Is there a best coupling cap location

[scstill]

In this point to point wired champ, will it matter if the coupling cap is located
closer to the pot, closer to the plate, or halfway in between?
The distance is about 5"

Also could I get away with unshielded wire or should I opt for shielded with single end gnd?

Included the schematic and would appreciate any feedback on the star grounds, MV design, speaker switch or input jack mods. TY

[mresistor]

Not sure why anyone would want a MV on a a 5w amp?  The 33K grid stopper should be located as close to the tube pin as possible, although Fender used them soldered onto the input jacks with no problems.


I would use a couple of insulated stand-offs for holding the .02 coupling cap, because I have a bunch of them and it would provide better support.

[mresistor]

I would keep the preamp ground separated from the Power Supply ground. 

[shaun]

My sort of rule of thumb on placing components in the circuit may not be "correct" but it sorta makes sense to me, which is this: if a cap blocks DC, I want it as near the source as possible. That way, there is less DC-loaded wiring in the amp, if you get my drift. So, if there is a 4 inch run from, say, pin 1 of a 12ax7 to the next connection, I'd put the cap as near Pin 1 as possible, because then there will only be an inch of DC loaded wire. (Unless it caused other problems due to lack of space or something.)

This idea seemed a bit silly when it first occurred to me, but when you add up all the wire in the amp, hum and buzz seem to be caused (in my builds at least) by poor lead dressing and layout choices, and using this concept has helped considerably. Hope that's of some help.

Regarding grounding, it looks like you have your pre section going to a ground lug nearby, which I would not do. So, another rule of thumb I've picked up is to float all my grounds off the chassis and then run a star ground from each stage to a single ground lug near the transformer - this means installing terminal strips. Poor grounding schemes have been the single biggest cause of buzz in my builds. With a good star ground, you can experiment if buzz continues, but it usually doesn't.

[scstill]

Not sure why anyone would want a MV on a a 5w amp?

5w in the house is pretty loud. In fact my 1w amps are pretty loud in the house.
With MV I can overdrive preamp without driving my family crazy.

[Williamblake]

5W is plenty to me, too for the same reasons, but i build meself a power soak to be able to play it loud and have the same sound muted or not, master volume turned up from practice level is a big dissapointment and to to cut 14dB at the speaker-out is two resistors.

[mresistor]

Not sure why anyone would want a MV on a a 5w amp?

5w in the house is pretty loud. In fact my 1w amps are pretty loud in the house.
With MV I can overdrive preamp without driving my family crazy.

I can play my champs wide open with no worries or people to bother.  My larger amps though I use a Weber Mass or Mini-mass attenuator. Headphones might be another option.

[Willabe]

My sort of rule of thumb on placing components in the circuit may not be "correct" but it sorta makes sense to me, which is this: if a cap blocks DC, I want it as near the source as possible. That way, there is less DC-loaded wiring in the amp, if you get my drift. So, if there is a 4 inch run from, say, pin 1 of a 12ax7 to the next connection, I'd put the cap as near Pin 1 as possible, because then there will only be an inch of DC loaded wire. (Unless it caused other problems due to lack of space or something.)

This idea seemed a bit silly when it first occurred to me, but when you add up all the wire in the amp, hum and buzz seem to be caused (in my builds at least) by poor lead dressing and layout choices, and using this concept has helped considerably. Hope that's of some help.

That's backwards, because;

After the coupling cap the plate wire is now a grid wire. Rule of thumb good lead dress is plate wire long, grid wire short. Grid wire acts like an antenna so you keep it as short as possible and/or use shielded wire for the grid wire run. You don't have that problem with plate wires.

The dcv on the plate wire acts like a shield. And it's an impedance thing, plate wire low Z, grid wire high Z.

I've never had a problem running the plate wire long and mounting the coupling cap as close to where it's going as I can, soldered to pot or tube pin. 

[shaun]

Thanks Willabe. That makes good sense. I'll take that idea into my next build.

[Willabe]

I should say that electro static fields are based on current through the wire, not voltage.

And the current in preamp plate wires is mA, so very low current.

[pdf64]

My thinking is that the current flowing will determine the electromagnetic field strength.
Whereas the voltage will determine the electric field strength (the term electrostatic, although used by tradition, seems a bit strange in regard of an AC field?).

[Willabe]

I may have the terms wrong, but the problem comes with current more than voltage injecting noise into surrounding circuitry in the preamp and PI.

And I was talking about plate wires with dcv on them as that was what shaun wanted to get rid of by moving the coupling cap close to the source; plate and having long grid wires instead.     

[shaun]

One of the very cool things about this forum is that it gives me a vocabulary to understand the amplifier and its phenomena.  And then I can research whatever is new to me. So, I found a simple explanation from wikipedia:

"The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. ... The force created by the electric field is much stronger than the force created by the magnetic field."

This still leaves me with a hellavalot to learn, but it's a good starting point, especially as I didn't even know the two things were separate :).

So we would have and electric field on the plate side of the coupling cap (which wikipedia says is a stronger field, relatively speaking), and we have a magnetic field on the grid side of the CC. My takeaway from Willabe's post is that, even though the electric field may be stronger, the magnetic field created by current flow can create more noise issues. Or it may simply be that because the lead attached to the grid acts like one big grid (a sort of antenna), it magnifies anything it is not shielded from. Very interesting.

 

[PRR]

> The force created by the electric field is much stronger than the force created by the magnetic field.

I'm not even sure what that is trying to mean without reading Purcell.

And "force" applies itself different ways. Are we forcing electrons?

Here's how I see it. The capacitance from anything else to either leg of the capacitor is 10pFd-100pFd. The capcitance from one leg of the capacitor to the other is, say, 22,000pFd. So it can't make even a half a percent difference which way you put it. IMHO, put it where and how you can see it clearly. We hear of far more problems from wiring and value errors than from supposed cap orientation.

[scstill]

I did it both ways with coupling caps close to the plate and then close to the grid.
I can't really tell a difference.
When close to grid, its connected to pot with shielded wire from pot to tube grid (gnd to preamp on one side).

This build is very quiet without guitar plugged in with vol & MV max
With guitar plugged in its noisy with vol & MV max. (but they all to do this right?)
Noise quiets considerably when vol @ 3/4 to 1/2 & MV max. As well when MV is turned down.
 
BTW - you will only have the choice of placement if its point to point.
If built on a board there will be wire runs on both sides.

[sluckey]

BTW - you will only have the choice of placement if its point to point.
If built on a board there will be wire runs on both sides.

And that pretty much answers your original question.   

[Mungo Park]

I would suspect your noise comes from the master volume wires to and from the pot.