This recent iteration of the 100w Marshall head arrived with no output. I surmised it had a blown power tube and thereby hangs a tale. A couple of tales, actually.
With today's move to more solid state devices and channel switching, well, you gotta get power from somewhere to run the little devils, and it's not always obvious to the eye when something in that line fails. I've also noticed that in the time it takes for a fuse to respond, a lot of damage can be done. Such was the case with a Peavey Valveking that I worked on a while ago.
Opening the chassis up I found a bulged power supply capacitor C49, a blown out bias resistor that had scorched a nearby capacitor and relay, a resistor that had turned from blue to brown R81, and I figured replacing those items would be required in any case and might fix things as well. In addition a new fuse and a set of new EL34s were installed.
The net result was zero. A quick check plugging a guitar into the effects return jack proved that the power section was working as advertised so the problem had to be somewhere else.
I was stumped.
As usual I happened to be trolling my favorite amp tech type chat boards and found a post by the ineffable Bill Machrone, a/k/a Billm, who's more or less the god of Blues Juniors. It was entitled
"How to make an audio test probe."
I was intrigued because I'd built something similar but a lot more elaborate than it needed to be. This was cooked up with a small amplifier from Radio Shack, a chunk of coax cable, a .01 capacitor, and a bit of heat shrink tubing. I knocked it out in about fifteen minutes. I already had a Leader audio signal generator from the last project-and it's a good one. Leader is some of the best test equipment i have ever seen at any price. This one cost about 25 bucks.
I fired up the amp and plugged in the signal generator. The first thing I noticed was that the tone didn't get past the first preamp tube .A quick voltage check revealed that the preamp tubes were not getting any high voltage although the phase inverter tube was doing fine. At this point I found it very useful to draw out the B+ circuit to see where I could intercept voltage, test for continuity and thus localize the problem. It was pretty quickly narrowed down to R82, a 15k 1w flameproof resistor that turned out to have about 100 megohms of resistance. A new resistor put voltage in all the right places, and luckily enough no other damage had been done.
The images show the parts that ended up being replaced and the signal generator and tracer rig.
The takehome for me was Bill's clear and concise explanation of what his signal tracer could do and how it could be used. It is HIGHLY recommended.