freestompboxes.org

I got my 6021-based Powerman amp running, and I like it a lot, but the tone is a little fuzzier or muddier (not sure of the correct term) than I would like. I am no amp expert (obviously), but I think what I'm shooting for is more of a Fender tone than a Marshall tone. On another forum, someone said something to me about using less preamp gain.

Now come the stupid questions.

1. Is there any hope that reducing the preamp gain will give me a slightly clearer, more trebly sound? It's hard to descibe amp sounds, but I think what I'm looking for is distortion with a harder edge, somewhere between a Tube Screamer and an OCD (Tube Screamers are a little soft for me). I am wondering if I can get it by toning down the preamp. Right now, I get the best sound by turning the amp way down (it only has one knob, which affects the preamp tube) and turning the volume on my pedals up.

2. Is decreasing the preamp gain a simple matter of increasing the size of the resistor upstream from the plate? The guy who designed the amp suggested increasing the resistance farther up, so all plate voltages are reduced, but I don't know if I want to change the output plate voltages. Right now the preamp tube plates are connected to 56K resistors.

I think I'll change the resistors to see what happens, but it would be nice to get competent advice.

Interesting project! You've got my attention definitely. Relatively speaking, one of the major reasons JCM800's are more gainier and fuzzier sounding than the JMP plexi models is due to the plate resistors being changed from 100k to 220k, so I am not sure if going up in size is what you want to do... but you could try it... who knows. The other reason for the gain increase has to do with the cathode bypass capacitors on the gain stages, but I am thinking that your problem has more to do with voicing than gain though. Do you have a schematic?

I posted the schematic in another topic. I assume the mods don't like it when you upload the same file over and over, so here's a link:

https://www.freestompboxes.org/download/ ... p?id=13077

Voicing wise what I would do to fix the problems you talked about is this...

1. Change C4 to something around .47uF give or take.

2. If that doesn't work or if you like what you hear and want more, try soldering a 1uF (give or take) cap in parallel with R6.

The second one will result in an increase in audible "gain" but the first mod should clean up the voicing of the amp enough to clean up the low end flubbiness and fuzz sound, so more gain probably wouldn't hurt at that point. Lower values on both mods will result in more high end, and higher values will result in more lows.

Thanks for the suggestions. I would give them a shot right now, but I just tore the amp open and stuck 22K resistors above the plates on the preamp tube, and I also replaced the 100 ohm resistors with better ones to cut down on noise. Seems like the resistors worked, and the amp seems to sound cleaner, although maybe what I did was just the equivalent of turning the pot! I was hoping to lower the plate voltage, but only on the preamp plates.

I just found a page explaining what the RC circuit below the tube does. I see what you're saying.

The amp sounds good; it just doesn't sound "sweet," and the sound is not as detailed as I'd like.

I don't understand this stuff very well. I'm starting to wonder if you really need a preamp tube when you use pedals.

If you lower the plate voltage, what that will do is reduce the headroom of the preamp and power amp tubes, making the clipping more exaggerated, one other thing you could try is substituting a 12AU7/ECC81 for V1, to get the amp sounding cleaner, you might have to increase the 56k plate resistors to something like 100k though....

I'm not totally sure people understand what I'm trying to do. It's not easy to explain, especially for someone who knows as little as I do.

I don't want a clean sound, per se. I want harder clipping, more detail, and a hot sound. If you've ever heard the way Les Paul used to mike Mary Ford, you'll understand what I mean by "hot."

I'm a little worried that the sound I have in my head may not exist in the real world.

Is it possible to stick a bigger tube in one side of the circuit? I don't know if a 12AU7 is a direct swap.

The resistors I added only affect V1.

I still think you should try the modifications that I suggested earlier. What that will do is clean up the gain voicing because what is happening now is that you are shoving so much low end and sub lows into the tubes that when they run hot those sub lows are muddying things up. I really think that this is the source of your problem and not the amount of gain per se. You could get a lot more definition out of that setup if you play around with the values on those cathode bypass capacitors. I would at least give it a try.

I'm looking around for a new capacitor right now. Some day I want someone to tell me why they don't just print the value on them, instead of long meaningless numbers!

I didn't have any .47uF electolytics. I have a box of electrolytics, and the tiniest one was like 10uF. I found a couple of fist-sized Orange Drops I ordered accidentally. They tested out at around .22uF, so I stuck one in the amp. Close enough for a test, I figured. I was going to parallel them, but I figured I would try one by itself to see if it was worth it.

I don't know why people use electrolytics so much. Perhaps someone could explain it to me. I assume big electrolytics are cheaper than other capacitors, since motors always have cans on them, but I don't know why they're used in tiny things like small amps.

Anyway, the amp is now considerably more brilliant. It loves my TS-9 and my Fat Sandwich. I am getting a much better range of tones out of it. I'm thinking I should go ahead and put the other capacitor in there, since I have no other use for it. I don't know if I should remove the resistors I added or leave them in. They don't seem to hurt anything, and I'm pretty lazy.

Thanks for the help. This is an excellent practice amp now. I can do Stevie Ray, Billy Gibbons, and even a decent Brian Setzer tone. And the amp doesn't even get warm, so I have it in a book-sized enclosure with no vents. It will go in a suitcase with no problem.

Now, if I can figure out what to do with all that spare circuit board real estate. I'd love to have something approximating a Tube Screamer or Fat Sandwich in there. Or maybe a Fender/Marshall switch, to change capacitors.