Building the Expression Kit — Part 6
by Mark ~ June 2nd, 2008. Filed under: Amplifiers.
At the end of this step we should, in theory, have a working amplifier. The step mainly involves straight one-to-one wiring of the main-board leads to the pots and tube sockets, as well as wiring the power supply leads to the appropriate main-board turrets.
This photo shows the final wiring, after I upgraded the yellow Mallory 150s supplied in the kit to Orange-Drop 600V polyester capacitors. I found the 150s bright/brittle sounding and noticed in my tweaking that they are quite microphonic as well. The orange drops should be the 6PS type from Vishay/Sprague or SBE or the PVC type from Mallory. You don’t want the 715 series polyproplyene capacitors.
You might also notice a grid resistor on the 2nd triode of V1. In my debugging I had played with a grid resistor here to help stabilize the amplifier a bit (keep it from easily breaking into oscillation). A grid resistor is a common tweak seen in real Trainwrecks as well, a common result of the tweaking process. I don’t think I need this resistor anymore, but felt it did no harm to leave it in place.
Another, earlier, overview shot of the main-board wired in place. Definitely refer back to the Francesca wiring pictures for this phase of the project (see Part 1)! You want to make sure that you get the lead dress (wire routing and shaping) correct here!
After completing all the amp wiring, check and double-check the finished product against your layout diagrams and wiring photos. It may help to take a break from the project and come back to do your final checks with fresh eyes. Resist rushing to plug it in and ’smoke test’ the device… you might get smoke! Next time we’ll talk about power up and debugging.
