Pull the tubes, one at a time and see if it stops.

Pull the 1st tube and let it warm up. See if there’s any hum. Then put the tube back in and pull the 2nd tube. It’s possible one of the 12AY7s is causing the hum. I found that in a friend’s preamp.

There’s a good possibility it’s the 12AY7. You can replace it with a 12AU7 to see if that takes care of the hum.

Hey there! I know this is a very old post, but I’ve recently bought a EHX Mic Pre and I’ve found myself with the same “problem” you describe. It’s not a problem per se. It happens that both outputs of this pedal are balanced. The hum noise appears when you you use the balanced 1/4” output and plug into an unbalanced 1/4” input. To eliminate that hum, you have to use a balanced cable (stereo jacks on both ends) and go into a balanced input.

Now, I want to process my voice using a lot of effects pedals (all of them use unbalanced inputs/outputs). The microphone goes directly to the EHX Mic Pre, and I cannot go from there directly to another effect pedal (because of the hum noise). To achieve this without noise, I have to take the balanced out from the Mic Pre and go into a compressor (or a mixer with balanced inputs). In my case, I go into a DBX compressor, that has an unbalanced output. So I use that unbalanced output to go to the effects pedal. In this way, I don’t get any hum noise.

I’ve tried making my own cable TRS –> TS but it didn’t help, the hum noise is still there.
So far I’ve found that the only way to eliminate the hum is go from the Mic Pre to a balanced input on a mixer or whatever device that offers an unbalanced out.

Hope this helps.
Let me know if you can think of another way to go out from the Mic Pre directly to an unbalanced input without that ugly hum noise :/

Thank you so much for taking the time to register and share this information with me, I really appreciate it! It’s great to hear that my post was helpful to you, and I’m glad you’ve found a solution for the hum issue with the EbTech Hum Eliminator. I hadn’t thought of using one of those, so this is very valuable advice!

If possible, could you share a link to the specific EbTech device you’re using? I just want to make sure I’m looking at the right one. I found this one on Amazon:
EBTech Hum Eliminator is this the one you’re referring to?

Thanks again for your help!

i keep trying to answer here and the replies are not showing up

sorry for so many replies, lol, the other issue (pasted from Joel Hamilton “…is that you really should’t be using the 1/4″ output, because you are actually going through an IC based buffer amp, rather than getting the actual tube goodness straight to tape (or DAW).

The 1/4” output is controlled by the knob on the right, and the pre itself is controlled with a single knob: the one on the left, for an all tube tape path… No IC’s, just great discrete tube goodness. The designer of this pre is really a creative, amazing designer, and this pre shows off his ability to make something at a price point, and have it actually rock! Even the “soft start” function for the phantom is unique, especially to this price point!”