Plexi clone, Noise when guitar plugged in..

[satsloader]

Greetings and Salutations,

Been away for a long spell, had to re-register..

My clone has always had a bit of noise when an instrument is plugged in, but seems to have gotten noisier of late ( Or I'm just now getting impatient with the noise ). It happens if my guitar is plugged in, dead quiet otherwise ( Either channel's volumes turned to max only produce the normal low volume hiss when no instrument is inserted ). It happens with any guitar. I swapped a few different 12AX7's into V1, no change. All voltages ( including cathodes ) are good. I inserted a grounded 1/4" dummy plug in either channel just for fun, it's quiet.

The noise is not so much a hum, it's more of a grating, almost hiss like noise with a very fast sort of ticking. Other than the noise, the amp sounds Bitchin.

The leads from the jacks are shielded, grounded only at V1's socket.

So, given the info above, where should I start to troubleshoot this? I have a scope and Signal generator..

Thanx,

Bill

[Don Moose]

What if you plug in a bare 1/4" jack (no cable, just the jack)?

If that's quiet, the amp is fine and your problem is in the cables, guitars or electrical environment (dimmers, ceiling fans, electronic motor speed controllers).

Otherwise, I think you need to concentrate on that first triode, and work back toward the instrument (see above).

Hope this helps!

[Andrei_T]

If the noise rasises and drops in volume as you turn around with your guitar than it's picked up by the pickups and is normal. What happends if you put the vol knob on your guitar at min? Check the wall plug from where you power the amp. It should be grounded. If not buzz noise may occur.

Andrei

[satsloader]

Thanx, Anrei and Don,

I should mention that I've tried different cables and guitars, different venues, and the noise is the same.

Don, I took your suggestion to plug in a cable by itself and observe.. The first thing I noticed, pulling the plug out of my ES-335 to try your idea, was that the noise, while still there, actually dropped in volume just a bit ( Note; I wasn't touching the body of the plug, but holding the insulation ). I found this interesting, as I expected the noise to rise after unplugging.

I then discovered if I place the plug anywhere close to the chassis ( with the exception of the PT, which generates it's own field noise ), the amp goes dead quiet, with nothing but the normal background hiss. I can place the plug next to V2's grid resistors, normally a spot that would generate a ton of noise with anything else.. dead quiet. I can put it 1/32" away from the heater wires.. dead quiet! It's as though I were re-introducing the noise, but in opposite phase, so that it was being canceled out. I would guess though that I'm simply grounding the original noise. If so that would seem to lead to a problem between the amp's inputs and V1's grids as Don mentioned. As for that section, it's Rean jacks, Belden shielded cable to 68k grid resistors soldered with short leads directly to V1's socket pins. The input grounds and V1 grounds are joined locally and a lead sent to the amp's main star ground. I'll try a makeshift chassis ground jumper to the various V1 component grounds and see if there's an issue there.

Just to be clear, the noise I'm complaining about hearing when a guitar is plugged in is absolutely the noise you hear when a cable is plugged in to an amp with no instrument attached , and as mentioned earlier, is actually a tad louder with a guitar attached than without, and goes away if I place an instrument cable's "guitar end" plug in the vicinity of the chassis. I should also mention that my testing area has no florescent bulbs, no dimmers, etc.

Also, Andrei, yes the noise does drop when I turn the guitar's volume down. The amp is 3 prong, and the wall is grounded.

Any additional help is greatly appreciated. I usually don't have too many problems shooting noise problems, and I'm sure the solution is right in front of me, this should be an easy one, but my old burnt cells aren't connecting the dots at the moment..

Thanx again,

Bill

[Don Moose]

The cable from input jack to V1.grid is shielded - try ungrounding ONE end of the shield (probably the jack end first).

You'll likely wind up trying both ends - remember to reconnect one if you unground the other.

I'd be surprised if this changes anything, it sounds like this is an artifact of having pickups.

Hope this helps!

[satsloader]

Hi Don,

Thanx for the suggestion. Unfortunately I already have the shields on the leads grounded at just the input jacks. Since I have the other ends of the leads shrink wrapped at the grid resistors I would have to replace the leads and grid resistors to swap the ground connections to the grid ends, but I might just try that..

Since my guitars are quieter when plugging into other amps, there is a noise issue in the amp itself.

I'm still curious as to why when I place the unplugged end of a cable ( that's plugged in ) close to the chassis the noise would go away. This sure seems to be a good clue, Yes?

Bill

[Mars Amp Repair]

My guess is that since the chassis is grounded, it tends to sheild the unplugged end. Not really a great clue.
Almost have to hold the amp up to the computer so we can hear it! glen

[Don Moose]

To put Mars' answer another way, the unplugged cable end is coupling to the chassis ground capacitively and/or magnetically. Without a pickup in the picture, the input impedance is high enough that it takes very little capacitance to make a nice filter.

I have to suggest swapping grounded ends on the shielded cable, so that the shield is coupling to SIGNAL ground (same tie point as the Rk/Ck for V1a) instead of the relatively noisy chassis ground. At least that's how I'd talk myself into it.

It may do nothing. It may make things worse. Consider doing it as a swapout of the whole jack/grid-stopper/cable assembly so you can switch back quickly if it turns out to be a step backward.

Hope this helps!

[Alex R]

...but did you try the bare jack as Don originally suggested? That would tell you for sure whether it was the guitar or the amp. A lead plugged in will make all kinds of noise as you move the tip around, from light hum to radio moscow.

[satsloader]

Hi Alex,

Ya know, I read Don's post a bunch of times and never even noticed the "No Cable attached" part of it, assuming he meant an unterminated cable, which at first I thought was a bit silly, I mean, who hasn't discovered the racket made when you pull the cable out of your guitar.. Then I discovered the relationship with the cable end and chassis, which had me chasing my tail as is pointed out above. A good learning experiment though.

I swapped in the replacement wiring, and she's quiet again. As for what was causing the noise with the old input wiring, I looked at the solder joints with a magnifying glass, they look good, everything measured good with a meter.. I've decided to stay ignorant on this one and enjoy my now normal amp..

Thanx again,

Bill

[Alex R]

Cool, I'm just glad it's working.