probably sacriledge

[phsyconoodler]

So why not add a pot where V2's 150k fixed resistor is?That makes the Express way more versatile and less noisy.
I'm going to try it and see if it floats my boat.

[solderstain]

It's been done - not sacrilege at all. I tried it on my 'mule' build before I built my 'proper' Express. Mounted the pot on the front of the chassis.

Didn't do much at all for the noise (mine wasn't noisy to begin with... choose your tubes carefully...), but it did allow me to mess with the amount of signal passed to the third gain stage. Fun to play with the balance between how much signal you send to the second gain stage with the normal volume control, and and how much you then send on to the third stage. It was fun to try, but after a lot of messing with it, I found I just gravitated to the original 150k value, so I left that feature out of my 'proper' build.

Worth messin' with, though!

[Allynmey]

I did that for a guy who wanted to dial in an instrument. I put a brass screw type mil spec (bar type) on the preamp board (100K) in series with a 47K. I wouldn't use a pot on the front of the amp though.

[ForcedFire]

I think it's a great idea. Maybe 10k resistor in series with 250k pot?

[phsyconoodler]

I tried a pot but didn't like it.
There must be a way to reduce the noise in these amps other than the preamp tube.they are pretty gainy and there may not be a way,but anything would help.

[dartanion]

There are many other causes of noise like fluorescent lights, dirty power, EMI, etc. IIRC, you said you used a smallish chassis. This can also contribute to noise as layout is so critical with these amps. Post some gut shots so we can help you figure this out.

BTW- Are you still hanging out at 18watt?

[ForcedFire]

For what it's worth I just redid almost all of the preamp runs to the pots with foil shielded wire yesterday. The before and after difference was amazing! The hiss is very minimal now.

My build is very cramped, I did my own layout and a smaller chassis with laydown PT. I had some oscillation but now it's gone with no grid stopper on v1b which I had needed before going to shielded.

The wires I shielded were:

-input to v1a
-volume to v1b
-treble to volume
-treble to bass
-mid to tone stack cap
-bass to tone stack cap
-PI to board, ground shield and PI seperate ground from preamp

[Zippy]

Any comments on the effect of the shielding on the highs? Is it still inherently bright?

[jaysg]

That resistor is part of a high-pass RC pair. A dual pole rotary switch with 6 RC pairs would make more sense.

[ForcedFire]

Any comments on the effect of the shielding on the highs? Is it still inherently bright?

It's very bright, has harmonic feedback all that's still there. I even have a 100k grid stopper on v2 leftover from tracking the oscillations. I'll take it out later. I also added to the input capacitance of v1a. I have no grid stopper to keep the hiss down but used 2 560pF 50V caps between the grid and cathode of v1a. I might clip one out but it's still super bright.