Marshall Class 5

This is a weird little amp. Unlike the Epiphone Valve Jr or any of its ilk (or even the good ol' Fender Champ) this guy has *two* preamp tubes. Only having one preamp tube (and thus two stages) seriously hinders a lot of Valve Jr mods. Sure, you can make a Champ (sort of; at least a Champ with an EL84) but if you want any more distortion than that without making the amp sound like an asthmatic 8-bit chainsaw, you're going to need to add at least one more gain stage.

A whole lot of amps make great sounds out of only 2 preamp tubes. Blackface Fenders, Tweed Fenders, classic Marshalls - even the JCM800 preamp, you only need 2 12AX7s. If you wanted a clean preamp, you could just follow the standard Blackface fender preamp and make yourself a nice little EL84 Champ. Let's ignore that one for now; this is a Marshall, remember?

 Okay, so let's look at some classic Marshall schematics.

So say you want a JTM45/Bluesbreaker:

So you have two channels, each gets one gain stage, then the signals combine and go through another gain stage and lastly a cathode follower pushes the signal through the tone stack and off to the power amp. Two preamp tubes.

How about a plexi? How about any of em?

Same deal - two channels, mix 'em together, slap a cathode follower and tone stack at the end, bam.

Since we really don't need two channels in this amp and we're keeping the power low so it'll be a sweet little distortion factory, how about the JCM800?

Slightly different! Only one channel, but two different paths. Low goes through one gain stage, then another, then the cathode follower & tone stack. High goes through one gain stage, and then right into the Low input. Kinda clever, yeah?

Unfortunately Marshall went out of business after the JCM800 was produced and they never went on to make channel-switching nu-metal fuel.

Okay, but we're seeing a pattern. It's a pattern that a whole lot of people like, and you could even go so far as to call it the crux of the Marshall.

• So you got your first gain stage. This just amplifies the signal enough for it to mess with other preamp tubes. There's a basketball term for this but I forget. Anyway, it just gets you started. 
• Then you have a volume pot, and then a second gain stage. When that volume pot is all the way up, that second gain stage is going to start distorting.
• Then we have a cathode follower, and this sucker started distorting a while ago. This'll be the first stage to break up.
• Now, after we've generated all that distortion, we shape it with the tone stack and send it on its merry way to the power amp. Well, to the phase inverter, but single-ended amps don't have those so it's moot. Because we're using a cathode follower to push the tone stack, the tone stack doesn't cost us much gain.

Compare that to a Blackface Fender preamp:

This is an AA165 Blackface Bassman and technically it uses 2 and a half tubes for the preamp, but if we were to make this a one-channel jobber we'd only need one and a half preamp tubes. We'd probably use two because most tubes stop working when you cut them in half.

• We go into a gain stage. Gotta start somewhere.
• SMACK dab into a tone stack. This one isn't driven by a cathode follower, so it's going to cost us a lot of gain. It's a fair tradeoff though: you can cut or "boost" the bass/treble much more this way. 
• Then through a volume control and finally a second gain stage. We're almost back to square one here. Not quite, but it'll be hard to overdrive this stage.
• Then into another gain stage that acts like a mixer. I picked the AA165 because the later AB165 uses a local NFB loop here to minimize any distortion this stage might generate. 
• Then we're off to the phase inverter & power amp.

So, this design makes sense for "Loud'n'Clean" Leo. Sure, you'll get some preamp distortion at the upper range of the volume pot, but the distortion will be generated after the tone controls, so we have much less control over how the distortion sounds. Distortion, as everyone intuitively knows, generates a lot of treble content, and wouldn't it be nice to tame that a little bit? Too damn bad!

Now, of course, there are other amps that are built for distortion which use a tone stack early in the preamp (though they often have a few more gain stages afterwards), and there are many other factors that make BF Fenders less than ideal distortion machines. My point is, Marshall has a recipe, and deviation from that recipe prevents you from getting a traditional Marshall sound.


So, now we finally get to the Class 5. You can find the whole schematic here.

Here's what Marshall decided to do with their two preamp tubes:

Where to begin...

