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[mclmk8d]

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[selloutrr]

try replacing tubes starting with the PI and working to V1 and see if that helps.

Bad resister in the B+ filtering

BAD GROUND

faulty input jack

Bad solder point



Is it a hum or a Hiss?

[selloutrr]

bad wire dressing try moving the wires around with a chop stick and see if it calms down after you check the grounds are good. and the AC is properly grounded

[paulster]

An Express with the volume on 0 still has as many gain stages operating as a plexi with the volume set on 10, and less attenuation between the stages to boot, so there will always be a level of hiss.

Bring the volume up from 0 and you add in another gain stage, so then you're looking at something more like a JCM800 but again with less attenuation between the stages whereas the JCM has quite a bit.

What I'd suggest is to look at tubes first. Pull the V2 tube and see how much hiss you have. That'll tell you how much is getting in through the PI, so swap this tube out if need be.

Put V2 back and set the volume to 0. You'll now be listening to V2 and V3 tubes only, so work on V2.

Finally, bring the volume up to add V1 into the mix. This is the most critical tube since any noise from here is going to get amplified significantly in the rest of the amp.

[mclmk8d]

The noise is a hiss, not a hum. I will try playing with the tubes today.
link to my photobucket album
http://s291.photobucket.com/albums/ll30 ... view_click
The last pre-amp tube seems to have some discoloration and the pins are dark...could that be the problem?
The builder's name is Jim Yorke in San Jose, CA; is he here on the boards or does anyone know how to get in touch with him?
Thanks for your help with the newbie...I'm not an electrical guy at all; I teach 6th grade world history, so the simpler the terms the better I understand..thanks

[sliberty]

Nice loking amp, and nicely built from what I can see. That preamp tube doesn't show anything obviously wrong, but swapping tubes is still called for as a test. If it doesn't help though, but that tube back in - it's a British made tube, and will probably sound pretty good.

[mclmk8d]

As I mentioned, I'm not an electrical type...which position is the PI tube on the chassis?

[gearhead]

It's the closest preamp tube to the power tubes (the bigger ones).

PI=Phase Inverter. It takes the signal from the preamp and provides an in-phase signal to one power tube, and an out-of-phase (inverted) signal to the other power tube.

In this amp it is in position "V3", or Valve #3. Valve is the British term for tube, and it stuck as naming convention for tube position. V2 is the preamp tube next to V3, and of course V1 is next to V2.

[mclmk8d]

I pulled V2 and turned it on with just v3 and V1...sweet silence. I then moved V1 (the British tube) to V2 so I had V3 and V2, but no V1; still silence. I then took the orginal V2 tube (ECC-83, made in Yugoslavia..Tung Sol?) to V1, and the hiss came back, so it looks like a bad tube. Any recommendations for a replacement?

You guys have been extremely helpful...thanks; you saved me from having to ship it back to the guy I traded with.
Looking forward to your recommendations.
Serge

[gearhead]

The crucial tube is V1. That's where the guitar signal first enters the amp, and any noise is amplified from there forward. The Express really, really needs a quiet tube in that position.

I'd borrow a tube from one of your other amps to stick in V2 and put the original V1 back in to see if it sounds good.

If not, Ken Fishers choice (and others on this forum) is NOS Tungsram. They are getting harder to find and more expensive tho.

[mclmk8d]

I put the British-made tube back in V1. I'll look for some Tungsram tubes.

Update; just put another tube in V2, a JJ ECC83, and the hiss is back...bad tube socket?

[ampdoc1]

I assume this is an Express type amp. When I built my first one, I loved everything about it, but the "hiss".

I ended up changing preamp tubes, and that eliminated most of the noise.

My preamp ended up as: 12AU7 (V1), 12AT7 (V2), and 12AX7 (V3).

This eliminated 90% of the hiss, but lowered the gain. I added another gain stage at the end of the preamp string, using the 1/2 12AX7 at V3. In the end, the amp lost none of the original harmonic content or touch sensitivity.

See attached drawing of my preamp.