Bright caps on EXP volume pot

[rooster]

sliberty - OK, I see your interest and intent. Consider this then. The amp gut is filled with stray electrons, looking for a place to land. A lead wire AFTER any coupling cap, or 'bridge' cap in this case is fair game. This wire is conducting signal - and it is unsheilded. A perfect mate. Typically you will see a resistor after a coupling cap, in this case the pot. (And let me qualify this, the actual resistive path to the bright caps at this point, the caps being attached to the OUTSIDE lug of the volume pot, is the TREBLE pot. In fact, the caps are ultimately attached to the wiper of the Treble pot, don't forget. Which, when you look at it, makes for an even bigger antena when wired like Francesca.)

Now, not all amps have the same layout. The BF Twin, for example, finds its resistive load on channel 1 way down the road from the coupling cap after stage 2 - and it seems pretty quiet relative. But then it is not high gain. This is the problem. In terms of amplitude, that EXP bright switch cap is a hot potatoe realtive to anything found on a BF amp. But note the signal chain: cap>resistor. In fact, if you could shorten up the signal path, placing that BF coupling cap right next to the 220K resistor before the PI signal cap, the BF would be quieter. It has to be because we removed 12" of antena wire. But then, in this low gain amp, the other components make even more noise and you would never realize that you had improved something. Again. not so with the EXP.

OK?

[57goldtop]

You've got it. The hook-up is either the 1st one or the 2nd one you acknowledged.
Terminate the cap leads that go to the pot lug about 1/2" from the cap
loop 'em and solder a single lead to the pot and shrink wrap. Ken apparently made these leads the same color.( i.e. The switch lead & the lead from the cap junction.)

Here's a picture of that 1st kine. FWIW, this is how mine is.

Is there any reason for terminating the cap lead and connecting a hookup wire ? I guess it's easier to do it like that.

I was able to connect the ends of the leads of both caps to the pot without hookup wire, most of the wire between the pot and cap is heat shrinked so there is no chance of shorting out.

[sliberty]

A lead wire AFTER any coupling cap, or 'bridge' cap in this case is fair game. This wire is conducting signal - and it is unsheilded.

OK, now I think I understand that you are referring to the lead that connects from the caps to the pot. But here is what confuses me.

In the Francesca layout there is a short lead that connects from the caps to the wiper and another short lead that connects from the center of the switch to the side lug on the pot. In the Kelly layout, there are still two short leads - they are simply reversed. The lead from the caps goes to the side lug, and the lead from the switch center goaes to the wiper. I am missing why this would make a difference. In both cases there is a wire (antenna) from both sides of the cap to the two pot lugs. What differes is where the switch sits in the mix.

Sorry for harping on this - its just that you seem rather confident that you understand this, and I want to understand too

[Fischerman]

Draw a simple diagram of the two ways...both with the volume pot about halfway up. See how one way those leads for the caps are basically connected directly to the grid of the second stage and the other way there is a resistance between those leads and the grid? See how one way has only one wire 'dangling off of the grid' (other than the actual wire from wiper to grid) and the other way has two wires there? Two wires/leads are more likely to pick up noise than one wire/lead. That's all it is really.

[leaveitalone84]

Just did this mod last night on my Ceriatone.

Noticeable improvement in the noise floor. Still need to find a less microphonic V1 grrrrrr.

[sst4270]

Okay so as they say: " A picture is worth..."

As rooster said, "any lead wire AFTER any coupling cap" acts as an antenna inside the chassis.

So, for example that would explain why some of us have had great results by incorporating a shielded cable in between volume wiper and Pin 2 of V1.

If I understand the jist of the original post... In the two pictures below I've circled (in blue) the leads from the two bright caps. Those particular leads that I've circled (after the coupling caps) should be made as short as possible. IOW: Solder them right to the wiper lug with zero lead length.

Disregard whether or not they should be going to the wiper or the outside lug. That is beside the point for this discussion. By minimizing the length of these wires you will reduce interference picked up by the leads.

Hope this helps. My $0.02

Steve

[jurgen]

The Francesca layout always leaves at least one unterminated

cap (basically a tuned antenna) hanging off the hyper sensitive grid wire

[sliberty]

Interesting. The diagram above which shows the caps connected directly (without a lead) to the pot is not the way that KF did it, at least in the photos I have seen. Perhaps this approach is better, but take a look at the picture I have attached here. This is an early 90's Express, and it clearly has a lead from the junction of the caps to the pot.

If this small length of wire is actually the contributing factor, I imagine that you could omit the wire regardless of whether you use the Francesca layout or the later Kelly layout.

The other point about having an unterminated cap attached to the grid - that's interesting too. The 90's approach does eliminate that (or at least move the unterminated cap to the side lug). I wonder how much this impacts noise though - since the lead to the grid is so long - and runs under the preamp board - I am thinking that the lead length is a bigger problem than that unterminated cap, but I am not sure.

Interesting discussion.

[sst4270]

SLiberty,

Based on your photo... The signal path goes from the Treble pot to the outer lug of the volume pot> The both caps go from there to the switch. Notice: Very short leads from the other side of the caps to the switch.
(downstream side of the coupling caps...)

For my illustration I took both the snapshots from the Express files section that Rworley posted... They obviously show the signal path going through the switch first and then to the volume pot... then to the grid.

I think either pathway is acceptable. However; the key is the length of wire AFTER the caps with respect to signal path. As you mentioned, the lead to the grid from the volume pot is long. And as I've found out on a couple of builds... using a shielded cable there has helped reduce noise. (Note photos of the Komet builds use a shielded wire there)

Anyone else have thoughts on this?

Steve

[leaveitalone84]

I used shielded wire on both of the V1 grids (setup just like a Komet 'over the board and zip tied together') and have just switched to the "revised" treble cap wiring.

Whisper quiet.

[angelodp]

can you post a gut shot please

ange

[leaveitalone84]

I can probably get one, but it'll be the weekend at the very least.

[rooster]

Now that Ron Worley is back from Europe, I wonder if he has seen this post?

Ron, I hope you think it worth the effort to update the Express building guide after reading this. I know its work but I think the bright switch layout could at least offer a second wiring option. For me, just knowing that Glen is running the layout that is not shown in the current guide is important. .........But its your baby, I understand.

[guitarsnguns04]

I installed my faceplate on my express lastnight and had the solder iron hot to remove the indicator bulb...I went ahead a switched my bright switch/volume pot wiring to the the non Frannie way or the Glen way if you will. I have to say I use the bright switch now where before I thought the amp was way too bright. Very nice improvement and is definately better to my ears this way. Not much difference in the noise floor for me. I dont find the amp to have any noise other than the light hiss they are known for. Anyways thanks for sharing this info..its a very nice improvement on a killer amp!

[Decko]

Hi All,

I just finish my build and wired the bright switch like the "unnamed" express. Can you tell me if this is correct? Allyn?