I am using 3/4" stand offs if that is what you are asking. 1/2" would be fine as well. Yes the 1/8" G10/FR4 board is very rigid. With the horizontal spacing between the stand offs at around 12" and the max overhang of about 2 1/4" on the power end of the board there is zero flex in the board on either end or in the middle. Being the overkill kind of guy I am, I would use (6) supports (even though (4) is enough) but it is really difficult to find places to put them with all of the transformers, cap cans etc, etc on the back side of the chassis.
Oh and by the way, for those of you watching this build, I was working on the final wiring yesterday and found a missing component on the board. The 0.1uf 250V coupling cap from the preamp to the phase inverter pin 7 had not been installed. Now I went over the board multiple times and didn't see that and this just goes to show that to err is human. You can see the added pink-ish 630 volt version I put on the board in the new attached picture which isn't present in the photo in my earlier post.
I've built so many of these amps now that one would think I wouldn't miss such a thing, but I did. So Doug's number one rule of amp building "if everything were correct it would be working." always applies. This is why taking your time and double/triple checking every component value and connection against a good layout drawing as you go along is essential to a trouble free start up. Now that I am twisting wires I am even ohming each lead from one end to the other to make sure the proper control wire lands on the proper board component. I'll be finishing the heater wires this morning and before start up I will once again check every component value and every connection against the layout to make sure all is correct. Then a voltage check before tubes go in. Check, check, check, check and check.
And one more thing, yes there are two wasted 100 ohm heater artificial center tap resistors on the board. This PT had a heater CT.
