I have a Marshall JCM800 Bass head. I use it to play guitar through. It has one volume knob right now.
I would like it to have some gain/distortion.

It is risky to have it modified?

Thanks,
me

sakes wrote:I have a Marshall JCM800 Bass head. I use it to play guitar through. It has one volume know right now.
I would like it to have some gain/distortion.

It is risky to have it modified?

Thanks,
me

risky? i dunno. expensive possibly . .
maybe you have seen the marshall-made thingy called the 'power brake'?
its like a master volume for between amp and cabinet.
so you can have the sounds-good-all-the-way-up amp up all the way and
not have your ears deafened.

In my experience of knowing quite a few people who have put master volume circuits in non-master Marshalls, most are not happy with the results. Some people like it OK, but it seems like most do not.

The MV usually makes the amp sound fizzier and impairs the visceral quality of Marshalls that are 50% of their charm. Instead of doing a mod, I would sell that amp and get something that broke up more to your liking, maybe even a 50 watt marshall, although the volume cut on those may not be what you are expecting at 1/2 wattage (they're quieter, but not dramatically so). Maybe try an AC30 or something with more gain like that?

i agree with nonno, and i'm not too into the powerbrakes either

having the amp turned into a 50W is not hard. i'm pretty sure you just pull out two tubes and rebias. don't do it yrself unless you know what's up. you have to pull the right tubes etc. and i might be missing something.

a more complicated mod, still not that hard, would be to have a hi/lo power switch put in. this would give you your full-on 100W for when you need it and 50W for when you don't care to blast.

tmidgett wrote:i agree with nonno, and i'm not too into the powerbrakes either

having the amp turned into a 50W is not hard. i'm pretty sure you just pull out two tubes and rebias. don't do it yrself unless you know what's up. you have to pull the right tubes etc. and i might be missing something.

a more complicated mod, still not that hard, would be to have a hi/lo power switch put in. this would give you your full-on 100W for when you need it and 50W for when you don't care to blast.

What Tim describes is a good place to start, and will do no harm to your amp, but there are some subtleties you need to observe:

Remove one tube from each parallel-wired pair, rather than two from the same pair. The tubes are arranged so that two tubes are connected to the "top" of the output transformer, and two are connected to the "bottom." If you take both off of one side, you will unbalance the output transformer, and this could cause core saturation or even burn out the primary side of the transformer.

Conventionally, there will be four tubes in a row, and if you pull the two end ones or the two middle ones, you'll be safe.

The output transformer is now seeing only two tubes, and this is a higher source impedance than four. You need to set the cabinet impedance selector to 2xthe cabinet load to maintain the proper winding ratio. If you use a 4-ohm cabinet, then set the amplifier to "8 ohms." If you use a 16-ohm cabinet, then set it to 16 (it won't have a 32-ohm setting) and hope for the best. There will be less power on the output transformer with only 2 tubes, so it is probably not going to cook, but the sound quality may change.

The bias tap will be loaded differently, so there may be significant voltage drift. This depends on the means of deriving the bias voltage. If it is regulated, you may not have any problem with this. If your bias pot doesn't provide enough throw to normalize bias, then you may need to add an additional series resistor with to the bias circuit.

In any case, once you find out if you like the sound, then these changes can be incorporated into a "half-power" switch, which will leave the tubes in place, powered and biased, but will remove the B+ from the two tubes you're cutting out. This will not harm the amp or the tubes.

Good luck! Have no fear!

It is a 50W head right now.

I've decided to let this dude here in Minneapolis named "zom" put on a knob.

Dude says he's the best.
We'll see.
His dog bites.

Thanks,
N.

...Or you could sell it and find a 50 Watt JMP Mark II Master Volume, they're around.

Jay

You should look at a pedal that Zachary Vex makes. It's called a "Super Hard-On." basically what the pedal does is makes the signal fron your guitar bigger whilst preserving it's integrity, and thusly takes that huge signal to slam the shit out of all those 12AX7's in your amp. Basically, it sounds like your guitar, and your amp, but bigger and more distorted.

Here's my bits of thoughts.

1. Generally cranking up a marshall and getting power tube saturation is more pleasing than cranking up the preap and getting pream tube saturation.

2. Preamp tube saturation in marshalls is a widely used sound and can sounds pretty damn good.

3. Non-destructive mods can be fun, and a great way to develop a unique, interesting sound.

4. Volume boosters like the Super Hard on, micro amp, etc slam the preamp tubes harder giving usually more preamp distortion... unless the amp is cranked, in which case it will give you more preamp and power amp distortion, but it'll still be very loud. These pedal also can give you a more satisfying sound due to what they do to your impedance.

5. the first mod I'd do to the marshall is the half power mob suggested by Tim and Steve. From there you can try a non-destructive master volume mod similar to where the master volume would be placed in a JCM800 or JMP 2203 or 2204.

that's it.

ben adrian

I actually have a 50 Watt Master Volume JMP I'd consider trading for your amp, if that interests you. You can email me at dogrange@yahoo.com if interested.