express hiss

[playloudd308]

alright. i bult an express a little over a year ago. and this whole time i have had no problems with it. But recently i started hearing a hissing sound which becomes more noticeable at louder levels. this also increases when the presence is tuned up or the bright is turned on.

any clue what this could be?

[mlp-mx6]

Have you swapped any tubes to see if the condition improves?

[playloudd308]

yeah. that was my first guess but that wasnt the problem. any other suggestions?

[tomrasdf]

Would I be right to think that because the bright switch has an effect on the level of the hiss that the problem would be proir to that in the signal path?

[paulster]

The bright switch is across the volume pot, which is after the first gain stage (half of V1) and the tone stack, so you're getting noise from outside the amp or from V1.

Depending whether or not you have a switched input jack which grounds itself with no plug inserted it can be a useful diagnostic tool to have a jack plug internally shorted plugged into the amp. This will make sure it's not external interference coming in through the jack socket.

With that ruled out it sounds like either a particularly noisy V1 tube or just the inherent Express hiss caused by the level of gain in the preamp.

Try all of your available tubes in the V1 position (rotating V2 and V3 if you only have the 3 12AX7s to hand) and you'll get a good idea of whether it's tube-related.

Paul