Fender 75 Tube Amp Wind Noise

Hi there,

Recently purchased an older tube amp Fender 75.  Great sounding amp.  Worked just fine for about 1 month.  First a power switch went, fixed that.  Now there is a howling noise, like wind blowing through a mic outside or rubbing a mic on a sweater.  When I turn off the master volume the noise disappears.  No noise on stand by.  Guitar volume all the way down- there is noise.  Tried to look at different blog and most of them is to replace tubes.  Changed them all: pream and power.  No difference.
Found few threads that pinpoint to capacitors or old resistors but how to find one from dozens of them?
Please help...

If and only IF you are comfortable working with the high voltages inside tube amps, I recommend opening the chassis up and having the speaker hooked up and poking around with a wooden chopstick.  Sounds like a lose wire or bad solder joint.

I am comfortable to work on an open amp though there are no loose connections or anything of that sort....

So poking around with a chopstick while the amp is plugged in with a speaker you aren't able to isolate or affect the sound?  Turn it up so you are hearing the sound and tap on every resistor and cap-including the power caps.

Tapped every single resistor etc.  Found one microphonic resistor that responds to tapping and even blowing on it.  It is over a a tube socket, preamp V1 12AX7.  One of those round brown things...

That "round brown thing" is a capacitor. You could try to determine where the noise is originating. With the amp on and noise happening, remove that first pre-amp tube. If the noise stops, the problem is before or at that first tube, I would suspect one of the jack switches isn't grounding. If the noise didn't stop, remove the next tube, remove each pre-amp tube to determine if and where the noise is in the pre-amp section. Hope that helps get you started troubleshooting.

Actually, after tapping with a chopstick more, I have noticed that everything is microphonic as it gets closer to the first tube V1, or the left side of the amp, even the chassis.  So far when I pull the first tube the noise stops.  Did not try to pull any other yet.

The noise stops when the first tube is removed, so that points to the first gain stage as the source of the noise. The input jacks have a grounding switch that shorts input to ground when nothing is plugged, the contacts may not be solid causing noise. If the switch has retained tension, the contacts can be cleaned, if the switch is loose, replace the jack. I use a contact burnishing tool, but just one or two swipes is enough. Here is a technique you can try: attach a coupling cap to ground, touch the other end to various points along the signal chain to short any audio. The noise will short to ground when you are downstream, this will further pinpoint the culprit. A .1uF 600v capacitor will be adequate, attach it to a chopstick if you need a handle.