Anyone have experience modifying amps?

[SausageofPower]

I'm looking at getting myself a JCA100H, or a JCA100HDM, however I was wondering if (assuming I can get the former cheaper) I should just snag the JCA100H and modify it to have the depth control.

So...school me. Please.

[Cygnus X1]

If you are buying it new then you shouldn't consider any mods at all.

[Shawn Lutz]

I agree with Cyg. If you're looking for a new amp buy one that will give you the tone your seeking instead of buying one that doesn't and modifying it.

[DonP]

Sure you can add a depth / resonance control. I was thinking about doing it to the Laney AOR head I have, but it sounds pretty good as-is.

You need an understanding of the negative feedback loop. The loop takes out-of-phase output and adds it back to the input in order to reduce distortion. Because it is out-of-phase, it reduces the gain.

The Presence control works by taking the "high end" out of the feedback loop. Less "negative" is more positive, so, when you turn up the presence control, you are grounding out more of the negative high end, and letting less of this negative high end reduce the gain.

The Depth control needs to reduce the bass from the negative feedback loop. There are many ways to do this (check amp schematics with this circuit for real world ideas), but one way I can think of is by sticking a lower value "blocking" cap in the middle of the feedback loop. This cap is sized to only let higher than bass frequencies through. Then for the control, have a pot that lets you bypass (go around) the cap. When the pot is turned to be open/high resistance, the "negative" bass freqencies cannot get past and you have full depth. As you turn the pot to lower the resistance around the blocking cap, more and more "negative" bass gets through and you get less depth.

[SausageofPower]

Thanks Don. That's just what I wanted to know. Looks like I've got some reading to do.

And yeah, I'm not looking to buy a piece of crap I'm not happy with and modify it to sound like I want, I'm just interested in all the stuff you can do with amps. Main reason being I might have a line on a JCA100H, which is a great amp on its own, and I'd be interesting in knowing what all I can do both for the sake of customizing the sound and learning. I love modifying/customizing stuff (I do it with my guitars, my guns, cars, electronics, etc.) so I'm curious to learn about all the crap I can do here.

That being said, I will probably just go for the JCA100HDM if this deal falls through.

[Cygnus X1]

Not sure if you know but I always advise safety first.
Tube amps "ain't no joke", they can kill even when unplugged.
Although I'm not a pro I spend a LOT of time working inside amps.

http://www.aikenamps.com/SafetyTips.html

[Chief_CrazyTalk]

I'd advise having hardcopies of the schematics handy, take plenty of notes, and photos. Double and triple check your work. The notes and pics I took saved my ass replacing a preamp tube socket today.