Using the pots as a ground bus anchor?

[polloelastico]

I'm looking at the Mojotone diagrams and they like to solder a wire across the pots as use that as a ground bus, and then have that wire connect to the input. However, does that make sense without having the pots isolated from the chassis with some kind of nylon or fiber washer? If the intention is to have a single ground point to the chassis, that doesn't seem to do it. Am I missing something here?

 

[King Fan]

Nope, you're right. Merlin points out each of those is a little ground loop. Look, many builders still do it; Marshall did, I believe. The ground loops are probably small enough to be insignificant -- the real downside is it's unneeded, soldering to pots is hard, unsoldering harder (I recently changed out a pot, twice). No big problem if you already did it or were strictly raised that way, but there is zero advantage.

Mojo's a great little shop, but their grounding and AC power wiring are both often idiosyncratic or less than perfectly modern. I always like to compare three drawings -- the factory layout for general geography, the kit layout (eg, Mojo), and a modern good-practices layout (Rob's are a common. useful reference for amps he's drawn).

 

[polloelastico]

Ok, thanks. I was hoping that's the case or else I would have been really confused. I agree about soldering to the back of pots, I really don't like to do it. For tweeds, I've been using the little lugs I get from AmplifiedParts, along with some 20ga wire I twist up to make a floating bus. I think it looks nice and I can build it outside of the chassis and only have to make the last few connections inside. I use nylon washers to isolate the pots and jacks from the chassis, except for the one input jack where it's connected to chassis.

I also tried wiring my preamp heater wires how Merlin suggests and I think I like it. It's a little tricky at first, especially with cloth wire as it likes to fray, but I'm getting the hang of it. It does make me wonder if it's better to do the power tubes the same way, but I can't say I've seen that before. I'm not a fan of flying them, not yet at least. I am, however, really enjoying yanking that 18ga wire and using 22 instead! And yep, I broke a tab on a 9-pin socket when I was cleaning up a previous build

 

[sds1]

I use nylon washers to isolate the pots and jacks from the chassis

As far as the pots go this is not recommended, when ungrounded the pot casings can/will couple noise to more sensitive parts of the circuit.

You have the right idea as far as not involving pot casings in the preamp grounding scheme, go with that but just put casings at ground potential and they'll act like shields instead.

 

[theprofessor]

I think you're on the right track. Your work looks excellent to me. You _can_ solder to the backs of potentiometers in amplifiers, but you can also find better ways to do it. For all the reasons @King Fan notes, I try to avoid it. That "under the lip" heater wiring looks good in that tweed chassis!

 

[mabinogeon]

soldering to pots is hard, unsoldering harder

Amen!

 

[Ed Storer]

I recently completed a Mojo PR kit. I didn't go with the bus soldered to the pots, but used a pair of terminal strips and elevated the bus wire next to the board. I had a little more room to work with that way and avoided lots of little ground loops (that are probably impossible to hear).

I used solid 22 gage red & white bell wire for the filaments. With two colors of wire it's easy to wire the filaments in phase. Probably makes no difference, but why not?! I find cloth insulation messy and I'm careful not to melt the plastic insulation when I solder.

FWIW the amp is very quiet and there's no audible hum until the volume is turned up past 8. At that point, the S/N ratio is still really good.