Components soldered to pots and jacks

[Paul P]

Is there any reason to solder resistors and capacitors directly to
the pots and jacks other than convenience ? In most cases I can
see that it saves a wire but in the case of the vibrato intensity pot
in brown fender amps the .1uf/200v cap could just as well have been
on the board.

I intend to use star grounding in my amp and it would be easier for me
to have the components on the boards rather than the pots and jacks.

Paul P

[Enzo]

But mounting a part on the jack doesn't negate the star ground. Wouldn't there be a wire to the star point from the jack anyway? And adding a brightness cap across a pot for example, will have zero relevance to a grounding pattern. And to board it might add unwanted lengths of wire. Can we find examples where it might be arbitrary? Sure. In which case, change it if you like. The design decision might have been made for reasons of space rather than electrical ones.

[Paul P]

But mounting a part on the jack doesn't negate the star ground.

In a lot of cases it looks to me like the part is mounted there because it can then be grounded
to the chassis right there, so you don't need a wire back to wherever you came from. But this
uses the chassis (or a buss bar) for grounding, not what I'd consider a star ground where all
the grounds are separate and converge back to a single point on the chassis.

Wouldn't there be a wire to the star point from the jack anyway ?

That's what I want to do. For example, for the normal preamp (in a Fender amp) I want to
isolate the input jacks from the chassis and ground them, and the resistor, at a point on the
preamp board. This point would be the ''end of the line" of a branch of the star ground system.
The ground wire would then go to the first triode's cathode ground then to the tone stack
grounds (the treble and bass caps on the pots would be moved back to the board and have
extra wires to the pots), then the recovery triode's ground at which point it would join with
the indentical ground branch coming from vibrato channel preamp. All the grounds from the
different circuits would converge this way all the way to the chassis ground by the power
transformer.

And adding a brightness cap across a pot for example, will have zero relevance to a grounding pattern.

I agree for the case of a brightness cap which doesn't go to ground. But the caps on the treble
and bass pots and the resistors on the input jacks do go to ground at different points on the
chassis.

And to board it might add unwanted lengths of wire. Can we find examples where it might be arbitrary? Sure. In which case, change it if you like. The design decision might have been made for reasons of space rather than electrical ones.

There is only one case which looks a bit arbitrary and that is the .1uf/200v cap on the intensity
pot which could have been placed on the board without running an extra wire. I'm wondering if
the operation of the cap is not affected by its distance from the pot (newbie here). I think that
sometimes you want a cap to be as close as possible to whatever it's shunting to ground and I
was wondering if putting the cap a fair distance away from the pot would have any adverse
effect.

Paul P

[Enzo]

The trem cap is about as uncritical as it gets, and it is not even in the signal path. One could easily put it on the board. Is there room on the existing board without rearanging things? it could be anywhere you want it. You mention a cap shuntng something to ground, but I don't think that is much of an issue. ANd I don't think that 0.1 is connected to ground anyway, is it?

The whole point of star grounding is to eliminate shared return current paths for different parts of the circuit. In my head, the ground that the input jack and first stage cathode share can be connected to the system ground at most any point on it. That is, once wired together, a wire from them to the rest of the works can be attached at the jack or at the cathode resistor bottom. There will be no current between them, so there is no worry. If you are concerned over like a 1 meg across the input jack, it disappears electriclaly when a guitar pickup is connected to the input jack.

When Fender grounded something to the pot cover instead of running a wire somewhere, it is purely a matter of convenience for Fender. It works well enough for their purposes. It would not have sold more amps to add extra wires and separate grounds, adding to labor costs. They were not pretending to have star grounds. You will also find amps where one side of the heater 6VAC is simply grounded, and only one 6v wire trails across the sockets instead of a pair. This works well enough, maybe not as well as a center tapped system, but they didn't care, it was good enough.

[Paul P]

Thanks Enzo. I wanted to make sure I wasn't missing something, given that
I'm straying a bit from the beaten path.

[jrfrond]

For the most part, you never really have to mount any components on the pots, as this is strictly a technique used in PTP wiring, and ALMOST never on units with PC boards. However, citing Fender as one of the benchmarks for PTP wiring design, there is nothing wrong with mounting ANYTHING on the pot. In a departure from Fender's design, I ALWAYS bus the pots together just in case one comes lose from the chassis, or develops resistance in the ground due to galvanic reaction between the pot bushing and chassis. But Fender's wiring design was rock-solid, quiet and stable (in the pre-CBS era anyway), and that was WITHOUT a star-ground system.