Help me understand the Express grounding scheme

[Firestorm]

I'm in the process of building an Express-like amp to use as a testbed for some of the circuit variations I'm curious about. Seemed like it was time to get empirical instead of posting theoretical questions.

Everything in a "stock" Express (as if there were such a thing) seems clear enough, except for the "wacky" tone stack (one of the things I want to experiment with) and the power supply grounding scheme.

All of the stages in the amp have decoupled power supply nodes (except that V1A and V1B share one), BUT all of the filters are grounded in the same place. This is a textbook no-no. So my questions are: 1) does anybody think meaningful signal interactions are happening at the common ground that I would screw up if I wired it "correctly"?; and 2) has anybody already compared the effect of grounding the caps in the "correct" places, versus using the single ground point?

BTW, once I get the chassis together, I'll stick a pic of it here and solicit ideas for things that might be fun to test.

[nickt]

Hey FireStorm - which layout were you looking at? I've only found one for the liverpool or pictures. Thanks!

[Firestorm]

Hey FireStorm - which layout were you looking at? I've only found one for the liverpool or pictures. Thanks!

I'm working off Francesca (and Dana's schhematics based on her).

[gearhead]

With the exception of the speaker output jacks being grounded near the Power Tubes, believe that drhulseys cleaned up layout (and schematic) are accurate. The output jack shown in the layout doesn't self-ground.

https://tubeamparchive.com/viewtopic.php?t=2930

[nickt]

Thanks Firestorm.

I think the ground scheme used in trainwrecks is pretty similar to the one Hoffman recommends. His take is here in case you haven't seen it.

http://www.el34world.com/charts/grounds.htm

I'm in the same boat myself with grounding - I'm not keen on the "stack-o-caps" and want to have a cleaner PS and add power scaling.

cheers
Nick

[Firestorm]

In my own designs, the power supply node filter grounds are separated: main filter, screen filter, PT center tap (if there is one) and bias supply ground all in one place; PI supplies and preamp supplies grounded sequentially along the grounding buss, ultimately grounded at the input jack. Essentially a two-point star ground.

But the question I was really asking was whether, in a KF Express, Ken A) (dare I say it) made a mistake; B) found it didn't matter and used common power supply grounds and random preamp grounds; or C) was doing something very deliberate in grounding everything the way he did. God knows he broke a lot of other "rules;" was this another?

DK

[Dai H.]

if it's working without hum and buzz then maybe it isn't a mistake. From looking at drhulsey's physical layout, I would guess it works because the green wires are separate to the ground point and the B+ center tap is towards the right direction on the chassis. Say if the green wire from C15-17 went directly to C19 and C20 negative terminals there would probably be a problem, but since those filter cap grounds are starred (avoiding a common impedance) that makes it okay. Maybe you could see the preamp filters as being on the edge of the preamp side grounds. Hmm... actually I've never built a clone or heard one that was built like an original up close. Are they quiet built like that?

[gearhead]

It really seems to vary from build to build, but yes. I finished one two weeks ago, and was expecting to have to spend a week chasing things down. But once it was up and running, other than slighlty readjusting 2-3 wire runs bychopsticking, it was spot on.

[Dai H.]

it looks very normal Marshall/Fendery. I don't know if this makes sense but sort of a bus using a giant flat conductor (i.e. the chassis) going diagonal from front to back.