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Old 20th August 2008, 10:03 PM   #1
pjanda1 is offline pjanda1  United States
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Join Date: Jan 2005
Location: Boulder, CO
Default Resources for PA - Guitar amp conversion

A few weeks ago I successfully converted a Rauland PA amp to guitar use. I found a couple of great resources, and I wanted to leave some links hear for posterity (so folks can find them in the future).

Here is a comparison of various tone stacks. I used the component values here to modify the stock Baxandall tone controls (the most common type) so they have decent contours for guitar use. I also added the mid boost switch described.
http://amps.zugster.net/articles/tone-stacks

I wanted an easy way to create distortion, and found several resources discussing "diode" distortion. I added this circuit. It sounds great and will be really useful for practice, as you get a nice distortion sound at low levels.
http://www.instructables.com/id/Add-...ur-Guitar-Amp/

If you've googled, you've seen this link:
http://www.geofex.com/Article_Folders/old2new.htm
It is useful none the less.

My conversion: I removed and bypassed the mic input transformers. I replaced all the electrolytic caps. I put a JJ 40/20/20/20 under the chassis to replace the stock can (30/10/10/10). (Which did nothing for my hum problem, it turned out to be a couple of cold solder joints left by a previous owner's botched mod). I measured most of the resistors and replaced 5 that were out of spec. I added a 3 prong cord. I replaced the bias supply diodes and one bad 12AX7. I modded the tone controls and added the diode distortion circuit. I also added switches to run two of the preamp channels in series. This makes it possible to overdrive one preamp channel or make up for some of the gain lost in the diode distortion circuit.

That's it. FWIW I heartily recommend the Raulands. They are built like tanks.

pj
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Old 21st August 2008, 07:23 PM   #2
m6tt is offline m6tt  United States
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Join Date: Aug 2007
m6tt's conversion tips:

-MAKE SURE IT IS NOT A HOT CHASSIS AMP!!!!!! This is often, but not always, indicated by the lack of a large filament transformer, by tubes which use strange voltages (35v, 50v, 117v) for heaters, and by the powerful electrical shock the guitar player will receive when playing your amp while standing in spilled vodka.

-WATCH OUT FOR 70V TERMINALS!!! These are average 75v, but can swing much higher. This is AC just like a wall socket, and can kill you dead just like a wall socket. For safety, you can internally disconnect the transformer leads and put a few layers of electrical tape over them.

-if it ain't broke, don't fix it: find the original schematics. Many aspects of a PA amp can be left unchanged. Check especially coupling cap values, but the PA is already designed to amplify a low level signal into a speaker. There's no point spending years turning it into an exact fender knockoff when with a simpler mod it may sound better & more unique than any fender.

-the original little black electrolytic (?) low-voltage caps for bypass are all trash.

-"measure" twice & solder once: make *sure* that everything that needs to be attached to a given tag on a strip or tube socket is attached so you don't have to resolder too much, leading to cold solders and mysterious problems.

-use solid core wire if you have any, it's easier to work with (braided wire may have a slight advantage electrically in some cases, but it can be messy to desolder when experimenting)

-for tube stuff, a high wattage soldering iron is always useful. Busbars can sink huge amounts of heat.

-From the fun facts file: Use the right tool for the job. A butter knife is a terrible screwdriver.

-take breaks, relax, and come back to it tomorrow (or next week!) if it's getting too unclear. Once you start getting tired and making mistakes, more are sure to come!
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