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Old November 26th, 2008, 11:15 AM   #1 (permalink)
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Uma Floresta's Avatar
 
Join Date: Jun 2008
Location: Monroe
Posts: 594
Cheap amp mods: Kustom Tube 12a

Because I can't just leave anything anything stock, I've been working on my crappy little practice amp. It's a solid state power amp, tube preamp, with a "gain" stage that engages a JRC4558D op amp and some clipping diodes to ground. It has an 8 inch speaker, and includes bass and treble controls. $100, a pretty good deal when I got it, though Crate's cheapy new series of tube amps appear to be even better cheap tube amp mod fodder.

What I've done is Fenderize the little guy. The weird tone stack values have been replaced with Fender values. I also added a 10K mid control pot. It had a bad sounding mid shift switch that went from okay to nasal. Now it will go from Fender cleans to a fuller sound with more lower midrange content. I actually prefer the fuller sound, though if it were a bigger amp it might be too full.

I added a couple of 22uF caps in parallel with the cathode bypass resistors, and changed those resistors to 1.5k for accuracy's sake. This drastically increased the volume from the amp, and added a little more bass as well.

There are several areas of high pass and low pass filtering in the circuit. In the high pass filter right before the tube stage, I initially increased the resistor to 1M to let more bass frequencies in. This helps make up for the small speaker size. It sounds great clean, but I wanted the option of a more focused, Tube Screamer type sound as well with dirt, so I changed that 1M resistor to a 1M pot in series with a 10K resistor. So that's an extra knob for cutting bass before the tube stage. Very handy.

I also cut the clipping diodes, so the gain stage just acts as a boost in front of the tube. It will boost the tube stage into overdrive, but you have to play with the bass cut mentioned above as well as the tone controls to get a good sound. I've got VERY good results mixing a slightly overdriven sound from the amp a little bit of bass cut and some mild overdrive from my OLC Professor Tweed and Umble pedals.



Here's a crappy cell phone pic. Most of the components on the amp look fairly low voltage, like parts you'd use in pedal. I used some massive 600v orange drop caps and big 2w resistors, which is overkill, but looks cool.



What does it sound like? Basically it sounds like a lot like a Champ. Before it wasn't quite loud enough to keep up with a drummer - it may be now, I haven't tried it at full volume yet.

There was a fair bit of hum, but after this latest round of mods, it's pretty much disappeared. I have no idea why.

Anyway, all in all, a very nice practice/recording amp, for very little money.

If you do mod your amp, those who own one of these, take proper precautions. Even unplugged, amps (caps really) can store lethal voltage. Mod at your own risk. Safety tips:

http://www.harpamps.com/micKdiy/B-pl...y-Tips%20.html

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