Isolating Input Jacks & Control Pots

[ALK2MT]

I am currently building an Ampeg R12RB and had a question about the need to insulate or isolate both the input jacks and control pots from the chassis. Is this necessary? Are there any recommended techniques to do this if so? And are there any recommended parts to source like rubber grommets etc.

Thank you!

Alex

[acheld]

Isolating inputs is good practice, as it avoids potential ground loops. And not difficult to do.

I do not isolate control pots.  Never had problems with that strategy as a general rule.   I don't think there is anything special about a Reverberocket in that regard, thoughI've not built one (they are cool amps though!).

And in fact, many people will solder a ground wire across all of the pots.   And while that is fine for having a ground buss, if you ever have to remove a pot in the future, it is a major pain point.

[pdf64]

There's no need / point, to isolate control pot body / bushing / shell from the chassis. I don't recall that ever being suggested, if you think youve seen that, please provide a link to that info.

To isolate the input socket sleeve from the chassis, the simplest method is to use an insulated style socket, eg typical Cliff rather than typical Switchcraft.

[RoadShow]

And in fact, many people will solder a ground wire across all of the pots.   And while that is fine for having a ground buss, if you ever have to remove a pot in the future, it is a major pain point.

Not really, I've had to do it.  Pull the nuts off of all the pots except the one being swapped, unsolder and pull the bus back from the pot letting the pots still attached pull back, and then swap it.  Push the pot assembly back in, screw the nuts back on, and solder the bus to the new pot.  Maybe an extra 5 minutes work taking the nuts off and back on.

FWIW, I also scuff the back of the solder point on the pot with my Dremel.

[pdf64]

Nonetheless, there seems no valid reason to solder to the pot shell.

[bmccowan]

Nonetheless, there seems no valid reason to solder to the pot shell.

Agree - and if you want a ground buss in that location, it's easy to use a fairly stout bare ground wire from one of the pots back to a grounding lug near the inputs. I used to use a 14 gauge bare copper wire, but taking a tip from sluckey, now use 17gauge electric fence wire. Preamp grounds can be connected all along that wire. Sluckey's page on the Plexi 6v6 has some good shots of that method https://sluckeyamps.com/6v6plexi/6v6plexi.htm

[RoadShow]

Nonetheless, there seems no valid reason to solder to the pot shell.

I'm not proposing for or against it, I was just offering a solution to the comment.

Anyway, the proposition is out there, see attachment 

[jjasilli]

There's no need / point, to isolate control pot body / bushing / shell from the chassis. I don't recall that ever being suggested, if you think youve seen that, please provide a link to that info.

To isolate the input socket sleeve from the chassis, the simplest method is to use an insulated style socket, eg typical Cliff rather than typical Switchcraft.

My thoughts:  I agree that pot bodies should not be isolated from the chassis.  This chassis ground connection allows the pot body to further shield the internals of the pot from ambient noise.  The question is where to wire the pot's signal ground.  There's a number choices which probably don't matter much in low gain amps; signal ground to:  back of pot body; buss ground wire across pot bodies; separate buss wire; etc., etc.


I don't think it's necessary to isolate the input jacks, except for higher gain amps.  Then where to ground inputs?  Probably at the Cathode R ground point of the gain stage being fed by that input.