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Guitar Amps...

Posted 15th December 2010 at 04:38 PM by chris661

Now then (does that make sense?).

Today, I played a small gig at a college (6th form) talent show. Maybe 100 people or so in the audience.

So, some background... I bought an old, faulty guitar amp off ebay. A resistor (1/2 watt one) had burned, as well as one of the output stage transistors. This had got hot enough to short one of the rails to the speaker output. This killed off the vintage Celestion driver in there. Lovely. Next up, the power supply was under-done. The mains transformer had been replaced before I'd got it (odd screws used to hold it in), and the old 2,200uF caps failed to meet half that value. The rectifier was a small chip, stood on the circuit board.

So, using a spare Eminence driver (modified Alpha 10 - felt dustcap, looser suspension), I fired it up, hoping there was some life in it. Life, there was. Lots of volts DC on the output, with a 100Hz buzz on top of that. Needless to say, I switched it off again.
Replaced the output stage, with some transistors rated for many amps, nice big heatsink. Also added a 150VA transformer, 400v@30A rectifier, and one 10,000uF cap per rail. Ca suffice. It'd then do 18v RMS into 4ohm without breaking a sweat. The fried (2.2 ohm) was replaced by paralleling 4x 10ohm resistors, each of 1/4 watt. The tolerances meant that 2.2ohm exactly was made. Brilliant. Sound wasn't bad at all.
Anyway, in search of the sound in my head, I played around with output transformers, after reading that they're often blamed for the valve (tube) sound. Found a 12-0-12 mains transformer, gave it a lovely sound. So, for some time, I was a happy teenager. I even made a 1x12" extension cabinet, to go from the 40w into 8ohm to 80w into the resulting 4ohms. The result was extreme volume, and the sound in my head was still there. Brilliant.

Anyway, forward several months to today.
During practice (an hour before we were due to play), the room was filled with an acrid smell. We all know that smell. The amp carried on working, so, with no screwdriver to hand, I carried on playing, figuring that, if it still worked, it wasn't broken.
We moved the drum kit, PA, etc, with the acrid smell following my amplifier around. I chose to ignore it, and encouraged others to do likewise.
We started practicing in the auditorium, and the smell returned. My solution was to stand further away. On the first chorus of the final run-through, the amp stopped. The light came on, the output fuse was fine. The heatsink was warm, but not glowing. So, after a while of hoping it'd resurrect itself, I gave up, plugged the headphone output of my pedal into the PA - a crude measure, but it worked. Anyway, I got the amp home to find the same resistor that had died first time had cooked again. In fact, no. Cooked is the wrong word. It was hot enough to de-solder the nearby speaker output connection. At least there's some speaker protection... Everything else on the (now even more scorched) board is fine.
When the speaker was re-connected, there was sound. Music, even, from my iPod. Now, I'm not an expert, but when bunch of resistors rated collectively at ~1w manage to burn off (there was no residue) their plastic case, and de-solder a nearby connection in the process, things got very hot indeed. How it has kept working after that, I don't know.

So, with a circuit design I've never seen before (not saying much, but neither has Dad, and he's seen a few), I plan to sort out the slight crossover distortion (though it does seem to complete the sound, so I might skip that part), then figure out how to stop that resistor cooking again.

Well, if you've read this far, congratulations.

Chris
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Comments

  1. Old Comment
    chris661's Avatar
    As an update, the amplifier has been pushed into bass guitar use. The open backed 10" driver even seems okay with it.

    Now using one of these, having modified the speakers slightly.
    permalink
    Posted 4th June 2011 at 09:48 AM by chris661 chris661 is offline
 
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