The amp is built on a 10”x6”x2” commercial aluminum chassis. The circuitry is mostly contained on three small double sided PC boards. One 2”x2” board for each channel holds the interstage circuitry, bias adjust pots and the current source for the phase inverters. A single 2.25”x4.25” board holds the power supply rectifiers and filters as well as the voltage regulators.
The boards were designed using “PCB Artist” software and manufactured by Advanced Circuits in Aurora Colorado. Each circuit board and major component is separately grounded to a single point near the input jacks to minimize hum and noise pickup due to ground loops.
I made a layout drawing on Visio showing centers for all the holes in each side of the chassis. I printed it full size, cut it up and glued it to the chassis then center-punched and drilled all the holes. A Step Drill Bit works great for the larger round holes. The rectangular hole for the power entry module was made with a handheld jigsaw. If you want to build this amp and have Visio, send me an e-mail and I will send you the original drawings.
The chassis were given a coating of clear Krylon before assembly to keep the aluminum shiny and resist fingerprints. Unfortunately Krylon is not what it used to be. The older formula retained it’s plasticity and made a really tough coating. The newer stuff is brittle and scratches easily. New environmental rules have caused paint companies to change many of their formulations and this may be an unfortunate consequence. The board and input jack spacers were made from round stock on the lathe.
Adjustments
The only adjustment required is the setting of the bias on the output tubes. Start with the arm of all four Bias Adjust pots at the -20Volt rail. Measure the voltage across the 10 Ω resistor in the cathode of each output tube and slowly advance the corresponding pot arm towards the ground end until the voltage reaches 200 mV. Repeat for all four tubes. That’s it. No other adjustments are reqiuired. Happy listening!