Expression trouble, need help

[geetarpicker]

On my original KF built Express amps, and my 2 clones the potentiometer grounding is critical. Even with the buss bar ground IF the pots get loose or the chassis surface gets dirty under the pot mountings oscillation can result. That said all my amps have bright switches which IMHO are quite important for the best clean to mean response. Only 1 of my 4 amps has grid stoppers and all can easily be run with all knobs full up with no stability issues. Also a stock Express has a Switchcraft input jack, and the ground is from the physical chassis connection only, then the shielded cable is floated ground wise at the tube. As I mentioned make sure your pots have a physical ground connection along with the buss bar, and you might try the traditional original KF grounding methods if the star ground doesn’t work out.




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[ivan]

For now

[ivan]

Less oscillations,sound is too bright.Its sensitive when tach guitar strings-,less hum, like ground issue?

[M Fowler]

It's sensitive in the Presence circuit on these Express amps. Try moving around the NFB wire.

I now replace the Presence with 250kL treble cut pot and ground the tail resistor in the PI section on my new builds.

Mark

[ivan]

Added 1n cap in parallel to 100k resistor in NFB and now harsh treble is gone.No oscillation, but for my taste is lifeless

[geetarpicker]

Your lead dress is considerably different than a KF build, same goes for the ground scheme. Many folks have also not had luck in these uncharted waters. I’d suggest to drop the star grounding and let the pots and jacks get their grounds from the chassis contact. This may require filing away any chassis coating inside to get a good connection, and also the use of non insulated jacks.

[ivan]

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[geetarpicker]

Are you running a grounded metal bottom plate on the amp chassis or some sort of ground plate in the headbox?

[Mark]

The enclosed plate on the chassis must make a difference as I could hear the difference on my Rocket build.

[geetarpicker]

Some amps are more prone to oscillation without a ground plate, especially ones with high gain. Original Express amps were built on an aluminum chassis that is fully enclosed once the bottom is screwed on.

In original Express amps a few wires were typically super glued to the chassis, to keep them from moving/lifting. The lead dress is critical on these amps for sure!

[Littlewyan]

Definitely try the grounding plate and also maybe try rerouting this NFB wire as Mark suggested. I've attached an image to show where I think it should go

[ivan]

Thanks to ya all!
I've found oscillation in second triode of V1.
I put ceramic cap 47 pF between grid and anode and I eliminated oscillation so far.
But it showed bad for the tone itself. It is not organic.
Do you have more suggestion?
I would like to take cause of this away more than consequence.

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[Littlewyan]

Does it go away if you push the grid wire down against the chassis? And did you remember to ground the shield in that shielded cable? At only one end?

[ivan]

Shilded cable is grounded on volume pot,only one side.Oscillation is still there,doesnt matter if i press grid wire down against chassis.

[geetarpicker]

A few things come to mind for starters, basically I'd suggest shorter lead lengths where possible but here are some more ideas:

I'd turn your output transformer around 180 degress so that the primary wires are closer to the output tubes, and then the primary wires can be shortened. My original '89 Express had some oscillation issues unless the primary wires were held down tight against the chassis. On your amp these wires are both elevated and very long, taking a much different path around the chassis.

You really shouldn't need ANY shielded cable anywhere in the amp except at the input, and you never know the shielded cable at v2 could actually be causing issues. I've had some amps where shielded wire actually did cause more harm then good.

Your negative feedback wire is very long. On an original TW this wire takes a very direct path and is also tight against the chassis for most of it's length.

The two wires from the phase inverter to the power tubes are as direct as possible on an original, pressed against the chassis, and don't even cross. Yours are not as direct as they could be.

Try cleaning up the pot wiring, and it appears a lug on your input jack doesn't have any solder!

I'd ditch the star grounding scheme.

Take a good look at the Francesca pics for lead dress. That said I just checked and those pics seem no longer available. You might instead take a look at the "undocumented 1990 Express" pics here>
https://ampgarage.com/forum/viewtopic.php?f=3&t=19

FYI I built 2 Express clones of my original '89 Express. I have not had to add any shunt caps nor inline resistors on the grids even V1,and I only have a shielded wire on the input. Both amps are fully stable when using the bright switches, and can even be fully dimed without any issue. I did follow the exact lead dress and grounding scheme of my original, which is quite close to Francesca. Some amps may require an inline resistor on the 1st or 2nd stage (my '85 Express uses one inline resistor) but typically if things are right you shouldn't need any shunt caps IMHO.

In general I've seen many similar threads to this one over the years, and even though some have had luck modifying the basic layout, lead dress, and grounding schemes most tend to fail. Personally I'd suggest taking more influence from how Ken Fischer did it originally on these amps, as it simply worked quite well!