5E3 w/ DC at input jack

[pdf64]

Has anyone else ever checked for grid voltage in standby? I wonder whether it's normal; there will be an electron cloud hanging around the hot cathode and some will likely find their way to the grid, sending it negative.
Don't have an amp with standby handy at the moment.

Regarding the grid voltage /scratchy guitar pot when operational, I wonder if the amp is oscillating; have you scoped it, if no scope, does a 12AY7 in V1, or ~220pF caps from 1st stage plates to 0V, stop it?

Bear in mind that the 5E3 was intended for a 12AY7. The 12AX7 is rather higher gain. Two 12AX7 sections set up for high gain, in cascade, with no inter-stage attenuation / tube base grid stoppers, will likely oscillate.
The gain will be ~3600 well into ultrasonics; with the input grid in the same polarity as the 2nd stage plate and in close proximity, wiring capacitances will easily form a positive feedback loop sufficient for oscillation, very good lead dress is needed to avoid it.

[Jazz P Bass]

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

In series to block the grid from the input jack.

It seems as if it was a good idea on the 5B3. (60 years ago)

deluxe_5b3.pdf

[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

Just an update to this thread:

I pretty much tried everything to get rid of this DC at the input jack, even replacing the V1 tube socket and nothing seemed to work. The DC at the input jacks was even there with V1 completely disconnected.

So last night I finally got fed up and ripped all the wiring out and started over again. This time I used modern stranded wire instead of the solid core cloth wiring that Weber supplied (which was actually fibreglass - very very itchy fibreglass) and that made things a lot easier. I didn't see any problems with my original wiring underneath the board or any potential problems. But regardless, after completely rewiring the entire amp the DC at the input jack is now pretty much gone. It' s still about 5mV though, but way less than before and not enough to make the guitar pots scratchy.

Problem fixed, but still totally perplexed.

[mozz]

ma or mv? What is the meter you are using to measure this? Tried to measure the same with a scope?

[waspclothes]

ma or mv? What is the meter you are using to measure this? Tried to measure the same with a scope?

Woops, mV not mA. This was measured with a Fluke 177 between the tip and the sleeve (with a jack in place). I did not measure with a scope, the amp is buttoned up and the only work I plan on doing it now is with a guitar

[mozz]

Glad it's working properly. Weird problem no doubt.