a little more. If you're willing to give up a watt or two, you could bias the tubes slightly hotter and go a little more towards the class A side of things.
I do not think that you can generalize that much with expected power -- running EL84s at that low a voltage as 175V B+ means you will get very very low power output, PP or otherwise!\\
Furthermore, running the tubes "hotter" does not give you any additional power! Actually, tubes will give you more power in one of the following cases>
a) higher B+ voltage
b) lower Ra (in SE) or Raa (in PP)
Running the tubes hotter (probably meaning "more current") only lowers the distortion, while reducing a little the bias and lowering the output power as well!
As I can read that some tube rectifiers are mentioned, I am inclined to understand that the main reason to try running a PP or whatever amp with a lower than usual B+ is lower voltage available when using tube rectifiers as compared to ss diodes. Furthermore, probably the power transformer at hand has relatively low HT on the secondary?
Well, if one or both of these reasons are involved, try using the secondary of the amp in a different way: i.e. do not connect the CT to ground, and use the rectifier in a hybrid graetz -- that way you will get way higher voltage output.
Post further questions, if you do not know how to do it...
Thanks this is exactly the situation howevr the application is for Headphones not loudspeakers so the lower power is not a problem however low distortion. can you post a schematic idea of the type of Power trans /Rect configuation you had in mind TIA
Attached with this post you will find a graphical explanation of the difference in connecting a tube rectifier (double diode) in a classical manner, and in the hybrid graetz (or bridge) manner.
If the amp is for headphones, you do not need too much power (unless it's some 32 ohm Grado or similia). But, while you can always build it as OTL, I still think the transformer output option is the preferable solution, since there is no large cap in the signal path (electrolytic, that could leak...).
Still, a PP of EL84s seems a trifle too much... maybe SE EL84? Take a look at my site -- although the amps are for loudspeakers, they can drive headphones as well!