JCM800 4211 mods?

[azrael]

A friend of mine picked one of these up. I'm not really impressed, tbh. Very different than the 2203/2204 800s.

This one is thin sounding, a bit buzzy in distortion character, and not at all evocative of classic Marshall tones.

What can be done to make it sound thicker, especially in the lower midrange?

How can I increase the gain? What the heck is up with all this transistor stuff!


I'd like to do what everything I can, short of extremely invasive ideas like adding a tube or something. I know there's not a lot of circuit to work with, as each channel doesn't have that many stages...but still, I'd like to hear some thoughts on the situation.

thanks!

[soundguruman]

You could do something like
change R2 from 2700 to 820 ohm
Change C2 from .47 to .68 uF
Install a 7 pf 1000V silver mica cap between plate and grid of V1A preamp stage. (this helps subdue the ringing microphonics of the first preamp tube)
any time you increase the gain, this tube will be a bit ring-ee.

But the whole thing is, only very small tweaks to this amp will give you way more gain.
You do not need to add stages, or do major modifications.
Just tiny little stuff makes all the difference.

like using higher gain 12AX7 tubes, huge change in sound.
Make the bias hot, huge improvement.
Humbucking pickups on the guitar, instead of single coil.

The Marshall 10 foot rule:
You have to stand at least 10 feet away from the speaker cabinet.
Remember this or go deaf.

[azrael]

Okay, those seem like good ideas. Increase the gain, shift the frequency response.
A small snubber like that can't hurt.

I haven't gotten inside it yet, to see how hot it's biased. It's still got the original GE 6550s haha.
I've found MESA 12AX7s to be quite high gain, so I may recommend those to him.

[soundguruman]

change the usual 5 resistors and install EL34 instead of 6550.
You know this stuff already, I can tell.

"a bit buzzy in distortion character,"
Bias is too cold.

Marshall power brake. Now you are cooking.
Get it to where the output tubes are over-driving.
Open the master volume almost all the way,
and reduce the volume with the power brake.

Now it sounds like a Marshall.

[azrael]

Ya, I was thinking of changing it over to EL34s. Not gonna bother changing the 150K pair of resistors, I don't think, unless it really cuts down the volume a bunch. 1k is still cool IMO for those screen resistors. I have a Peavey VTM, another close relative to the JCM800, and I'm using 1k screens with E34Ls. The voltage divider for the bias control should be fine, should still have enough play for me to get to 70% dissipation, I would think?
I think my friend would probably prefer to have to replace 4 EL34s rather than 4 6550s, too haha. Much less expensive.

Actually, some KT77s might be really awesome, give it a fat bottom end.

What do you think about changing the clipping diodes?

[soundguruman]

Change the 150K to 220 K
This allows more overdrive of the output tubes.
What you want is output tube overdrive,
the preamp should be clean when the output overdrives.
The sensitivity should be:
2.5 mv sine wave input for over-driven output tubes
if you do not have 2.5 mv sine wave in to drive full wattage out, something else is wrong.
(this is basically with the controls wide open.)

The clipping diodes are LEDs?
unless they are bad, you don't need to change them
(these are not shown on a lot of schematics, but these are in the circuit?)

The whole idea behind the Marshall sound is clean preamp, over driven power amp.
Lowering the master volume encourages the PI to distort, and that's wrong.

If your customer wants the opposite,
over-driven preamp, distortion from PI, then you don't want a Marshall.

You want a Mesa Boogie*.
Don't try to make a Marshall into a Mesa Boogie, just buy a Mesa Boogie.

*It takes a huge amount of tweaking to make a preamp overdrive,
without sounding muddy.

[azrael]

Oh, okay, I thought the 150K pair limited the output more for 6550s. I'll do that one, at least.

The clipping diodes are 1N4001s in series. Using something with a lower voltage threshold might sound more gainy.

[soundguruman]

That is correct,
6550 is 150 K
EL34 is 220k
I thought you would use EL34.
Stop trying to make the preamp grainy, you want silky smooth.
The sound is the output tubes, screaming for holy mercy. That IS the sound.
That's why the Power Brake.
Are you OC about making the Marshall sound like another amp?
It can't be improved, that's what it is.

Look, just give it a chance.
1. Snubber cap on V1a high gain preamp tubes.
2. Change V1a cathode Bias
3. Re-bias output hot.
The notch of crossover distortion should appear just slightly "after" the output tube barely starts to clip (full output power).
OR if you are KK, the notch appears wayyyy after the output tube clips, and you buy plenty tubes.

There's one thing I'm adding, a tube damper.
This is going to help the microphonic tendency of V1. (the squealing noise)
1X Fender Hot Rod Deluxe Foam Tube Stabilizer part number 0059718000
(this is a piece of high temperature high density foam with two holes in it, that slides over the preamp tubes)
Now this foam will be cut in half, and I will install both pieces on V1, stacked up.

