Funky input wiring

[hylaphone]

Hi folks -- could this input scheme be changed to a conventional 68K style without effecting the tone switching?
It is rather noisy as-is!
Old Guild amp, btw:
guild-thunderbass-4-tube-preamp-diagram.pdf

[loudthud]

The input arrangement looks to be an integral part of the tone switch which uses feedback around the input stage. You might try metal film resistors for the 330Ks and any other resistors in the feedback network.

[nickb]

On a quick look I'd say that the input stage has a very high input impedance and the tone switch won't affect it much. Therefore changing 330k input resistors to 68K won't affect the tone switch. The lower resistors will reduce the input noise a little. They will also affect the 3.5Khz HF cutoff introduced by the 300pF cap. You might wan't to change it to 680pf or so to keep things the same.

It seems like this would be easy to just try out and see.

[nickb]

A quick simulation of the two schemes for all three switch positions. The feedback does affect the input so comparing 68k with 330K you get x1.8 the gain in switch positions 1 and 2 which will reduce the noise, and x1.3 in position 3. There is a slight effect on the frequency response but IMHO, I don't think it's much. Try it out out and let us know. BTW, the 300pF was changed to 1.5nF for the 68K case.

As LT said, metal or carbon film resistors have lower noise than carbon composition.

[hylaphone]

Nickb, thanks very much for the sim --
I'll give this a go!