Plexi - 2204 cross sort of

[printer2]

They guy at work wants a 15-20W Plexi but from hearing some of the type of guitar he likes I think he really wants a higher gain model, more of a 80's-90's sound. So I started to mess around with the Plexi schematic and came up with this with the thought of using a 500k pot I have with a dual push-pull switch on it. While the hi gain position is not 100% true to the 2203 circuit I think it might be a good compromise. The schematic has the switches in the Plexi position.

While the 500k pot should be a 1M, it is harder to come by 1M's with the switch, I think it loading the first stage more would be less detrimental than after the second stage. In 2203 mode I am going to loose signal after the second stage with the 470k and 220k combination already, normally there would be a pair of 470k's. The first stage is also loaded with the other 220k resistor, the price I have to pay for using two switches and wanting the Plexi circuit to remain true to its roots with the 470k mixing resistors.

Also on the 2203 the coupling cap after the first stage is a 0.022uF while the Plexi value is 0.0022uF. I think the reduction of bass should not be a detriment in high gain. Other than that just tune the PI for 6V6's or maybe some EL84's. At least that is the theory. OK guys, now tell me where I am going wrong.

[soundguruman]

You put a 500pf across the 470K in the mixer which helps thin out the "almost DC" from the input stage. Otherwise it sounds too muddy. The over-bass response tends to produce too much distortion at the grid of V3A.
You use 10uF and 1.5K for the cathode of the input stage. Otherwise your bass response goes too far, and it sounds too much like a mesa boogie and not like a Marshall anymore. Again, the grid of V3A comes out too overloaded.
The 2X 220K should just be 470K with a 500 pf across it...
The 500K volume should be 1M, the grid of V3A should just have a 470K to ground instead...
But when you switch the gain, it's going to have a fair POP noise...so you are going to need a 1M resistor to ground on that V1A grid.
The 500 pf cap across the 1M pot is TOO bright and it's treble overkill for that stage. Lose the bright cap it's overkill.
There is going to be some need to dampen the tendency to oscillate by adding capacitance between plate and grid on the input stages. 2X 15pf 500V in series across plate to grid is a suggestion. Or twist the plate and grid wires together.
after that, you could actually sneak the plate resistors of V1 up to 120K-150K

It's really better to run the output tubes triode. Just extend your screen resistors all the way to the OT windings. You will get about 13 watts. You can crank it up for better saturation at a lower loudness. I mean, 50 watts will blow your ears out, and you will miss the good output tube overdrive. 50 watts is too much.
And you will be surprised how loud it is in triode...and get all that EL34 crunch without being too loud.
Otherwise, at 50 watts, you tend to keep the master volume down, and it won't sound as much like a Marshall should sound. You really gotta get the output tubes saturated for that Marshall crunch.
And I would for sure use EL34. Yeah, ground pins 1&8 on EL34.

[big_teee]

Nice layout.
I would lose one set of jacks, and wire the cascade circuit like the stock 2204.
You could still do the switching like you have it.
I like the 68k input instead of the 470k, but I leave the 500pf cap with it.
Sacrificing one set of jacks, will give room for the PPI-MV on the front panel.
I wouldn't have a 2204 without it. It allows playing at near Bedroom levels.
EL34s or 6L6s both sound good, I've built it both ways.
Here is the last one I made, from a modified mojo 18w Chassis.
The PPIMV is between the presence and the pilot lamp.
The preamp is JCM800 2204, with Classictone fender iron, and TAD short bottle 6L6s.
My Friend I built it for Loves it.
The Case is a mojo extra small 24 inch.

[printer2]

Boy you guys are quick and gave me a lot to digest. Just for others convenience I'll post the schematics I am using as a guide.



I like the idea of using EL34's but I want to put together a test mule to sort out the preamp with tubes and iron I have. Pretty sure to have a PPIMV or a VVR, put in the MV before the PI just in case I go that way. Triode at 13W, sounds interesting.

[printer2]

OK, iteration #2. Took some of the suggestions and tried to fit them with my goals. First off and foremost I want this to mimic a Plexi. With the normal and bright channel you kind of want to jumper the inputs. Also need the mixing resistors and 500p cap on the bright channel, good call I missed that trying to switch the input stages in series only using two switches. I came up with a fairly bizarre way of accomplishing it, not all together happy about it as it requires the user to know they can not plug into the normal channel when they are in series. Oh well, at least it will not hurt anything. The Plexi bright channel has a 0.002uF coupling cap and I think it after the first triode will somewhat make up for the treble bypassed resistors in the 800 series amps. Might be worth trying out.

