Hello to all,

I'm fairly new to this forum and untill now I have read a lot of posts to get a bit acquainted.

For normal bias phones, I use a SRM-1/Mk2 which I would like to upgrade (actualy a P.P., but I fitted 1 normal bias socket). I have read about swapping out the power transistors for 2SC5466 units, currently used in Stax amps. However, in a SRM-323 thread I read that 2SC3675 transistors are to be preferred over the 2sc5466 units. Why is that and which transistors are best for the SRM-1/Mk2 (p.p.)?

For pro bias phones, I use a SRM-727II on which I want to perform the "spitzer mod". What resistors are to be used? metal film, carbon film, others? Are 1/2 Watt resistors "big" enough?

Does it make sense to swap the 2SC5466's in this amp with 2SC3675's (see above)?

Are the modfied 727 units still going strong? No side effects till now?

I don't really see any need to swap out the 2SC5466's. Their only "problems" are the low operating voltage and that they are pretty hard to find. I used them in my SRM-1 Mk2 rebuild just because I had them at hand and none of the what Stax did use back in the day.

You should use metal films for the mod, 1/4W and rated at 350V is more than enough. I used Vishay RN60's because that's what I had at hand and they are hard to top.

Just finished modding the 727. Quite a difference. The bass hump is gone indeed, much more control.

BTW: I placed 330 Ohm resistors parallel to the 560 ones instead of removing them. I read that earlier in a post of Kevin Gilmore and I liked the idea of destroying less of the original.

If I can find the time, I will put myself to the SRM-1/Mk2 (balanced inputs, new pot, low ESR caps, new transistors (2SC3675).

I also have a SRM-T1 which I want to turn into balanced, etc...

It looks like it has a a zener in the input stage? Have to take a close look at the input section though.

There should be two zeners in the input stage per phase.