Plexi 6v6 Hum

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JoeCon
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Plexi 6v6 Hum

Post by JoeCon »

Guys
While my Plexi 6V6 sounds great when playing it does have quite a bit of hum.
I tried moving grounds around but no difference...
Then I put on a bottom plate and now it's quiet. With the plate off if I move my hand over the preamp tube area the hum gets louder.
Yet if I hold the chassis with my hand it goes down.
Any ideas where I went wrong?
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Stevem
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Re: Plexi 6v6 Hum

Post by Stevem »

Yup that's normal if you have grid wires that are unsheilded, or you have grid leads too close or laying on top of the heater wires, but as long as the noise level is good with the bottom plate on you can choose to leave it.
Sheilding the wires on the first two gain stages will make the most difference in this issue!
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JoeCon
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Re: Plexi 6v6 Hum

Post by JoeCon »

Steve

OK so shielded wire from the socket to the two volume pots, right?
I got shielded input.
I tried moving the wires around and no diff so it's gotta be a shield...

Thanks...
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Re: Plexi 6v6 Hum

Post by Stevem »

Yup, sheild the input lead to the tone stack and the output of the stack, in short any grid lead untill you get to the PI input should be sheilded in my book!
Just remember to land the sheild at one end only, and that should be the upstream end of the wire, and if you can land it to the chassis near the tube it's coming from.
At times it helps to run a common ground across the back of all the pots in case the pot the pot that is doing the main preamp ground had a flacky ground from being bolted down.

Note that once the output signal of a tube leaves the down stream end of a coupling cap then the next stages grid lead starts right there!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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xtian
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Re: Plexi 6v6 Hum

Post by xtian »

Remember that if you add shielded cables, you're adding a small amount of capacitance, and it will dull the highs a bit.
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Littlewyan
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Re: Plexi 6v6 Hum

Post by Littlewyan »

It is normal for any amp to have a bit of hum without the bottom plate on. If its quiet with the plate on then why is it an issue?
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Re: Plexi 6v6 Hum

Post by Stevem »

Having that metal plate on the bottom did not stop all of the original non master Marshall plexi amps from osscilating when you cranked the amp wide open and had the presence up full, if you had at least two strong preamp tubes in these amps then you had that issue!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Littlewyan
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Re: Plexi 6v6 Hum

Post by Littlewyan »

We can probably solve that by moving some wires around. I do know that the reissues came with shielded wire on the inputs and the volume pots but nothing else and they are dead quiet.
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Re: Plexi 6v6 Hum

Post by Stevem »

Well may be its just that I hate hearing all that buzz going on when a service a amp that's off that plate!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Littlewyan
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Re: Plexi 6v6 Hum

Post by Littlewyan »

Try servicing a Trainwreck!
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M Fowler
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Re: Plexi 6v6 Hum

Post by M Fowler »

Speaker jack ground goes to same place as PT CT and main filter cap ground.

Power tube imbalance? Change tubes
Baddog01
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Noise issue

Post by Baddog01 »

Try grounding each channel separately. Ground the input and tone stack of each channel to the back of that channels volume pot. It's the only way I could quiet an issue in my 18watt Marshall tmb clone.:)
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Structo
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Re: Plexi 6v6 Hum

Post by Structo »

Search for Larry's Grounding.

Wait one......

This is supposedly the best scheme for Marshall type amps.

The guys over at Metro really liked this.
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Paulo P
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Re: Plexi 6v6 Hum

Post by Paulo P »

I had the same issue with my plexi, it was the tone stack capacitor. Spent 2 months on ground, change the 500pF/33k to 250pF/56K and the noise gone.
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rp
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Re: Plexi 6v6 Hum

Post by rp »

Having that metal plate on the bottom did not stop all of the original non master Marshall plexi amps from osscilating when you cranked the amp wide open and had the presence up full, if you had at least two strong preamp tubes in these amps then you had that issue!
If this is occurring try using tubes shields. I know it's a peculiarity of the circuit but I hate it occurring in something I built. Tore my hair out over it, last was tube shields and bingo. Never thought about gainier tubes making it worse and tube rolling, that must be why some Marshalls have it worse than others or don't experience it.
I had the same issue with my plexi, it was the tone stack capacitor. Spent 2 months on ground, change the 500pF/33k to 250pF/56K and the noise gone.
Maybe a bad mica cap, if it was mica, and not the values?
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