[azrael]

I'm familiar with the attenuator method.
I'm actually considering building a dummy load for myself so I can reamp my own amp cranked.

But I don't think my friend wants to buy a Power Brake (I actually prefer the Weber reactive speaker attenuators).

I am giving your ideas a chance, it just will depend on when my friend has some money to put down. I'm just trying to gather some ideas to present to him when he is ready to do it.
Thank you very much for your input.

[Alex R]

I'm not sure the effort to make a JCM800 preamp 'silky smooth' and clean is going to work, there is some clipping in that preamp when you turn it up isn't there?

JCM800s do vary and there is always a tendency for them to get too thin because there is so much filtering-out of low frequencies going on in there. This is done to keep the distortion from sounding muddy and as the guru says, little changes to that preamp make a big difference. it is very easy to let too much bass through and that ruins the Marshall sound. Try your changes one at a time. That c1 change might be all you need. If it doesn't work, put the old value back and try your next change, maybe trying some different values for c4, the bright cap (remembering that with the gain fully open these changes are negated) - 470pf in there will certainly tell you what a bright cap does, and .001uF, though extreme, is a value some plexis used which, whilst making the amp seem pretty wild, doesn't stop it sounding like a Marshall (but let's not forget this isn't a plexi, it has way more preamp gain and clipping). But if you don't go one at a time and back to stock each time you're not happy with the result, you are likely to lose your way, as this is a quite subtle game you're playing. Remember that this preamp is meant to filter out a lot of low frequencies, and if it doesn't do that it won't sound like a Marshall, it's just a question of degree.

[soundguruman]

I'm familiar with the attenuator method.
I'm actually considering building a dummy load for myself so I can reamp my own amp cranked.

But I don't think my friend wants to buy a Power Brake (I actually prefer the Weber reactive speaker attenuators).

I am giving your ideas a chance, it just will depend on when my friend has some money to put down. I'm just trying to gather some ideas to present to him when he is ready to do it.
Thank you very much for your input.

These are not my ides. Everything I told you comes from the Marshall factory.
We just combine the stock designs into one amp.
The sensitivity figure of 2.5mv was given to me by the Marshall Engineers, who work in Milton Keynes.
This is how they test that the amp is working / biased correctly.
Trivia FYI the sensitivity of a Plexi / Superlead should be 5 mv.

People misunderstand how a Marshall is supposed to sound, That's why I explained it.
With the advent of amps like Mesa Boogie, people have forgotten what the true sound is.

I am sure a Weber / Power Brake / Hot Plate will work just fine.
But remember it's the power amp that overdrives, not the preamp. That is the true classic Marshall sound.

It's so funny that people try to alter the original intentions...
The original is what sounds best by far.

[azrael]

I don't think i'll be able to get much overdrive, even with an attenuator, I'll have to bring down the volume a bit to get 4 6550s to overdrive. not to mention the clipping diodes.

The 4210 isn't like other Marshalls.

I'm familiar with the "Marshall sound". I modified my Peavey to get me there, I can get it to overdrive the tubes. but 2 E34Ls is very different than 4 6550s...


And no, the original is what sounds best to you. I understand that you are trying to help, but every person has their own idea of their ultimate tone, you can't just generalize like that.

[wizard333]

I have a series of comprehensive mods for that amp, I'm very familiar with it.

I'll give you this warning: There are all kinds of mod "suggestions" to be found if you search this site and others.

None of them make that much sense or really do what you're asking.

You don't need to change the biasing, or the output section, or run it at ear splitting volumes for one thing. It is easy to add more gain, and it is easy to not have the amp be "buzzy" as is. As for stock gain, here's a hint: The Gain Channel Volume isn't, it's a 2nd gain pot. Treat it that way. Look at the circuit.

My advice is, ignore all the suggestions, think about what makes an amp sound the way it does, and spend some time in that circuit.

The whole idea behind the Marshall sound is clean preamp, over driven power amp.

That is mythology. With an older Marshall, when you turn it up loud, the whole amp is one big overdrive circuit. That is PREAMP as well as power amp.

I guarantee you that a 4210/4211/4212/2205/2210 can be turned into a very tonally versatile, high gain fire breathing monster with a killer clean channel and pure Marshall tone on the gain, definitely *NOT* a Mesa. If you're anywhere near Nashville, you're welcome to come see one in action. Ear shattering volume not required.

[azrael]

I'd love to hear what sort of mods you do to yours.

I'll definitely be playing around with the amp more before doing anything, like I said, it belongs to my friend, so I don't have a lot of face time with it. But next I see him, I'll try.

[wizard333]

I'd love to tell you but the 30 minute response I just wrote got eaten by the forum and I'm not writing it again now. Maybe some other time.