Hiss in Express Build

[MarshallPlexi]

I have built an Express, and while the amp sounds wonderful, there is a TON of hiss when I dime the controls. Is this normal?

Do you guys have the same effect when you dime your Express builds?

Thanks!

[Mark]

Dear Marshall plexi

I do recall my prototype amp did display some hiss. Are you using coaxial cable on the input lead and the volume pot?

This might help the hiss.

Also try plugging your guitar into the amp with these settings.

I don't think you'll feel so bad about the hiss.

[MarshallPlexi]

Thanks Mark. Yes I do have shielded cable on every grid including the PI grids. The amp sounds awesome but it has a LOT of hiss, not a small ammount A LOT of hiss when the treble and presence are dimed. Is this normal? Did your Express "variation" have this same trait?

[Mark]

Dear Marshall plexi

That is a tough question as it was some time ago that I built the prototype, and mine was only about twenty watts at the most, but I didn't think the amp was unreasonably hissy at the time.

Have you tried a different set of pre-amp tubes in the amp?

[MarshallPlexi]

I'll try some more tubes and see what I find. I may also try the Cut control and see if that helps.

Thanks again!

[Allynmey]

What kind of resistors did you use? Carbon comps hiss more than others.
Allynmey

[MarshallPlexi]

1W Carbon Film all the way.

[Elcabong]

What about your layout and wiring. I have had some hiss problems with an amp where the OT and its wires were too close to the board.

You can use a chopstick and move some wires around to see the effect on hiss. A picture of your build could help to spot potential problem.

[Funkalicousgroove]

[MarshallPlexi]

is it Hissy without the controls all dimed? Almost every tube amp will hiss a bit with all the controls dimed. Make sure that your grid wires do not run paralell with plate wires, and if they must intersect that they do at 90 degree angles, also, plate wires should be long and grid wires should be short as possible, and shielded where ever possible-only connect 1 side of the shielding to ground to avoid ground loops-try many different preamp tubes and let us know your findings.

The amp does hiss a lot when the controls are dimed. The hiss is not as bad as the RUSH sound it makes. It sounds like rushing water when the controls are dimed. Any ideas what that is?

I'm going to try the Star Ground idea and see if that works, if that doesn't help, I'll try rotating the OT and see if I can get that to work better.

I've done the shielded wire as you recommend.

Thanks!

[PeteRH]

I wouldn't waste any time with star grounding trying to eliminate hiss.

Grounding problems cause hum....not hiss.

[MarshallPlexi]

I wouldn't waste any time with star grounding trying to eliminate hiss.

Grounding problems cause hum....not hiss.

I agree, but there is still some hum there when the volume is all the way down. I'm running DC heaters on the first tube and AC on the rest and I'm thinking I'll trying running all AC and see if that helps.

Also I'm going to try star grounding the system, rotating the ot and choke, shorten the NFB wire, play with the filtering and see if those help. That should help with the hum.

What helps with hiss and running water sound?

[Robert]

I wonder if your expectation for no hiss when dimed is appropriate for a Marshall, but unrealistic for a Twreck. I have read others describe their experience with the amp, several note background hiss and some state they barely turn up the treble and presence to get the best tone, unlike other amps. Also noted have been descriptions of hiss when not playing or with no hands on the strings, but that goes away when playing, yet returns a second after one stops. My experience has been similar. I am not able to dime my controls either but use a PPIMV to balance volume with hiss when not playing. However playing eliminates all hiss. You might consider looking for a cold solder joint or a poor/defective switch. I had to remedy both with my build and each decreased background hiss.

Robert

[Funkalicousgroove]

[MarshallPlexi]

Ground the preamp near the input jack, that includes filter caps, ground your presence network and screen supply near the center of the chassis, and your 1st filtering stage, PT center taps, and output tube cathodes at the far end of the chassis. The green ground wire from the wall should have it's own ground just inside the chassis. NOW, get your schematic and a highliter, start at one end of the schematic and check your wiring from one end to the other joint by joint, check resistor values, cold solder joints, ect. Are your trannies currently at 90 degrees of eachother?

The PT is a laydown type, the OT's end bells are each pointed to the left and right side of the chassis, and the choke is arranged the same way. I'm wondering if the OT is picking up something from the choke given they are running in parallel.