amp layout
Thatch,
I agree with you that the RCA input jacks, the input selector switch and the tubes are clustered close together and away from the power xfmer for low noise performance. Since Wards is wanting to eliminate a whole bunch of input wiring and stray pickup capacitance (these lines are not shielded as I understand it) then with the theoretical noise budget improvement this would bring I figured he could afford to run an input line (shielded) from the back apron from a single unswitched pair of inputs, bringing the noise figure back to where it was in the trade-off for esthetics.
Wards,
Thatch has valid points, however someone with experience in low noise layouts could play with the layout and probably even improve on it. If your design art can tolerate it I'd agree with Thatch and leave the RCA input jack up near the tube.
Be aware that the power transformer and its wires, especially the HV wires to the rectifier and the rectifier components in the AC part of the circuitry all radiate electrical hum and need to be kept physically as far as possible from the amplifying circuits. You could even pick up some reduced hum performance by slipping a grounded shield around the filament supply wires (only an advantage if the filaments are run on AC) up to very close to the pins on the tube socket. The shields for the shielded wires must be insulated so that they cannot contact the chassis anywhere except at the one end where it will go to a selected ground tie point. This is a general true statement that applies to most shield wiring whenever the shield is not required to actually carry signal or power. An RCA patch cord is a good example of an exception.
I think that running a single shielded line to the back apron is OK but it must be dealt with carefully. Cluster these input jacks (2) on the opposite side of the rear apron as the AC power cord entry point. You can put your output jacks between the power cord end and the inputs. Try also to keep the output jacks away from the power cord, fuse etc. You should isolate the ground of the RCA jack from the chassis where it is mounted (to avoid ground loop hum pickup). This can be done by installing a small plastic panel around the jack. You must also shield the wires from the jack. Twisted pair inside a shield will be fine. Don't twist too tight. One turn per 3/4-1 inch is plenty. Float the shield (do not connect) at the jack end and at the tube end connect to the same ground point as the 1/2 of the twisted pair that represents the jack outer shield connection (ground) goes to.
The figure 8 tube shield sounds cool (pun intended). How about forming it from spaced stacked plates of identically punched sheet aluminum with uniform spacers. Like the metal lens on a JBL horn speaker?
Rob