Bright caps on EXP volume pot

[Noel Grassy]

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.

[castor]

Yesterday evening I had Lara opened for installing vvr, so I did this mod and change from francesca layout to the other one...
really you can hear the difference in sound.. now it's rounder and less bright.

castor

[strato17]

Wow.... I can already hear a huge difference! And I havent even got a chance to crank it. Its alot quieter, and the bright switch doesnt pop anymore.

[geetarpicker]

On my original the lead going from the center terminal of the pot to the center terminal of the switch is white, and the other wire is yellow. Also on my amp the two caps are 500pf and 50pf, not 500/100. That said, I hardly ever use the 50pf position anyway.

Interestingly we are still finding little details that do make a difference in these things!

I had my original amp right on the bench as I was building my personal pair of clones. "Monkey see monkey do" basically. As I copied the original layout I had noticed the bright switch wiring differences in the Ceriatone layout vs my original. At the time I probably thought it was just an error in the layout drawing. I also hadn't noticed that the Ceriatone layout did match Francesca and that my amp was different in this department.

[funkmeblue]

I'm curious of all the little tweaks ken did along the way, would be cool to hear an early express against a newer model. what about the resistor after the volume, do all the "newer" ones have that?

[Noel Grassy]

Yep, I said a resounding, "Doh!".
I guess I thought Ken couldn't have built this amp, it's wrong.

I'm going to wire mine like this too.

Thanks Glen & rooster.

[Fischerman]

I'm glad this came back up...I thought about this a while back but I guess it wasn't so interesting at the time.

[sliberty]

So, the differences as I understand them are shown in my layout snippet a(attached).

If you visualize the signal chain when one cap is selected, the only real difference is whether the cap is to the left or to the right of the switch (the other cap is not in the signal chain at the time).

I suppose that the mere fact that the other cap is connected to the unused switch lug and to the other cap at the tie point could have some impact, but I can't really understand why that would be the case.

So why would left side vs. right side make a difference electronically? Anybody?

[stevek66]

I had one of my amps open yesterday and tried the switch.I guess I prefer the original Francesca layout.I use the 500pf cap as well,like Glen.
I ran both my clones through an ab switcher and compared that way.
I seemed to lose a little bottom end and a little top end wired the revised way.Just sounded thinner on mine.
Amps were both set at pretty much 12:00 on everything.Like I said I always use the 500pf cap as well.Neither of my amps suffers from being overly bright.Both my cabs are 2 x 12 Scumback H75/H55.Could be why they aren't overly bright.YMMV.

[dansamp]

I used a DPTT on-on-on switch just by chance,
it was the only switch I had and it seems to give me three separate settings
the middle position sounds different than the up or down position
FWIW

Dan..........

[sliberty]

A three position switch is actually correct (usually center off though). The middle position is "no bright boost", and up and down are the two different levels of bright boost as the two different caps are switched into the circuit.

[rooster]

OK, lets get this clear: the bright caps in either position effect the brightness of the amp at low to medium volumes. This is called a 'treble bleed cap' circuit, meaning just that. (Of note, with a typical PEC 1 meg pot, running a 10% audio taper - the norm these days - the bright cap will still be doing something when the volume knob is past 12 noon. Glen probably has a 30% taper in his original amp and this may be why he doesn't notice it past 12 noon. I don't know what is in his other amps, whether he was able to find the same pots? I suspect he did but I don't know for sure. In any case, a 10% taper means the pot is sitting at 100K at 12 noon. This leaves plenty of resistance to limit the top end.) The Science: the volume pot has a huge relative resistance - 1 meg - that eats up the top end. When you turn on the bright switch, depending on with one you engage, that spectrum of the signal is allowed to 'bleed' across the resistor without attenuation.

That said, running the switch either way, Francesca or Kelly, the particular cappage will do the same thing frequency-wise. So it doesn't follow that some of you say the treble or bass of the amp is now different with the change. This should be constant in either version.

Now, what DOES change is the noise floor of the amp. One way is quieter than the other. Wired like Francesca, the lead wire acts as a virtual pickup (antena) within the amp, grabbing stray electrons and whatever, and adding then to the signal chain. And this is why I say it seems KF learned something along the way regarding his build. He moved away from one thing and did something else. Oh, is does reduce the 'pop' - and probably because of this antena effect, but this is not the main reason for doing the swap. The amp just sounds quieter with the switch engaged.

[rooster]

Oh, fischerman, yes I see you were on to it. I, too, wish it had some impact on the Build Guide. I bet if you had tried either version, and then stated a preference, somebody would have listened. Hard to say.

For me, if somebody were to post here with something that they were running with, particularily something that was a KF alteration, and it was said to be quieter in the EXP? Man, I would be all over it.

Anyway, fischerman, you still rock!!! And for that matter, have you found anything else?

[sliberty]

Now, what DOES change is the noise floor of the amp. One way is quieter than the other. Wired like Francesca, the lead wire acts as a virtual pickup (antena) within the amp, grabbing stray electrons and whatever, and adding then to the signal chain. And this is why I say it seems KF learned something along the way regarding his build. He moved away from one thing and did something else. Oh, is does reduce the 'pop' - and probably because of this antena effect, but this is not the main reason for doing the swap. The amp just sounds quieter with the switch engaged.

Please understand, you aren't getting an argument from me - I am trying to understand the theory. So please bear with my questions.

I assume that the lead you are referring to is the one that connects to the wiper. Why would swapping the positions of the caps and switch (which together connect across the wiper and the right side lug of the pot) have any impact on whether or not (or how much) the flying lead picks up noise? I would think that thislead would be equally susceptable to noise in both cases. That should be more related to what the lead comes close to. No?

Steve

[Fischerman]

Oh, fischerman, yes I see you were on to it. I, too, wish it had some impact on the Build Guide. I bet if you had tried either version, and then stated a preference, somebody would have listened. Hard to say.

I never tried it the Franny way...I decided I didn't like the extra stuff dangling off of the grid so I did it the way I thought was best (even though it was contrary to the way I had seen KF do it). That's the way I do any amp I build...the way I think is right (which isn't to say it is...but ya gotta go with your gut as they say).

I've read that KF had 'tried everything to make the Express more stable'. This was from other amp builders that knew him and they talked about amps. Apparently Kenny had lamented that his Express builds didn't always turn out the way he wanted and he had struggled at times to get them stable. So anything that appears to have changed over the years (and remained) could just be a lesson learned along the way. I assumed the PI being laid out 'backwards' from many other amps is an example.