Low Frequency Noise (Express)

[mumford]

I've got my Express pretty well dialed in, except for a bit of preamp hiss, (which I doubt I can get out) and a low frequency hum.

The low frequency hum is only audible when I'm not playing/unplugged, and I would say is slightly below the guitar fundamental frequencies. I realize it's not too specific, but any ideas?

[Firestorm]

It's hard to tell the difference between 60-cycle hum and 120-cycle hum without an analyzer, but it would sure narrow it down if you knew for sure. 60Hz will be related to the power transformer, the filament circuit or environmental hum finding it's way into the amp (we are absolutely surrounded by 60-cycle hum). 120Hz will be related to ripple current in the power supply.
To narrow down the source, pull the phase inverter tube. If the hum stays, it's in the power amp. If it goes away, it's in the PI itself or in a preamp stage. Zero out the volume. If the hum goes away, it's between the input and the grid of V1b.
Do you have any pix of the build for the eagle-eyed guys here to squint at?

[peesinstew]

Sounds like you're describing 60Hz hum. There are a couple of possible causes.

Does the volume of the amp control the hum?
If not, then it could be your transformers coupling and your OT is picking up signal from your PT.
If the hum goes away with the volume down, then it could be coming from your heaters. If this is the case, then you could elevate the heater center tap, try a hum balance pot, or convert to DC heaters.

Ground loops can cause hum too.

Alot of other people have had this problem, including me. if you search around here, there are some good solutions.

otherwise, give us some more details.

[drhulsey]

...The low frequency hum is only audible when I'm not playing/unplugged...

I get a low freq hum until I plug the guitar in. I haven't worried much about it, since it goes away after I plug in

[jjman]

.........The low frequency hum is only audible when I'm not playing/unplugged.....

If it hums with nothing plugged in, and doesn't hum when plugged in, the jack is not "shorting" properly when unplugged. Did you use a shorting type jack and ground the shorting tang?

[mumford]

I don't hear it when I'm playing, but I do when I stop or when the input is unplugged.