Grounding Input Jack, Speaker Jack, Pots

[dbishopbliss]

I'm still planning out my Bandmaster restoration.  I've read the Grounding Info page, but I don't understand how soldering a buss wire across the back of the pots, to the jacks etc. will improve the ground when they are already bolted to the chassis.  Or, is he suggesting that the jacks, pots, etc be isolated from the chassis.

[eleventeen]

Part of the answer to this question is, it's an inexpensive "best practice" thing to do that might cut hum & noise by a coupla percent. After all; if I am dependent upon the steel of the chassis (steel: not the best of conductors, but OTOH, there is a lot of it) then that chassis subject to all manner of currents running around in it, in quite the unpredictable basis. Those pots are in and/or before the high gain part of an amp. You would really, really like for them not to be a problem, I suspect.

If you look at the traditional and most (by far) widely used flavor of control, CTS; or, if you have ever taken one apart, you will see that the enclosure ("can") of the pot is held onto the front (where the threaded bushing which goes thru the chassis and is hopefully star-washer mounted) by four fold-over tabs. Undoubtedly, those are folded over by machine which stomps on them pretty well during manufacture and probably makes an excellent connection to the bushing part of the pot.

Until you move to Los Angeles and get a little bit of smog in the air, maybe. Fast forward several years. Hot > cold > hot > cold > hot > cold, inside an amplifier. Vibration. Chemical reaction/corrosion possible. Throwing that ground wire across the back is just an OCD way of ensuring that the grounding scheme that existed the minute your last solder joint firmed up persists on a permanent basis, and you are sure of it. >>IF<< it turned out that this was a necessary thing for you to do to reduce hum; it's maybe 5x easier to it while you're doing the build than afterwards.

[Willabe]

but I don't understand how soldering a buss wire across the back of the pots, to the jacks etc. will improve the ground when they are already bolted to the chassis.  Or, is he suggesting that the jacks, pots, etc be isolated from the chassis.

If you solder the pot tab that goes to ground to the back of the pot then it is grounded through the pots frame to the chassis. Like 11 teen says you could lose your ground connection because of corrosion. If you solder a buss wire to the back of the pots and attach 1 end of the buss wire to the chassis then you won't lose your ground connection.

They did it that way originally for ease of manufacturing, less time. I doubt anyone at the time thought someone would still be playing the amp long enough for corrosion to corrupt the ground connection.

I don't like to solder a wire to the back of the pots for a couple of reasons. One is it's a hassle to change a bad pot latter and 2 it forms a ground loop by definition. The pots frame is grounded to the chassis and then it's grounded again through the buss wire. Granted it's a small ground loop and it might not cause any problems but it might as it is a ground loop.

If you don't solder the pots ground tab to the back of the pot just run a wire instead to ground. You can still use a buss wire.

If your still worried that you'll loose the shielding that the pots shell/can frame provides after many years you could also add a separate ground buss wire to the back of the pots but I think that's over kill.

Here's a link on more grounding that's worth reading;

http://www.valvewizard.co.uk/Grounding.html



                        Brad              

[floyd]

I'll never solder a buss to the back of pots again. I've NEVER had a problem when I use a lock washer between pot and chassis.

[Willabe]

I've NEVER had a problem when I use a lock washer between pot and chassis.

It takes 20 to 30, maybe more years for the corrosion to become a problem.


            Brad     

[Ed_Chambley]

I never solder to the pots back for obvious reasons.  I use a 2 point ground star.  One only has earth from the cord, the other is at the transformer bolt.

Granted, in years time it may have problems.  Like recently I purchased a 65 Fender Twin.  I had to remove all the pots and clean the surfaces and replace some of the grounds fender made.  Big deal, it took 30 minutes and did not change the floor noise, but changing the plate resistors did.

There are different grounding methods that work well.  I isolate jacks and use a star ground and never intentinally great a ground loop.  BTW, I am very partial to a quiet amp and on any of my builds, I am inside the chassis doing something often.

On Marshall builds I run a copper buss through the pots lugs bending them forward and at a right angle, but not to the input jacks.  Easy to remove the pot if it goes bad by splitting the lug with side cutters.

It is fine to find what you feel works best, this is just a proven way and it works.  Doug Hoffman is a great amp builder and I will assure you anything in his library works and works well.

[jjasilli]

I'm still planning out my Bandmaster restoration.  I've read the Grounding Info page, but I don't understand how soldering a buss wire across the back of the pots, to the jacks etc. will improve the ground when they are already bolted to the chassis.  Or, is he suggesting that the jacks, pots, etc be isolated from the chassis.

I had the same question when I joined the Forum; plus a concern re ground loops.  But the empirical evidence is that Doug's grounding scheme works.  This is important for production work.  Also, it enables all the pots to be temporarily placed in a jig (such as backwards into the amp chassis).  This provides easy access to pre-wire lead wires and components to the the pots.  The ground bus wire keeps the pots in place & all lined-up for final installation as a group into the amp.  This also facilitates production.

[sluckey]

Here's an alternative to the pot buss...

[Willabe]

But the empirical evidence is that Doug's grounding scheme works.  This is important for production work.  Also, it enables all the pots to be temporarily placed in a jig (such as backwards into the amp chassis).  This provides easy access to pre-wire lead wires and components to the the pots.  The ground bus wire keeps the pots in place & all lined-up for final installation as a group into the amp.  This also facilitates production.

All good points and all true IMO.


               Brad