It's Alive! The Amp Rebuild Part 3.

Open chassis ready for a line check.
Outlet for the lightbulb series limiter in top left corner. 

After soldering the last few connections to the output tube plates, and connecting the various B+ voltages to their selected points, it was time to power up. Not a full power up, but a graduated one that would start with a 75 watt lightbulb in series with the amplifier to prevent destroying any of the precious components: transformers, tubes, capacitors, or staring a fire.
For the first power up, I would do so without any tubes installed, just to make sure. 


If you are curious about putting a 75 watt bulb in series for debugging, read my previous blog post "Amp Rebuild, Part One." 


With the 75 watt bulb in place, AC cord in the power outlet, and safety goggles on, I flipped up the mains switch into the ON position. To my relief, no sparks, no fire, no snap, crackle or pop. With the necessary precaution, I measured the voltages across the primary and secondary winding with the voltmeter, as well as the B+ voltages, with everything checking out just fine, if not slightly under voltage due to the loading created by the 75 watt bulb. It is important to be wearing rubber shoes in order to prevent your body from being the path to ground, and electrocution . 


I then turned the amp off. Since there are no tubes installing in the circuit, the smoothing filter capacitors do no discharge, which means they can be storing up to 450 volts in some cases. With one hand in my back pocket, I connected a short clip lead to the chassis and proceeded very cautiously to touch the positive (+) terminals of each of the polarized capacitors in the circuit. Remember, it is very important to be wearing rubber shoes in order to prevent your body from being the path to ground, resulting in electrocution. Be advised, when discharging polarized capacitors, large sparks are likely to leap from the positive terminals to your clip lead as the capacitors ground out their charge. 


Still with the 75 watt bulb installed, I repeated the power up, except this time with tubes. Again with the precaution of wearing rubber shoes and keeping one hand in my back pocket and other holding the voltmeter test probe (making sure not to touch any B+ voltages and ground points simultaneously, which would cause a short), took all the measurements again. This time the voltages were even slightly lower, to be expected with the tubes installed (drawing more current, lowering the voltages). Again no sparks or pops and voltages to be expected with a 75 watt bulb in series.


I then repeated the same test with a 150 watt bulb in series, then full line current/voltage, and everything looked great on paper. The true test came when I plugged my Stratocaster in and turned up the volume controls just to see if I got any hum from the single coil pickups. Nothing. Not a peep; the amp was dead silent. Problems. Hopefully not serious ones, but problems nonetheless. 


It's alive, but it's still not talking. The saga continues.