10 Groovy things you can do with a Plexi board.

The Marshall 1987 circuit, also known as the "Plexi" is by far my favorite amp design. Many folks know this started out as the Fender Bassman. Many folks don't know that this circuit was originally invented by some guys at Western Electric back on the 30's. For telephones. Whata trip, huh? This basic circuit, and dozens of variations, has been used by dozens of amps and amp companies over the years. With a few part changes this can sound chicken-pickin clean or as heavy as Slayer (the Marshall Kerry King 2203 model.) I'm even gonna show you how to make this a channel switcher. Scaled up to 100 watts or down to 20 it always sounds great. You darn near can't screw it up. If I could only build one turret board it would be this one because there's so many things you can do with it. So let's break it down.

1) Build it stock -1987/ 50 watt Plexi
Marshall model 1987 schem (from schematicheaven.com)
Ace Pepper Plexi/1987 turretboard layout
Let's start here. This is the 4-input, 70's icon we all know and love. You Marshall freaks probably know there were several versions of this design over the years. JTM45, JTM50, JMP50, 1986, 1987, etc. I'm showing the part values for the 70's style 1987, but you could build any version you like. If you're really interested in the difference you can find the various schems on the web. Check out Brian Wallace's table of Marshall part values to get an idea of all the tweaks Marshall did over the years.

2) Ab-normal inputs
We've got a pair of "Normal" inputs feeding V1, pin2 (or V1.2). This stage has a 100k plate resistor, .02uf coupling cap, and 820ohm cathode with a 330uf bypass cap. This makes for a "hot" biased stage (the low 820 ohm cathode) with tons of bass (the high 330uf bypass cap.) Bassmans and JTM45s had both cathodes tied together to this setup. I never liked this sound, too much gain and bass. Try a smaller bypass cap. 20uf is still a lot and will tame some of the ultralows. Try 1.5k cathode resistor.

3) The Bright ones
The "Bright" inputs are feeding another stage (V1.7) where we have 100k plate, .02uf coupling, and 2.7k cathode res w/ .68uf bypass. I read somewhere this was a mod done for Hendrix that became a standard part of the circuit. This is the classic Marshall lead tone, the channel most folks use. I like a 1uf bypass cap here for a bit more lows, fatter input tone. You can change the cathode res too. 2.2k-1.5k will make it scream!

4) Mix this
These normal and bright inputs come together with 470k mixing resistors that feed the next tube stage, V2.2. The res from the bright pot is shown with a bright cap around it. This one can stay or go but I like the way it adds some crunch to the sound. The cathode of this stage has an 820 ohm res and a .68uf bypass. That's not bad, or it could go up to 1k or 1.5k w/1uf bypass for a bit more bass/fatness. That triode is tied as a cathode follower to the other triode in V2. In this configuration the 2nd triode doesn't add any gain but acts as a buffer for the tonestack, which is very cool. It keeps the tonestack from loading the preceding triode stage too much, giving clearer tone.

5) Tonestackin' goodness
The tonestack is a great place for voicing changes. Most guitar amp EQs take a lot of mids out of the tone. The value of the treble cap (500pf shown) effects the frequency where this happens. The slope resistor (33k) effects how much mids are "dipped" in the EQ curve. The best way I've seen to demonstrate this is Duncan's Tone Stack Calculator, a freebie download for PCs (sorry Mac guys.) A lot of the difference between Tweed Bassman/old Marshall tone and newer Marshall tone is different values in the tonestack. I like the Bassman values (250p/.02/.02/68k) for fatter/darker tone.

Another cool tonestack mod is to up the value of the mid control. Stock is 25k and doesn't change the tone very much. Try 50k for both more AND less mids. More control sweep and a mid control that actually changes the tone!

