Need help deciding on the new Bassman build

[Colossal]

I have another question... I am putting together the list of all the parts for this amp and I saw this strange capacitor between the plates (anodes) of the PI tube. I circled it on the photo. It says 47 something, and on another schematic it said .00001. What is it for? Do I really need it (cause other schematics do not have it at all)? If I do need it, what size cap should I use?
Thank you!

That small fixed capacitor in between the plates in a Bassman and many Marshall circuits helps to prevent oscillation and trim a little bit of high end hash. Also look at the Cut Control circuit in a Vox AC30, which is for cutting high end, is located just after the grid leak resistors.

[martin manning]

I saw this strange capacitor between the plates (anodes) of the PI tube. I circled it on the photo. It says 47 something, and on another schematic it said .00001. What is it for? Do I really need it (cause other schematics do not have it at all)? If I do need it, what size cap should I use?

The PI anode-anode cap, sometimes called a "Fizz" cap, is for squashing parasitic oscillation. On the 5F6-A schematic it says 47 mmF, for micro-micro Farads, which is an old fashioned way of writing 47 pF. On the other one, the 0.00001 is assumed to be in uF, which would be 10 pF.

[Helmholtz]

The cap between the LTPI plates is used for HF phase compensation within the global NFB loop.
As said by others it's meant to prevent HF oscillation.
There are a number of things which influence HF phase but the main contributor to unwanted phase rotation is the OT.
So a different OT might require a different value compensation cap.

[chikov]

I am trying to find the time to start drilling the chassies, but in the meantime I found an excellent resource of info on 5F6A amps. https://robrobinette.com/5F6A_Modifications.htm
Do you think their schematic for PPIMV is a working one?
here is the shematic:

[stratoblaster80]

Any of the 6G6 varieties gets my vote! Also Lyle from Psionic Audio on YouTube has modded a few w/ the AB165 preamp and the AA864 power section - could be interesting.