5E3 w/ DC at input jack

[johnny99]

Hey guys,
I recently finished up a Weber 5E3 build. Pretty simple, works great. But today I finally had a chance to crank it and noticed that the guitar's volume pots were extremely scratchy. I tried the guitar on another amp and the pots were silent.

So I have it back on the bench right now, and I'm measuring some DC on the input jacks:

Standby:
Normal 1: -.76V
Normal 2: -.52V
Bright 1: -.83V
Bright 2: -.61V


On
Normal 1: .07V
Normal 2: .005V
Bright 1: .16V
Bright 2: .013V

What's weirder is the pretty significant negative voltage on the input jacks while standby is on.


I tried running a jumper wire from another ground point (PT bolt) straight to the ground of the jacks, and the DC remained unchanged. The DC offsets go away when V1 is removed, so I tried another tube in V1 and the DC level was unchanged.

I can't say I've experienced this before. Any opinions? Amp sounds pretty killer otherwise.

Mike

[JazzGuitarGimp]

What's the plate voltage on the first stage of each channel? Did you install a vvr in this amp?

[johnny99]

What's the plate voltage on the first stage of each channel? Did you install a vvr in this amp?

No VVR. I'll measure the plate voltages and report back.

[johnny99]

168V and 163V respectively on Pin 1 and 6.

Here's a diagram of all the voltages:

[JazzGuitarGimp]

Your plate voltages look reasonable. I have more questions:

Have you double checked the input jack wiring? In particular, are the 1M resistors present and properly wired?

Are you using a guitar with active pickups?

You said you noticed the scratchy pots when "you finally had a chance to crank it". From this, I infer that the DC voltage at the inputs increases as the amps' volume ontrols are turned up. Am I understanding this right?

Have you tried a third tube ar V1?

[Stevem]

Your input jacks are wired wrong!

[johnny99]

Your plate voltages look reasonable. I have more questions:

Have you double checked the input jack wiring? In particular, are the 1M resistors present and properly wired?

Are you using a guitar with active pickups?

You said you noticed the scratchy pots when "you finally had a chance to crank it". From this, I infer that the DC voltage at the inputs increases as the amps' volume ontrols are turned up. Am I understanding this right?

Have you tried a third tube ar V1?

I've double checked the input jack wiring, and it all looks to be correct including the 1M resistors. With the amp off, plugging into each jack individually I get a resistance measurement of 1.06M on input 1 and 138k on input 2 to ground. And I'm pretty sure that's what I should be getting.

One thing that might be different in my build, is that I've grounded V1's cathode resistor/cap to the ground-lug of Normal 2. Comparing it to some photos of a Victoria build, my input jack wiring is the same, but they attached V1 cathode resistor/cap ground to the chassis near the input jacks.

The DC voltages don't increase as the volume goes up, the scratchiness of the pots just increases in volume.

I tried a third tube in V1 and exact same problem.

[JazzGuitarGimp]

The DC resistances you measured at the high and low inputs jacks are correct, so I am betting the input jacks are wired correctly. I think the next thing I'd do is measure the DC resistance from V1 cathodes to ground. This should be 820 ohms.

Are you using active pups? If so, is your battery fresh?

[johnny99]

The DC resistances you measured at the high and low inputs jacks are correct, so I am betting the input jacks are wired correctly. I think the next thing I'd do is measure the DC resistance from V1 cathodes to ground. This should be 820 ohms.

Are you using active pups? If so, is your battery fresh?

I only have standard pickups in my guitars. V1 cathode to ground is close to 820 ohm. I also removed the bypass capacitor and measured again, same result. For funsies, I fired it up with the bypass capacitor removed to measure the DC at the input jack. It changed a bit, I don't remember if it got higher or lower (don't have my notes handy at this moment).

I also unscrewed the fibreboard and backed it away from the other board underneath it. The DC at the input jack didn't seem to be affected.

What's strange to me is that it's -.7V when the amp is on standby (only heaters on).

I tried jumpering pretty much all the grounds as well, moving the V1 cathode resistor/cap ground to different parts of the chassis, etc. All ground points are measuring pretty much 0 ohm on my fluke.

[martin manning]

...Oops!

[JazzGuitarGimp]

That is very strange. I know weber sometimes has issues with their PTs. Hopefully, this is not your problem. But I would be inclined to disconnect the the 6.3V winding's center tap from ground and then measure the DC resistance from the center tap to ground. Use your meter's highest ohm scale (if it isn't auto-ranging) If you measure any resistance at all, pull the tubes (and the pilot light, if it is run from the 6.3V winding) one at a time and re-measure. If you've pulled all the tubes and still measure resistance, i would suspect either a heater string miswire, or the PT.

Other than that, I'm afraid I'm running out of ideas. So hopefully someone else will chime in with some good advice.

[The Ballzz]

johnny99,
Please be sure to post the solution/error when and if you find it. I eagerly follow all 5E3 build and troubleshoot threads, as I am getting ready to "do" one myself! I wish I had some tips for you, but sadly my knowledge and skills pale in comparison to the masterful heavyweights who are already guiding you!

With that said, I'm betting that you'll find either a faulty cap or solder joint "somewhere" or other!

I'm not sure if on this site or another, but a well known builder was chasing a similar issue that actually "went away" after changing a pre-amp tube socket. The socket displayed no visible defects and showed no faults with a meter. There was a lot of speculation as to what the fault may have been (oxidized pin grips, microscopic cracks, etc.) but no definitive smoking gun! My money is on a faulty solder joint!

I'll see if I can locate the thread and link it here. It would really suck to find that there is a batch of defective "name brand" sockets floating around, but it would at least be helpful to be aware of them.

Best Of Luck,
Gene

[The Ballzz]

Here's a link to the previously mentioned thread over at the Les Paul forum:

http://www.mylespaul.com/forums/squawk- ... oblem.html

My $.02,
Gene

[johnny99]

johnny99,
Please be sure to post the solution/error when and if you find it

Well, I'm happy to say that this problem is now fixed!

The only solution I could come to was to remove the entire circuit board and rewire everything. It took a few hours, but it also allowed me to upgrade the Weber supplied wiring (fibreglass insulation that caused a lot of itching) with modern PVC insulated wire.

This time, as I went along I fired up the amp at different stages to see if any DC appeared at the input jacks and it never did.

[Colossal]

Here's a diagram of all the voltages:

FWIW and off-topic, I wish everyone with an amp problem looking for help did what you did; posted a clear, concise schematic with voltages before doing anything.

Excellent and glad you got it fixed.