5E3 Volume Control Issue

[cminor9]

Just finished my very first 5E3 build. Powered it up, and it worked well except that when plugged into either channel, with all knobs at zero, I get sound when I play my guitar. As I turn up the volume knob of the channel I am playing, it stays at the same volume (somewhat low and muted, very thin and hollow, but definitely not what it should be) until about 2, then it kicks in fairly loud and sounds how I'd expect. Something is obviously amiss here, though, because with all knobs set to zero, I should not hear my guitar, right?

I read another post here about crosstalk or leakage (http://music-electronics-forum.com/s...ead.php?t=5657), which sounded a bit like what I was experiencing. Is that what's happening here? Bad pots? I did ground the pots to themselves by bending the tab onto the body and soldering it, and that was mentioned in the other post as a culprit. Could a bad solder joint cause this? I have triple checked the wiring of the tone stack and it's all right. Anyone have any ideas?

[loudthud]

Solder joints are always suspect on any new build. (Power off, amp unplugged from mains) I would connect one lead of an ohm meter to the negative side of one of the filter caps and ohm every point that is supposed to be grounded. Most meters are not very accurate on resistances below 1 ohm but readings should be close to the reading obtained by shorting the meter's leads together. Does your amp have the brass plate between behind the front panel ? Make sure the pot nuts are tight.

[cminor9]

actually, working as designed, I think...

loudthud, thanks for the tip. I did that and wasn't getting more than about .3 ohms (yes, 0.3 ohms) anywhere. So, I checked the solder joints on my tone stack. They weren't the greatest, but I redid them and added a ground wire from the pots to the star ground point. I do not have a brass plate, the pots are mounted directly to the aluminum chassis.

I plugged in, and fiddled with the knobs. Interesting, actually...when the active channel's volume is at zero, I still get sound IF the other channel's volume is turned up. Looking at the schematic, this makes sense and would seem to be a side effect of the interactive channels and 1M pots. When I turn down both volume knobs, it is dead silent and the volume knob works as I'd expect (the tone knob is irrelevant in this). Am I making a correct assumption, that this is simply a side effect of the interactive channels? It seems a little...quirky to me, but maybe that's part of the amp's charm. Right?

One new problem, though, I am getting a slight hum that gets louder with the volume and more intense when I turn up the tone knob (and interestingly peaks at about 3 o'clock on the tone knob and then tapers off). It's not a loud hum, but the amp used to dead quiet. I can only assume that what I added is the problem. Sound right? Just trying to exercise my troubleshooting skills, so forgive me if I made you read all that and I am way off track.

Thanks again.

[loudthud]

Yes, the channel interaction will allow a signal on one channel to come thru on the other's volume pot. Two possible paths for this are the common cathode bypass cap, and inside the tube, plate to plate because there is no shield between the two halves of the tube. There also may be some small amount of crosstalk because of layout issues and lead dress. All in all it would seem you amp is operating normally.

The slight hum may have been caused by the ground wire you added or another lead dress issue. If you searched this forum I'm sure you could find thousands of posts (sometimes conflicting) about grounding. It's one of those things like making a good beer that generates seemingly endless debate. Instead of the star point, try running the new ground wire to the ground side of the 2nd stage cathode resistor.

For lead dress you can try moving wires around while the amp is on with a chop stick or some other insulated probe (don't use a lead pencil) to try to find which wire is picking up the hum. Some people have success using coax cable on grid wires. I've never found it necessary but do a search if you are curious.