[big_teee]

Do you have a chassis criteria.
A standard 17 inch JTM 45 Chassis is what is easiest to get and build on.
It will be hard to fit all that on a standard chassis.
If you go to a large marshall chassis you get big and heavy.
Just Saying.
Here is a couple of the schematics, and layouts I like.
http://mhuss.com/MyJCM/JCM800_2204.pdf
http://www.el34world.com/charts/Sche...l_50w_2204.pdf
http://site.triodestore.com/JCM8002204V.2NEWlayout.pdf

T

[printer2]

Do you have a chassis criteria.
A standard 17 inch JTM 45 Chassis is what is easiest to get and build on.
It will be hard to fit all that on a standard chassis.
If you go to a large marshall chassis you get big and heavy.
Just Saying.
Here is a couple of the schematics, and layouts I like.
http://mhuss.com/MyJCM/JCM800_2204.pdf
http://www.el34world.com/charts/Sche...l_50w_2204.pdf
http://site.triodestore.com/JCM8002204V.2NEWlayout.pdf

T

Other than the master volume it is the same number of controls as a Plexi, the switch is one one of the preamp volume controls. Chassis is not a problem, we have a small sheet metal shop at work. Thanks for the links, I'll take a look at them.

[big_teee]

Other than the master volume it is the same number of controls as a Plexi, the switch is one one of the preamp volume controls. Chassis is not a problem, we have a small sheet metal shop at work. Thanks for the links, I'll take a look at them.

You're right, and you can put the second MV in one of the Jack holes.
TubeDepot does that on there 50 watt plexi kit.
If you drill a blank chassis, you can consolidate a bit.
Good Luck on your build.
T
**Edit
You left off the regular 2204 Pre Master Volume.
It goes right before the PI.
I use both pre and post on my version, and using the the two MVs together makes the volume much more controlable, on a 50 or 60 watt build.
Your version may or may not need it.

[silverfox]

Is it possible to combine both channels and mix the pre amp outputs with a single tube mixer. I know this isn't the goal here but I've often wondered. Will that result in a clash of waves at the PI, destructive interference, cancelations? Maybe that is what the JMP amp is. Two channels jumped... Don't know.

Silverfox.

[printer2]

Is it possible to combine both channels and mix the pre amp outputs with a single tube mixer. I know this isn't the goal here but I've often wondered. Will that result in a clash of waves at the PI, destructive interference, cancelations? Maybe that is what the JMP amp is. Two channels jumped... Don't know.

Silverfox.

Well that is what the second stage is doing, mixing the two input channels along with driving the tone stack.

[silverfox]

I should have been more specific. Drive both channels in parallel so you get the different channel voicings combined through a mixing tube prior to the PI.

Silverfox.

[soundguruman]

Is it possible to combine both channels and mix the pre amp outputs with a single tube mixer. I know this isn't the goal here but I've often wondered. Will that result in a clash of waves at the PI, destructive interference, cancelations? Maybe that is what the JMP amp is. Two channels jumped... Don't know.

Silverfox.

Although many people jumper 2 channels inputs together, it hardly makes any difference at all.

What you want is to put 2 channels in series. This is also called "dime-ing" the channels.
The channels are "dimed" together...
This is how Marshall made it's first over-driving preamps.

the output of the first preamp goes to the low gain jack switch.
So, plugging in the "low" input takes the first stage out. The preamp is all clean.
Plugging into the "high" input puts the 2 preamps in series for more input gain. (through the jack switches)
The Marshall method has been the most stable and proven of all designs, is more use-able, and has less noise and oscillation..
That's why everybody wants to use that JCM 800 design.

Later Marshall started using op amps to buffer the signal, before the first tube.
This was also copied by just about everybody....
The idea was (IMHO) to use something more stable, that was less prone to ringing and oscillations. There is all sorts of negative feedback in the circuit...
It also works pretty good.
That idea is the basis for JCM 900...

[silverfox]

Thanks soundg- I really need to start a separate thread on this change in topic as it is diverse from what the intent here is. But thanks for clearing that up. I do understand the jumping concept as the Fender channels are good for.

Silverfox.

[printer2]

Thanks soundg- I really need to start a separate thread on this change in topic as it is diverse from what the intent here is. But thanks for clearing that up. I do understand the jumping concept as the Fender channels are good for.

Silverfox.

There is no reason why it is wrong to jumper channels on a Marshall and OK to do it on a Fender. Even the 18 Watt, Spitfire, think a Dz Z amp but can't remember which offhand, do not have the first stages cascaded. It really depends on how much preamp clipping you want. At around 6:10 he shows the effect of jumping the channels.

Some people want more gain than a Plexi and they go for a cascaded amp or a booster in front. Really depends on the style of music you are doing. Personally I have little need of a cascaded preamp.

[soundguruman]

Although many people jumper 2 channels inputs together, it hardly makes any difference at all.

You may think you are going to get overdrive. No...that's not overdrive.

You have to put 2 preamps in series, one after the other.
This requires re-wiring and tweaking.

Anybody can chain the preamp stages together...
It's the layout and the tweaking of the circuit that's difficult.
To get more gain, with less noise or oscillation, is another thing altogether. This requires time, patience, etc...
You should be ready to make this sacrifice, before you commit.