SVT 4 Pro hum

[lowell]

Here's a link to schematics.
http://music-electronics-forum.com/t15870/

This amp has a loud hum coming from the 1st preamp stage, first half of V1. Turning down the GAIN rids of the hum. Also when the mute switch is pressed in there is a fairly loud pop which seems abnormal to me. V1 grid is 17mv when unmuted and 0v when muted. I initially found V1 to be a bad tube and replaced it. Now voltages are correct but hum remains. I'm thinking something having to do with the mute circuitry is off. The gate of J176 is +12v when unmuted and 0v when muted so it seems as the jfet shunt switch is working. Q3: Vc is 0v when unmuted and 2v when muted. Any help would be great, thanks!

[MarkusBass]

I would start with simpe things first. Is the ground pin of the input socket connected to the ground of the amp? Is the hum with or without a cable plugged into the input jack? If without, is the IN-pin shorted to the ground?
Is the hum on the TUNER OUT output? Do you have +15/-15V voltages on input opamps?

I initially found V1 to be a bad tube and replaced it. Now voltages are correct but hum remains.

Which exactly voltages were incorrect and now are correct? I would rather measure anode voltage instead of the grid voltage (and looked at the anode resistor). I would also check V1 valve socket.
If you are concerned with the J-Fet, you can just desolder it for a moment.

Mark

[lowell]

Markus good point. I often overlook the basics and need to be reminded of them. The input jack was not shorting to ground with nothing plugged in. I retensioned the jack and it's good now. The amp still hums when the gain and master are at 11... but what amp doesn't do that?? Thanks!