Building the Ceriatone Expression Kit — Part 5

by Mark ~ May 18th, 2008. Filed under: Amplifiers.

Input jack and bright cap wiring

Some of the most critical wiring in a high-gain amp like the Trainwreck is around the 1st tube stage… including the Input Jack, the Volume Control, and the route to the grid of the 2nd stage triode. In my project I found the placement of the bright caps and damping microphonics associated with them to be a challenge. The lead dress in this area, including the ground wire and the blue wire leading from the volume control is very critical and I tacked the wires and caps in place with hot glue at several points on the chassis.

Showing the wiring of the presence cap

It is very convenient to pre-wire all the pots and get the ground-bus established before fitting in the main board. One of the hardest things was to get a good reliable connection from the back of each pot to the brass rod provided as a ground bus. The brass rod takes solder well, but the pot cases are another story. I filed the pot cases to roughen the surface and take off any lacquer, but I think you also need a really powerful heavy-tipped soldering iron to get enough heat to take solder well. If you have pots with stainless steel cases (like mil-spec clarostat or PEC) you wouldn’t be able to solder at all and you’d need to find special lug rings that go over each pot shaft to make the chassis ground connection. Separate wires go from the pot connections to the ground bus as required. The yellow wire is the negative feed-back to the presence pot circuit from the 8-ohm speaker lug on the impedance selector switch (see other posting).

Input jack, two-position bright switch and volume pot wiring

The coax lead from the input Jack to the grid of the 1st amplifier stage has its shield grounded right at the jack and left open at the grid to avoid a ground loop and potential noise from that.

Treble, middle, and bass pots wired in purple

Overview of pots and the prewiring

In the overview picture we see the chassis now complete except for the main board.

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