The first thing to look for in case of feedback noise is a defective tube. The 1st preamp tube(V1) in the signal chain is usually the one that's prone to get microphonic. V1 is the tube that every channel uses to amplify the guitar signal and it works harder than any other preamp tube.
The phase inverter tube used to drive the power amp is the second most hard-working tube but this rack preamp does not have one.
The Valve No1 was the one to blame in this case and we changed it with a new 7025/12ax7. The feedback was still there at high volumes though, so the 2nd place to look was the Ground!
The ground is very important as it's here to protect you and your equipment from voltage socks, it reduces hum /noise and it serves as a point of voltage reference for the circuit (the ideal ground sits at zero volts). Every metallic part of an amp should be grounded unless otherwise noted. I noticed that the back of the control pots on this preamp were not grounded. That's because the paint of the chassis where the pots were mounted was not conductive... I wired a ground wire at the back of every pot and the feedback was gone!
To ensure that the hum and noise stays low, you have to star-ground your equipment. But, what's "star ground"?... Star grounding is a wiring method where every ground wire from the circuit and the controls is soldered/connected to a single ground point(usually somewhere on the metal chassis).
The other thing you have to notice is the Farad cage effect: When every side of a closed metal box is connected to ground, no noise or electromagnetic interference can penetrate the box. This box is called a Farad Cage. So, by grounding the back of a control pot or shielding your guitar with foil, you actually create a Farad cage which reduces hum and noise.
Here are a few pictures of the preamp: (open in new tab to see the full-size pics)
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