5E3 w/ DC at input jack

[waspclothes]

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

[The Dude]

The input jacks are shorting jacks. The inputs should be grounded with nothing plugged in. Or, do you have something plugged in when you are measuring? If not, I'd say you don't have a good ground to the jacks.

[waspclothes]

The input jacks are shorting jacks. The inputs should be grounded with nothing plugged in. Or, do you have something plugged in when you are measuring? If not, I'd say you don't have a good ground to the jacks.

Hey Dude, yes this is measured using a 1/4" jack plugged in.

[The Dude]

And does the DC disappear when the plug is removed? I'm still betting on a grounding issue and the DC is some "floating residual junk". I don't see how it could otherwise get there. The only other idea I have is leakage on the board itself.

[waspclothes]

And does the DC disappear when the plug is removed? I'm still betting on a grounding issue and the DC is some "floating residual junk". I don't see how it could otherwise get there. The only other idea I have is leakage on the board itself.

Yeah, the DC on the tip is removed when the jack grounds itself. The resistance from the jack's ground sleeve to the other ground points on the board is .5ohms (same as my meter's probes by themselves).

[loudthud]

A low plate Voltage on a 12AX7 or 12AY7 will cause grid current to flow. That's why you see grid current (Voltage on the grids) when the standby is in the warmup position. What is the plate Voltage on that input tube? (pins 1 and 6)

[waspclothes]

A low plate Voltage on a 12AX7 or 12AY7 will cause grid current to flow. That's why you see grid current (Voltage on the grids) when the standby is in the warmup position. What is the plate Voltage on that input tube? (pins 1 and 6)

168V on Pin 1 and 163V on Pin 3. Here's all the voltages:

[Randall]

If it's a fiber board, check to see if you can measure voltages from ground directly to the board on the power supply side of it. If you can see anything over a few volts, then you have a conductive board. It's happened to me.

[TimmyP1955]

Check the ground on the 1M resistor(s).

[pdf64]

Try yet another, known good tube in V1; some just have excessive grid current and don't suit applications where there's a pot on their grid.

Are those input Vdc in standby mode 'solid', or do they gradually fade to zero?

To borrow a phrase, guitar amps need standby like a fish needs a bicycle, see http://www.valvewizard.co.uk/standby.html
If you might use a real rectifier tube in there, such an arrangement that facilitates 'hot switching' the reservoir cap on to it may result in current surges above its limiting value, reducing its service life and possibly promoting a catastrophic failure mode.

[loudthud]

168V on Pin 1 and 163V on Pin 3. Here's all the voltages:

Those Voltages should be high enough to avoid the problem of which I spoke.

[waspclothes]

Try yet another, known good tube in V1; some just have excessive grid current and don't suit applications where there's a pot on their grid.

Are those input Vdc in standby mode 'solid', or do they gradually fade to zero?

To borrow a phrase, guitar amps need standby like a fish needs a bicycle, see The Valve Wizard
If you might use a real rectifier tube in there, such an arrangement that facilitates 'hot switching' the reservoir cap on to it may result in current surges above its limiting value, reducing its service life and possibly promoting a catastrophic failure mode.

Hi there. I've tried 3 different 12AX7's in there and at AT7 and all with the same DC voltage at the output. Those voltages are solid in standby mode - they start at 0, and rise to -.7 as the preamp tube warms up and remain there until the standby is switched to the 'play' position and then they jump to positive voltages and remain there.

Thanks for the heads-up on the standby switch. I'm currently using a Weber copper-cap though. I did switch to a real tube rectifier briefly, but the DC voltages on the inputs were still present.

[waspclothes]

If it's a fiber board, check to see if you can measure voltages from ground directly to the board on the power supply side of it. If you can see anything over a few volts, then you have a conductive board. It's happened to me.

Thanks for the reply Randall. I checked the board and the highest was 20mV at any point.

[Jazz P Bass]

Put a .05 cap on the input to the grid.

[waspclothes]

Put a .05 cap on the input to the grid.

Hey Jazz, so I'd be bypassing the 470K input resistor with this cap - or putting it in series?