6) the Splitter
Next up is the "splitter". Or "driver". Or the "phase inverter" if you're not into the whole brevety thing. It takes your single guitar signal and "splits" it, "inverts the phase" of one of those signals, and "drives" the power tubes. It provides the "push" and "pull" for that push-pull power section. This configuration is called a "long-tail" splitter and is used in LOTS of amps, Marshall, Fender, everybody. The values shown here are typical and sound really good. Tweaks in this section affect the feel more than the tone of the amp. Some folks think changing the 10k "tail" resistor to 20k or 24k gives a tighter feel. You can also try changing the 470 ohm cathode resistor to 820 ohm to bias this tube a little cooler.

On the output side of this, feeding the power tubes, we have a couple .1uf coupling caps. You can try .02uf to tighten bass if youre running a lot of gain (2204s use .02 caps here) Get several caps, measure the actual capacitance with a meter, find two that match/measure the same. Matched coupling caps here give smoother tone.

You'll also notice 82k and 100k plate resistors. This part of the amp is running a lot of voltage a lot of the time. I like to upgrade these resistors from 1/2W (350v rated) to 1W (700v!). I really like the "World's Best Resistors" from westlabs.com. Low-noise, hi-fi metal film resistors are a nice upgrade for any plate resistors in the amp.

7) Feedback and Presence
The negative feedback resistor is another one to look at. This is connected to one of the speaker jacks at one end (usually the 8 ohm) and to the splitter and presence control at the other end. This takes a little of the final output signal and "feeds it back" into the splitter. They came up with this to make the amp tighter/less distorted. Stock value is 47k, but you'll notice (100k) right next to it in the schem drawing. Some amps had the 100k value and it gives the amp a different feel. You can try both and decide which you like. You can put both on a switch and switch between them. You can even replace the resistor with a 50-100k pot to make it adjustable.

There's even things you can do to the presence control. Remember how you've got this full-range signal being "fed-back" from the speaker out? This presence control kinda filters what frequencies get "fed-back" and mixed into our sound. You can change the value of the presence cap (stock .1uf) for a different sound/tone. You can even stick a 1M pot and a 25uf cap in there to create an extended bass, or "resonance", control. See the "Earth" control in my BuzzBomb amp to see how this is done. I've done this control in 20, 50, 100 watt amps and it rocks!

8) Let's get small
So far we've been talking about a 50 watt icon of tone using a pair of EL34 tubes. If you're building your own amp it's just as easy to use 6V6 or EL84 tubes for around 15-20 watts of output. Just use smaller power and output trannys to make the little tubes happy. While you're at it, you could even make the amp cathode-biased! Just look at a Marshall 18w, AC15, or my BuzzBomb for examples.

9) Go Big!
Go the 100 watt route! Four EL34s roaring in unison is a wonderful thing. Just ask... well any rock star;-) The power section hookup is a little different, see model 1959 schematics, but the board remains the same. Almost. The 100 watter uses a bridge rectifier instead of the full-wave. So, tie the backside of the diodes together and ground 'em, connect the HV to the middle of the diodes, and bob's your uncle;-) You've got a bridge rectifier. The 100 watters usually have a seperate tap for the bias voltage, so that's an easy hookup.

10) Go for the gain!
Why don't we stack up those normal and bright stages? Techs were doing this for a while by the time Marshall got hip. For many of us this should have been Mod #1. See Marshall 2204 schematic for the basic idea.

See, ya start with the "lead" or "bright" input voice (cathode res = 2.7k/.68uf), take that out thru the low-gain input to a gain control. That gain control feeds that other triode stage (V1.7). This one is running a lot cooler/lower-gain with a 10k cathode res and no bypass. This feeds our 470k mixing resistors, only now one is inline with our signal (and has that 470pf bright cap) and the other is grounded. This feeds our V2 with the same cathode follower/tonestack setup.

If you've ever played one of these 2204s, or the later JCM800s, you know they're cool, but could be better. See my TopFuel 50 for ideas. See also Pignose G40 and Top Hat Emplexador. I could (and will soon) write a whole article on cool stuff you can do with the 2204-type setup.

