twisted/isolated/shielded wires?

[2bz]

I am building a SE stereo tube amp, a la Tubelabs.

I have a few questions: (1) Which internal wires would benefit from twisting? (2) Would using a shielded wire eliminate the need for twisting and finally (3) Which wires should not be run close together?

Thanks in advance for you help.

[HollowState]

If you are using George's circuit board, most of the critical wiring is done for you. If you're doing point to point, the filament wires should be twisted and run close the chassis and away from signal wires. If a signal wire (or componet) must cross the filament line, do so at 90ºs. Input and output lines should be kept reasonably separated. Putting the filament wires inside shielding can't hurt, but I've found that it generally isn't necessary if you adhere to the above procedure.

With point to point, grounding will be a concern. A central "star" ground or a short ground buss (made from heavy solid copper wire) grounded at one point is proper practice. That point is usually at the input or centralized on the chassis. It can even be near the power supply if that is convenient (depending on location) and produces low hum. I prefer the ground buss approach because it provides for neatness with a convenient place to anchor componets and make signal grounds. The final chassis ground point can be easily found (for lowest hum) with a single wire connection. This is somewhat like adjusting a hum pot across the filament.

Obviously channel 1 and 2 signal wires should not be closely paralleled together to avoid crosstalk pickup. Input signal lines will be helped by using shielded wire, especially if they are long. For short runs, twisting together with a ground wire will provide some shielding.

Follow these simple guidelines and you'll be OK. You could also purchase the book "Building Valve Amplifiers" (Morgan Jones) for more detailed information regarding placement of transformers and other parts.

[2bz]

Thanks for the quick and thorough response. So, assuming that I don't mix wire types, i.e., output and mains, filiment and input, etc., is it safe to go ahead and twist everthing?

[HollowState]

Originally Posted by 2bz

is it safe to go ahead and twist everthing?

No. Don't twist everything. It isn't necessary unless you really like the twisted look. Only the filament wires require it. And this is only necessary if you are using AC to light them. You may twist the input connection together with a ground wire to help prevent hum pickup if you don't use a piece of shielded cable. If the run is short (couple inches) this may not be needed, especially if you are using a metal chassis which provides some shielding itself.

Without knowing just how you're building this, I'll mention one more thing regarding grounding. If you use a metal chassis and plan to mount your input connectors on it, they should be insulated from the chassis. This way they can be grounded by wiring them to the central point and avoid a ground loop. This, of course, assumes your ground point is somewhere other then the the input.

[2bz]

Thanks again. I am using a particle board chassis and plan to install a ground bus and connect the case of all the metalic components (power xfmr, OPT, choke, capacitor, PCB, etc) to it then connect the bus to the mains ground.