Building a new Express check the pix !

[Curranproducer]

OK got the shielded wire in place and the osculations are gone! I don't know if I am being weird of not but I think the tone has changed a bit with the shielded wire. I have to be imagining things at this point in the day!

[Curranproducer]

OK got the shielded wire in place and the osculations are gone! I don't know if I am being weird of not but I think the tone has changed a bit with the shielded wire. I have to be imagining things at this point in the day!

[geetarpicker]

You do have a ground plate for the bottom of the chassis? Keep in mind the originals had a closed box chassis, with a screwed on aluminum bottom. It helps keep the amp stable, and you may not need to resort to shielded cable and grid stoppers with it in place. Make sure you try it first with some sort of grounded bottom plate in place, then start tweaking and/or working with the lead dress if needed. Yes shielded wire can sometimes not be a good thing, and my two clones (and two original KF built Express amps) have shielded cable on the input jack leads only. Also only one of my four Express amps (my '85 original) has a grid stopper.

[ampgeek]

Great stuff...and well done Curran!

It is not uncommon for shielded wire to "bleed" off a bit of treble.

Also, in my experience, it isn't uncommon for an amps tone/feel to change subtly on its own (i.e., breaking in).

The problem is that we (me at least!) typically want to start tweaking stuff before that break in so it's hard to tell what really caused a change!

Rock on,
Dave O.

[Curranproducer]

I do not have a bottom plate for the chassis, It is an aluminum hammond 16x8x2. so I May have to try a non shielded wire on the 12ax7 pin 2 but dress it OVER the board like the shielded wire.

[geetarpicker]

Give it a try.

BUD, the makers of the original off the shelf chassis that Ken used also make a bottom plate that screws on forming a complete sealed aluminum box. The plate shields the circuit quite well. That said, even old Marshalls have an aluminum plate in the cab, and blackface Fenders even have metal screen material stapled into the cab for shielding. You should have enough stability that you can dime all the knobs on an Express, but you will probably need the plate on to do so.

[Curranproducer]

well I am going to have to order one and see what happens after i put it back to stock with the wiring!!!! I want to drop a PPIMV in there as well can I use the 250Kdual pot with the 2M2 resistors like on my AC30/rocket i built a few months ago?

[Curranproducer]

OR would a VVR be cooler on this

[martin manning]

Yes the same PPIMV set-up will work on your Express. Opinions vary on whether that or a VVR works better, some will say neither. The nice thing about the PPIMV is that when full-on it's out of the circuit. I've said before that a Vox-style cut goes well with the PPIMV.

[Curranproducer]

cool I am going to do that today! haha!!!

[Curranproducer]

OK so to drop in the PPIMV i will need to remove the 220K resistors and route the leads to the dual gang pot. BUT I will need to put jumpers to the bias turret from the PI caps to keep the bias connection running correctly, yes?

[martin manning]

The bias supply connects to the CCW lugs of the pot elements, the PI coupling caps connect to the CW lugs, and the wipers go to the power tube grids. The 2M2 resistors go from the CCW lugs to the wipers.

[Curranproducer]

and the bias supply is only like -33mv so that is safe right?

[martin manning]

Bias voltage is around -33V, and there is very little current flowing through the pots. Are you worried about the power dissipation? If so, it's not a problem; audio pots are usually 1/4W. What kind of wire are you using for the MV?

[Curranproducer]

I was going to use my cloth push back stuff rates at 600V, but I can use shielded instead