Capacitors replacement for tube stage

Dear community,

I am reviewing an old modification (very poorly executed…) that was done on the two tubes modules representing the final stage of my hybrid AMC CVT3030 amp.

I don’t have those modules anymore and I am thinking of applying the same mod to the present ones since it’s only about replacing four capacitors and one resistor.

I highlighted in the schematics the components that were changed and I am adding also some pictures.

Could you please take a look, possibly indicate if the changes make sense to you and which capacitors I should buy?

For instance, I could not find online the green Russian caps but I’m sure there are good alternatives.

C507: 10k100 white bulky cap, no idea what that is
C502: 3u3 400vdc +/- 3% Mundorf MCAP
R520: 12k
C505 / C506: K71-7 0,1 mk +/- 2% 250B8604 green Russian caps

Thanks a lot in advance

Fabrizio

russian cap

mundorf cap

bulky unknown cap

The green ones are russian polystyrene caps. They were hyped online a few years ago. I bought lot's of them and put them in my console. Later took them out again.  You can use WIMA FKP or polystyrene or lot's of others readily availible.

IMHO there is little or no benefit in replacing the highlighted components with exotic types.

Cheers

Ian

C507: 10k100 white bulky cap, no idea what that is
C502: 3u3 400vdc +/- 3% Mundorf MCAP

In most of the amps I've worked on .. The Neg. Feedback cap (C507) is normally a film type cap but is most amps the value to this part is .1uf or less. At 10uf, Electrolytic will be the easiest to get choice. There are some film types that reach these capacitance but they are physically very large. I don't think the type here will greatly affect the amps sound here.

C502 Im not sure about but the M-Cap is a nice cap. I'd probably stick with some sort of film type here.

15K 5w resistor I'd probably replace with a wirewound type. I dont think the type here is going to make a whole lot of difference.

C505 and 506 on the other hand will have a noticeable change when you change them out.  They should be replaced with a good film type cap (Polyester , Polypropylene , PIO, something of these types) The most commonly used are Orange drops and Mallory 150s. I've used SonicCaps in the phase inverter coupling with great results but know that Polystyrene caps cant take a lot of heat.

Thanks everyone for the feedback!

I agree with Ian. These caps are so far back in the circuit that there is no sonic  goodness to be expected by going exotic. In fact, I would reverse those mods to the original design. Get caps which suit the original size intended on the PCB.