What do these Friedman mods in the Smallbox do?

[VintageCharlie]

Hi everyone,

i do like how the Friedman Smallbox plexi (clean) channel sounds a lot. It sounds like a lead spec Marshall but without some of the aspects of rawness/scratchiness that i do not like personally.
Looking at the schematic it looks like for the most part it is a lead spec Marshall, similar to e.g. a 1968 lead, but with lower filtering (32uF for the most part, 16uF in the preamp), which is also something i like - makes it squishier and saggier.
These are the main changes that i find:
- fizz cap increased from 47pF to 100pF (filters out more high end i guess)
- both sides of the PI coupling caps are .022uF (less bass, more even PI response?)
- bypass cap on V2A is 22uF instead of .68uF (more gain in all frequencys, not just mids/high mids?)

And then there are these 2 500pF caps that i have not seen on any Marshall schematic (please see attachment). I have no clue what they do?
I would guess they might help with some filtering? (I am guessing this because the amp is noticeably less shrill than some Marshalls tend to be).

Thanks for any input!

[sluckey]

They are both anti-shrill caps.

[pdf64]

…
- both sides of the PI coupling caps are .022uF (less bass, more even PI response?)
…

When heavily overdriven, having the same time constants on both grids may help the LTP to recover more evenly, and, in the case of the common grid side, 5 times faster.

[ampmike]

I bought a used PT and that little head I’d really sounds good.I would love to hear the combo. Thanks