Shielding Plate Wires

[defaced]

I've got a high gain design that requires that the plate wires from the OT pass under the entire preamp board, including the input stages. Based on some stuff Steve has mentioned about shielding plate wires, I've got an idea to shield them by running them through a copper conduit under the preamp boards. This will provided shielding and a nice easy way to lead dress. My question is this: for this to work, is it required that the conduit be grounded? If so, should I ground it to the chassis or to the preamp ground (logical flowing buss bar with tight local decoupling and such).

[dai h.]

yes I don't think you would leave it floating. It would have to be grounded. I'd just connect to chassis. IIRC Steve (C.) was doing something like that in one of his homebrews, and seen something like that in one or more amps by Larry (of Larry Amplification--try his site or perhaps metroamp bbs. for chassis pics/examples. I think he used a plate and not a tube however). And Bogner (Ecstasy) might be another one to check out (*I think* that one had the OT placed on the other end of the chassis relative to the PT). Also, the OT wiring should probably be at a right angle relative to the preamp stuff so that they don't interact as much as possible (as they should if tightly paralleled with no shield).

[black_labb]

i often put copper shielding tape (for shielding guitars) over sensitive wires and the heater supply. not sure how much of a difference it makes, but since i started doing that my amps have been quieter, but i also started to learn more about correct grounding as well so its not all the shielding.

[jrfrond]

Or, you could use tinned copper braided sleeving like the did in the old Ampeg B15N design.

[Enzo]

Or drill a couple holes and run the wires outside the chassis and back in

[Steve Conner]

Yes, it should be grounded, ideally to the chassis rather than the preamp bus.

I've done a few variations on the theme, because I like to put the OT at the input jack end of the chassis, the reason being that it lets me put the power supply at the other end. I've used sheet metal shields and copper tape ala Black Labb.

I think I once even tried two lengths of RG-174 miniature coax, with the cores connected to the primary ends and the screens carrying B+ to the centre tap. B+ is the same as ground from a shielding perspective, because it's bypassed to ground by a big capacitor. Wouldn't risk this with ordinary audio screened cable, though.

If your OT is a foot mounting type, you don't have to lead the wires into the chassis right at the same place where it's mounted.

[novosibir]

... and seen something like that in one or more amps by Larry (of Larry Amplification--try his site or perhaps metroamp bbs. for chassis pics/examples. I think he used a plate and not a tube however).

Dai, that's correct!

I do it this way in all of my amps, because the OT is sitting on the input side of the chassis and then the incoming primaries & secondaries would interact with the highly sensitive preamp section.

I'm using an aluminum shield beyond the incoming cables, so that they're running like in a sandwich (chassis & shield) until they reach the rear chassis' edge. The shield is between the cables and the board.

Another aluminum shield is located between the OT's cables and the 3 jacks of the effects loop.

Check also the attached photos - 1-st still without, 2-nd with the board!
The example is one of my DINO939 amp heads.

Larry


PS: Had to diffuse one area and some components, because I want to avoid, that maybe anywhen a 'Dynosy' kit will be offered from a company located far east

[defaced]

The grounded copper tube has worked out well. So far this build is going pretty good considering the changes I've made. I can't wait to hear what it sounds like cranked, but first I need to find out what is causing unpleasant distortion on low volume notes - like rolling the volume on the guitar back or turning the gain down.