DC on input jacks?

[potatofarmer]

There are the typical cathode resistor and cap on each cathode.
THEY are grounded to the input jack ground lugs.

Sorry.
But like I said it's a 5E3 circuit, so of course it has 820ohm/22uf on each cathode and they are grounded to the input jack ground lugs.

of course I tried other 12AX7's - with no change.

On the stock 5E3 schematic V1 pins 3 and 8 are connected and share a cathode resistor and cathode bypass capacitor. There's nothing wrong with splitting the cathodes and biasing them individually, but if you intended to follow the stock layout you might want to double check that.

I can't really propose how DC wound up on your grid (accidentally bridging grids to cathodes?) but to me this suggests you should double-check the wiring on V1.

[52 Bill]

Fiberboard leakage? If you pull the tube out does the voltage go away?

[ngteleblues]

I have exactly the same problem with 2 builds. A 5E3 and a Tweed Pro. If I place a reverb pedal between the guitar and amp the scratchy pot noise (voltage) is blocked. I can't figure it out.

[drewl]

Are they Weber kits?
I replaced the input jacks and resistors and that fixed the problem.
Even though the 1M and 68K input resistors all measured fine, they were somehow causing the problem.

[ngteleblues]

Both are mojo kits except the transformers on the 5E3 are Classictone. Both amps show the same voltage at the input. I have had some trouble with the jacks on the Pro and I replaced them recently but reused the resistors.

[Randall]

Even though the 1M and 68K input resistors all measured fine, they were somehow causing the problem.

This I would need to be explained to me.

[Steve A.]

I replaced the input jacks and resistors and that fixed the problem.
Even though the 1M and 68K input resistors all measured fine, they were somehow causing the problem.

I think that it was the process of replacing the part that fixed the problem. With your 5E3 kit did you get the brass plate that goes between the chassis and the pots and jacks? Just wondering.

So when you were measuring voltage did you just check between the tip and sleeve of a guitar cable? Or did you also check for voltage from the tip or sleeve to chassis ground? I have a hunch that there might not have been perfect continuity between the jack sleeve and the chassis so grounding the cathode resistor and bypass cap to the jack created the voltage that you measured.

Just a shot in the dark...

Steve Ahola

[Steve Conner]

Sounds like a bad connection somewhere that got fixed in the process of replacing the input resistors.

I've seen many 12AX7s that leak a little grid current, causing the guitar volume pot to scratch. Maybe this was acceptable back in the Fender era, or maybe the tubes were better back then, I don't know. Anyway I think the most I ever saw is about 30mV across the 1M grid leak resistor, not 0.6V.

[ngteleblues]

What ever I did wrong I did it twice because two amps have the same problem otherwise the tone is great but I started using a volume pedal again and the noise is very annoying.
Both amps were built without the brass plate on the panel, they both have a 16 gauge copper buss wire soldered to the back of the pots per Mojotone. I did a Harvard scratch build last year with the brass grounding plate and no voltage/noise is present. I have checked my grounds and lead dress and I am puzzled. I am going back into the 5E3 today. As I told Drewl I am a paint by numbers builder and I suck at trouble shooting. Maybe I'll post some photos because fresh eye can usually see stupid mistakes and there is probably something obvious that I have overlooked. Whatever I did wrong, I did it twice at least I'm consistent!

[ngteleblues]

[J M Fahey]

If you add a good 22nF cap in series with the input, does the scratching go away?

I mean between the jack metallic lug and the resistors (68K/1M net).
The grid DC path to ground is necessary.

Just for kicks: do you read any DC voltage between the jack ground leg and the PSU "common" ? (All the filter cap cans)

[Leo_Gnardo]

If you add a good 22nF cap in series with the input, does the scratching go away? I mean between the jack metallic lug and the resistors (68K/1M net).
The grid DC path to ground is necessary.

To maintain input Z at @ 1M, I'd put a 2 or 2.2M R at the grid and another at the jack when trying this solution.

[ngteleblues]

5E3 Photos by ngteleblues | Photobucket
I hope this works. Should be about 5 gut shots. Please be gentle with your critique.

[drewl]

This I would need to be explained to me.

I'd like it explained to me too!
It was more likely the jacks not having a good ground connection, but I tightened the old ones, reworked the solder joints, tried clipping test leads to several different grounds and still had the problem and always measured zero ohms from the jack grounds to the chassis.

[ngteleblues]

Anyone see any glaring mistakes? Suggestions? I don't possess the suggested parts to add to the input but I will order a few things shortly.