The Fenton Project VII

Everything is wired and time to test!!!!!

As with any new build or extensive overhaul it's best to power up slowly. A variac works well, to apply power gradually. I prefer the safety light approach. It reacts faster than I can read a current meter and it's more or less automatic. A 100 watt bulb will limit maximum current to something under one Amp and that is low enough that no serious damage would result from any problems.

Good thing I didn't trim the Output Transformer primary leads to length - as feared they were out of phase with the rest of the circuit and needed to be reversed. This is one of those things that are easier to identify with a function test than by pulsing the tranny and testing phase.

Now that everything works let's run it up on the scope and see what we get. Ooh, lots of gain. Bias check shows the power tubes dropping 13V across the 150 Ohm Cathode resistor; we've got 370V at the plate for a static dissipation of 15.5W per tube. A bit hot but no hotter than a lot of other EL84 amps. We'll run with this for now. Once we've got a baseline on the sound we'll see if cooling them off doesn't hurt.

Running it up into an 8 Ohm dummy load we get 18WRMS at 400Hz as the top and bottom of the waveform start to round off. Pretty much what you'd expect. Run a few different frequencies and some square wave to test the tone control. Time for a half power / one hour burn in. Everything appears solid and stable, nothing overheats and no extraneous noise appears on the 'scope.

Now for a real test - the test instrument is a Marshall 1936 2x12 cabinet. Plug in, power up, dead quiet. Turn everything up and there's the expected shhhhh of gain noise, but that's all there is no hum, sizzle or buzz. Now the guitar ...