Express hiss (fixed)

[AcornHouse]

Due to the search function being out of whack for the time being (results lead to empty posts, if you haven't tried recently), I need to ask a perennial question again.

Finished my first Express build, and have worked out some mechanical noises, now it's sounding good, but it has a persistent hiss. What are the likely suspects to start looking at?

[M Fowler]

It will always have that hiss because of it's high gain characteristics.

You can try lower gain preamp tubes such as 5751 or 12AY7 in V1 but that changes it to lower gain and a little more smooth but might not be what you want.

Can also use 12AT7 phase inverter tube as well.

I find different brands of preamp tubes make a difference but this requires you to have a selection to choose from and have a tube roll party.

Mark

[AcornHouse]

Ok, that gives me a place to poke at. I wondered if it might be the preamps; I originally had an RCA in V1 while I was working out mechanical noise issues, but switched to an Amperex (dimpled disc getter) I got in. I didn't remember it quite as hissy with the RCA, but it had been awhile.

Thanks.

[M Fowler]

Yeah I tried many different combinations in the Express but eventually just went back to my standard JJ ECC83 tubes.

I think I used some Northern preamp tubes in my PCB style build but I don't think the added cost of those tubes was worth it.

NOS tube suggested by Ken F. is worth trying though.

[geetarpicker]

An Express will have some hiss. Still, what amps are you comparing to? In my experience if you compare an Express to other amps with similar gain (like a non master Marshall JMP 50 or SL 100) the Express should typically be less noisy. For example set the Express to all knobs on 1/2, and fully dime the non master Marshall. At that point the gain should be similar but I'd expect the Express to be quieter noise wise. One trick for bedroom or around the house playing (with an Express) is to use a power Attenuator. Then when you play the amp at lets say 80db peak levels the noise will almost be gone. Think of it this way, an Express has a bunch of gain and a typical playing levels may push well passed 110db. Are you used to such levels and the noise one would typically have with a cranked single channel amp?

Other than that be sure to try many different preamp tubes to find the lowest noise ones you can find. I don't typically switch out the preamp tubes for totally different gain level tubes like 12AU7, etc., but it can be done. Another thing is IF your amp pots come on too quickly you may be cranking the gain a lot higher than the knob positions indicate. I've noticed that 10% taper pots are just about mandatory in an Express or Liverpool. Not 10% tolerance, 10% taper. Search through my prior ampgarage posts to find more details.

GK

[AcornHouse]

An amp tech friend had a sleeve of 5751s that he let me borrow. A Sylvania Gold Brand was first and the hiss was non-existent! I didn't notice any ill effects from the theoretical gain drop, was still plenty loud and the basic sound was the same. The RCA and GE 5751s didn't test as well, both a little hissy, though not as much as my Amperex 12AX7 (I suspect it's this particular tube, rather than the brand in general.)

[Tone Lover]

Differant tubes even from the same brands have a varying levels of noise . Certain tube testers can test for noise I found a good seller with that particular tester. It was either a maxi matcher or amplitrex 1000. Now i only buy from those type of people . A super quiet tube in V1 makes all the differance. Even though there more expensive there worth it.
Glad to see your friend had one that worked. A 5751 and 12ay7 always seem to be alot more quiet for me. I know some of it is because of lower gain
Bill

[Phil_S]

You might want to try increasing the wattage rating of the plate load resistors. If half-watt, try 1W. Above 2W I think you are wasting your time. The reason I suggest this is that many 1/2W resistors are near or above the maximum voltage rating in guitar amplifiers. The Express, being what it is, seems to like to pick up the hiss and amplify it.

This is a cheap fix. If it doesn't work, there's no reason to reverse it and you won't be any the worse for it.