Cap outer foil connections in Express

[eazilyled]

I bought a set of PVC caps from RJ that I plan to sub into my Express build.

I don't have a scope, but I managed to determine which lead on each cap was connected to the outer foil as follows:

I plugged a guitar lead into an amp set fairly loud. I then gripped each cap between my finger and thumb, and placed one lead on the tip and the other lead on the ground of the guitar lead jack. I then swapped the cap leads over and listened for where the hum was quietest. The outer foil lead was then the one touching the ground of the jack in the quietest position.

One initial question is whether the above procedure was correct.

My main question, however, is where to connect the outer foil leads in the circuit.

I have read that the outer foil needs to be connected to the lowest impedance side of the circuit.

So in the Express, is it as follows:

C3, C4, C9, C10, C13, C14 (the caps that are vertically placed on the board) - outer foil closest to tube sockets.
C8 (coupling cap between 2nd and 3rd stage) - outer foil closest to V1.
C11 (presence cap) - outer foil closest to ground connection.

One thing I found with the PVC caps I have is that they don't don't have an outer foil marker even though it looks like they do. It was 50/50 as to whether the mark matched the outer foil. This is consistent with what others have reported previously.

I don't know whether the PVC caps originally had an outer foil marker, but interestingly the Francesca pics show that the marker is in the opposite positions to what I have listed above (apart from the presence cap).

[martin manning]

Your procedure using an amp to find the quietest orientation works fine. Usually the outer foil will go to the upstream (signal-wise) node. In the tone stack, I would put the outer foil to the wipers of the bass and mid pots.

[eazilyled]

Thanks Martin.

So, to recap I need to orient them as I said except for the mid and bass caps.

I was just looking at the schematic and I can see why you'd want to put the outer foil to the wiper of the mid pot, but was trying to understand why you would want to do this for the 1M bass pot?

Is it something to do with the fact that bass pot is audio taper and is usually set in the middle of its travel?

[martin manning]

So, to recap...

<rim-shot>

I was just looking at the schematic and I can see why you'd want to put the outer foil to the wiper of the mid pot, but was trying to understand why you would want to do this for the 1M bass pot?

Is it something to do with the fact that bass pot is audio taper and is usually set in the middle of its travel?

Yes!

[eazilyled]

I wish that cap pun was intentional

Thanks again

[Cliff Schecht]

Your procedure using an amp to find the quietest orientation works fine. Usually the outer foil will go to the upstream (signal-wise) node. In the tone stack, I would put the outer foil to the wipers of the bass and mid pots.

If I am hooking up outer foils I always place the foil side towards the lower impedance node. IME anytime something is shielded you hook the shield side up to the lower impedance side (whether this be ground, a DC voltage or the output of a stage). When I'm connecting caps to a tonestack I make the outer foil point towards the output of the driving stage. If any side of a cap connects to ground, I put the foil to ground. You never connect the shield of a cable to a high impedance (that is if you want effective shielding) and I believe the same rules apply for a caps outer foil.

[eazilyled]

Interesting explanation Cliff, thanks

[Cliff Schecht]

I'm interested to see if Martin agrees with this as it seems putting the foil towards the wipers of the pots is backwards from this logic. The wiper side of the pots all point towards a grid input with a typically high input resistance so this seems backwards to what I understand to be correct.

I've seen circuits that are essentially active shield drivers for cables to guarantee a low output impedance and make the shielding more effective. Granted stuff like this is typically used in high sensitivity applications like medical devices (in my case it was when I was amplifying nanovolts from the brain) but the idea of driving shields with the lowest impedance node carries over to what we are doing.

[martin manning]

Cliff, the path to ground through the tone pots and the following volume pot is I think slightly lower impedance than the path back through the slope resistor and the Zo of the driving stage. I have a whole-amp Express sim handy here so I injected a "noise" voltage on either side of the caps and monitored the amplitude of the resulting signal at the OT secondary. The bass cap is almost a push regardless of where the bass pot is set (treble, middle, and volume at noon), but putting the noise at the wiper wins by a few mV at the output. The mid cap is more obvious because of the smaller value pot, and indeed the amplitude at the output is about 10% less with the noise injected at the wiper.

[Clyde]

It makes no difference which side you place the foil on. The cap is part of the connection (wire). It has no shielding to speak of.

[Cliff Schecht]

It makes no difference which side you place the foil on. The cap is part of the connection (wire). It has no shielding to speak of.

There are some caps which are made with a deliberate inner and outer plate. This allows the outer plate to be used effectively as a shield. Google it if you don't believe us, it can clearly be seen on a scope which side is an effective shield.

Martin I see what you mean. TBH I think this is damn near negligible and I'd probably point my outer foils towards the plates just out of habit. A few mV of noise after the first stage is relatively negligible, the SNR of the second stage is much better than the first.

[eazilyled]

Cliff, the path to ground through the tone pots and the following volume pot is I think slightly lower impedance than the path back through the slope resistor and the Zo of the driving stage. I have a whole-amp Express sim handy here so I injected a "noise" voltage on either side of the caps and monitored the amplitude of the resulting signal at the OT secondary. The bass cap is almost a push regardless of where the bass pot is set (treble, middle, and volume at noon), but putting the noise at the wiper wins by a few mV at the output. The mid cap is more obvious because of the smaller value pot, and indeed the amplitude at the output is about 10% less with the noise injected at the wiper.

Martin, I'd be interested to see what happens in your sim with the treble / bright caps, assuming you switched them from silver mica to polystyrene or something (and that you are inclined to look at this any more).

[martin manning]

First, I had a large slope resistor in the sim for some reason, so the bass cap is still a toss-up, but actually comes out a few mV better with the noise on the input side (all controls at noon). The Mid is an even bigger difference than I quoted before, though, and putting the outer foil on the wiper results in ~100mV less signal, which is >40%, at the OT. These are all using a 1mV noise at 60Hz.

The treble cap is connected directly to the plate of the input stage, so it clearly goes with thre outer foil on that side. The bright cap is realy obvious, because one side of it is connected directly to the grid of the following stage which has maybe ten times the impedance to ground of the 1Meg volume pot. Caps used in that position usually don't have an outer foil, though, as you noted.

In summary, the "put the outer foil towards the driving stage" rule is good, with the exception of the mid cap.

[eazilyled]

The treble cap is connected directly to the plate of the input stage, so it clearly goes with thre outer foil on that side. The bright cap is realy obvious, because one side of it is connected directly to the grid of the following stage which has maybe ten times the impedance to ground of the 1Meg volume pot.

That makes sense, thanks.

It's time to install those caps!

[Colossal]

In summary, the "put the outer foil towards the driving stage" rule is good, with the exception of the mid cap.

Hey Martin,

I thought I would illustrate this. Would you mind auditing the drawing for accuracy? PM sent.

Thanks,
Dave