Okay… point to point wiring is hard work. I’m starting to understand why it’s not done in mass produced amps.
No surprise there I suppose… but it really does take a lot of planning and the process itself is a lot slower than mounting components onto a board. Still, I’m making progress. Last night I finished the wiring of the power tubes, including the master volume and prepared the power and ground for the inverter circuit, which promises to be one of the more complicated portions. I haven’t tested this new work yet, but I hope to do so tonight before pressing on with the wiring. There is a small chance that I’ll be finished this weekend, but it is more likely to be next week, particularly if I want to have the attenuators installed… which I do.
Speaking attenuators, I’ve updated my design to a three-level attenuator, mounted in the doghouse on the top. It will be controlling it via a 2 pole, 4 throw rotary switch mounted on the back next to the output jack. Unfortunately, I will not be able to install it in the small hole I drilled for the toggle switch so now I need to decide whether I’m going to live with that imperfection or try to manufacture a new rear plate. I’ll see how much energy I have this weekend.
The four positions on the switch will be: bypass, -3dB, -6dB and -10dB. The odd switching arrangement is an attempt at noise mitigation. These resistors are a fair distance from the output jack and the wires will pass over all sorts of noisy circuitry, so I’ll be running a 4 pair shielded cable to the doghouse. The switching arrangement is set up so that each par will carry one send and one return. (Although now that I look at it, this might put any noise it does pick up into a 'push/pull' arrangement... which double the problem rather than mitigate it... I'll have another look later).
Pair 1 carries the outward signal to the bottom set of resistors (parallel) and the return for the top set of resistors (series). Each other pair carries the signal to one of the top resistors and the return from the corresponding bottom resistor.
The capacitors on the top resistors are intended to allow some high frequencies to bleed past and increase the treble at greater attenuation levels. I probably won’t install these initially but am making provision for them in case I want them in the future.
The plan in the dog house is to put the top resistors on one side and the bottom resistor on the other. This means that only one resistor on each plate will ever be conducting at a time, which should help with heat dissipation.
Unfortunately it doesn’t look like there is space for the dummy load in the dog house but it’s a lot easier to find space for a single resistor elsewhere in the chassis and as I’m not worried about noise on the resistor I’ve got fewer restrictions. I still may elect to omit it, we’ll see.