11) our friend Master Volume
Now that another triode of gain has been added, which is a lot, we need a way to control how much signal hits our splitter. Otherwise any little turn of the gain knob would produce truly stupid amounts of volume. This is where the master volume comes in. Looking at the 2204 schematic we can see, after the treble knob, there's a 1M master volume control feeding the input of the splitter. They call this a "pre-phase inverter"-type master. I like it a lot because I can drive the preamp as hard as I feel the need and have control over how much signal hits the input of the splitter and balance the preamp/poweramp overdrive. When you think of it this way it gives a finer control over the whole amp's tone and feel.

Some folks don't like this type of master because it sounds thin at low volumes or it doesn't sound the same at every volume. The easy fix is to add a 100k-220k resistor between the 1M pot and the input of the splitter. Boom, a master volume that sounds great at all levels! My turret boards even have a spot for this resistor!

No doubt you've heard lots of stuff on the new about "post-phase-inverter" master volumes, or PPIV. There's about 4 or 5 ways to do this and they work... well, OK. Sorta. I guess. The idea is that you can turn up the stock (or modded) preamp as much as you want to get overdrive and then control the signal feeding the power tubes and have the same tone at any volume. The problem is if you crank the preamp, with no master volume in the way, you'll overdrive the phase inverter/splitter. And phase inverter overdrive really sounds like crap! Sure, you can dial back the volume and control the power tube drive, but... And all the PPIV masters are more complicated than the 1M pot/220k resistor. I feel I get better tone leaving the splitter to work as designed. Your milage may vary.

12) Channel switching Marshall, done right!
Ace Pepper Powermatic 50 layout and schem
For years players have wanted a way to switch between 2 great Marshall sounds. For some reason Marshall misunderstood this and made a series of progressivly crappier chanel switching heads with too many options nobody wanted and a reputation for breaking. A lot. All most players wanted was a way to switch between the classic Plexi sound and the high-gain 2204-type sound. It turns out this is not that hard.

The basic idea is the input stage (V1-A) is set up like a Marshall lead channel, or 2204 (2.7k/.68 cathode resistor). that stge feeds 2 "gain" controls, for clean and dirty. the "clean volume" goes straight to our mixing resistors, like the 1987/Plexi. The "dirty gain" control feeds our 2nd triode (V1-B), like the 2204-style. This stage's coupling cap is connected to the other 470k mixing resistor. Note that neither of these is grounded yet and at this point both gain controls are active.

At this point we need to add a relay to do our switching. There's lots of types of relays but all we need is a double-pole, double throw (DPDT)-type switch that'll work with 5 volts or so DC. There's several ways to do a relay and it's power supply. The simplest is a relay switching kit from Weber VST. There are also channel switching kits from mojotone.com and possibly from others too. There's also a great way to wire your own at hoffmanamps.com (who also sells really good relays for amp channel switching) and another variation from blueguitar.org (look for "ac-to-dc.gif" for a way to power your relay.)

The idea is we're gonna use 1/2 the relay to shunt-to-ground/mute our clean volume or the output from our dirty gain and it's extra gain stage. We're gonna use the other 1/2 to feed our splitter straight from the treble pot (in clean mode, like the Plexi) or from a master volume (in dirty mode, like a 2204) In clean mode you've got a clean volume only and it's like the "bright" or "lead" chanel of a Plexi, in dirty mode you've got an extra gain stage switched-in and a dirty volume to balance the level of your clean and dirty sounds. The footswitch jack on the back is wired so that when nothing is plugged-in switching is controlled by a front panel SPST toggle switch. When a footswitch is connected the front panel switch is disconnected and the footswitch takes over. The footswitch is a normal grounding-type and can be connected with a normal guitar cable. You can even have an LED that lights when the amp is in "dirty" mode.

So there ya have it! I know that's more than 10 tips, call it bonus stuff. Tons of stuff you can do to these amps! For even more ideas check out:
thegearpage.net - forum for gearheads
ampage.org - forum for amp builders
blueguitar.org - TONS of info. Look for "Plexi Mods" or "JCM800 Mods"
plexipalace.com - forum for Marshall lovers
MetroAmps forum - forum for Marshall-style builders

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