Plexi 6v6 Hum
Moderators: pompeiisneaks, Colossal
Plexi 6v6 Hum
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?
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?
In theory, theory is the same as practice. In practice it's different.
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Re: Plexi 6v6 Hum
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!
Sheilding the wires on the first two gain stages will make the most difference in this 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!
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!
Re: Plexi 6v6 Hum
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...
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...
In theory, theory is the same as practice. In practice it's different.
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Re: Plexi 6v6 Hum
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!
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!
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!
Re: Plexi 6v6 Hum
Remember that if you add shielded cables, you're adding a small amount of capacitance, and it will dull the highs a bit.
I build and repair tube amps. http://amps.monkeymatic.com
- Littlewyan
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Re: Plexi 6v6 Hum
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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- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Plexi 6v6 Hum
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!
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!
- Littlewyan
- Posts: 1911
- Joined: Thu Sep 12, 2013 6:50 pm
- Location: UK
Re: Plexi 6v6 Hum
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
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!
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!
- Littlewyan
- Posts: 1911
- Joined: Thu Sep 12, 2013 6:50 pm
- Location: UK
Re: Plexi 6v6 Hum
Try servicing a Trainwreck!
Re: Plexi 6v6 Hum
Speaker jack ground goes to same place as PT CT and main filter cap ground.
Power tube imbalance? Change tubes
Power tube imbalance? Change tubes
Noise issue
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.
Re: Plexi 6v6 Hum
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.
Wait one......
This is supposedly the best scheme for Marshall type amps.
The guys over at Metro really liked this.
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Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Plexi 6v6 Hum
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.
Re: Plexi 6v6 Hum
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.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!
Maybe a bad mica cap, if it was mica, and not the values?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.