• So we have our first gain stage. Fair enough. But wait! That grid-leak resistor is only 470k instead of the usual 1M! Kill it! Well, okay. It's not terrible at 470k, but compared to a 1M your guitar is going to start losing highs when you turn the guitar's volume knob down. It might also not play nicely with every effects pedal ever made. Reducing this resistor makes sense when you're getting close to the maximum grid circuit resistance for a preamp tube and you don't want to send DC through the guitar (6SJ7 I'm looking at you) but I honestly can't see a reason that this should be 470k. 
• Then we have a 47p capacitor to ground. Not sure why they put this before the grid stopper. After the grid stopper makes more sense; you could lower the grid stopper to reduce noise and keep RF interference from getting into the amp. It's not doing anything the Miller capacitance isn't already doing and it's so small that even with the guitar's volume control turned down almost all the way it's going to be rolling off treble that the guitar can't produce. 
• The first gainstage itself is fine. It's going to amplify a bunch of bass, which is okay in the first stage but usually not ideal.
• Then we have a gain control and immediately following it another coupling cap and voltage divider. I have no idea what they were trying to accomplish here. Grid stopper for cathode follower = good. DC path for grid pulldown = good. It would've been easier and cheaper to just put that 220k resistor between C9 and the volume pot and gotten rid of R7 and C13, so there must have been a reason, but I'm guessing it came after a few rounds of tweaking. 
• Then we have a cathode follower pushing a tone stack. So early in the preamp? We're not losing much gain, thanks to the cathode follower, but we also don't have a huge adjustment range for the controls, and since it's so early in the preamp it's not going to have that much influence on the distorted tone of the amp. Kind of the worst of both worlds. Cathode followers distort really interestingly in the traditional Marshall schematic, because they're DC coupled and biased quite warm so they're always tugging on the gain stage they're coupled to. Merlin goes on for a handful of pages about this in both editions of his preamp book and he does a much better job of it than I could, but because this is an AC coupled cathodyne, you're not getting that signature smooth distortion out of it. It's kind of a wasted stage here.
• Then the signal is attenuated in half and sent to a gain stage. Fair enough. The top half of that voltage divider is also acting as a big grid stopper, which minimizes blocking distortion and also rolls off some high treble, which we need to do because there's no tone stack coming later.
• And again, the signal is attenuated in half and sent to a gain stage. This one's a high-gain stage though, and since we're at the end of the preamp it's clear they're trying to get the power tube to break up before the preamp, and that's a noble goal. There's a whole ton of grid stopperage (two 470k resistors in series) which, in addition to the higher gain (notice the plate resistor is 220k) is going to roll off a good chunk of treble. I don't feel like doing the math right now, but this seems a little extreme, a little sloppy, and excessively noisy.
• Speaking of excessively noisy ways to cut treble, they put a cap across the anode resistor. This means any AC ripple is going to be injected right into that high-gain stage. The power supply seems pretty well designed, but this is a single-ended design so we really should be extra careful about that and put it after C5 if we really need to cut even more treble. You know, we could've just put the tone stack at the end of the preamp instead. Just sayin'...
• Then finally there's one last voltage divider and then the power tube.

I got to work on one of these recently, and the customer wanted these mods: Some Mods for Marshall Class 5, which I highly recommend as an easy and cost-efficient way to make this amp a whole lot better. The guy clearly knows his stuff, and while I think I would've tried some different things, I can only imagine he already did and these mods sound better.

It's not really a bad design, but considering the MSRP on these when they came out I expected a little better. Really going to town on one is pretty cost-prohibitive - who wants to dump another couple hundred rebuilding an amp that only cost a couple hundred to begin with?

I was tempted to come up with my own schematic, something I'd do if I were using a Class 5 as a donor amp and had more money than sense. Well, I guess I already meet that last criterion - based on the length of this post alone, I could be homeless and still have more money than sense. But really, if you were to gut this thing and build something inside of it, just build a Plexi or a JCM800 and call it a day. Maybe add a "preamp out" jack and run that to a beefy power amp. Or if you want something a little more unique, build a High-Octane inside. If you were really nuts and didn't mind buying all new iron, you could go for a Single-Ended Lead and really blow minds.

Alright, that's enough. I can think of a few more mods you could try on this thing - bootstrap the cathode follower to max out the gain of V1, add a freaking NFB loop for the love of god to get a crunchier overdrive and better cleans, DC-couple the cathode follower and move the volume pot to after it ( in place of jumper LK 21 perhaps), change C12 to a 1F cap for good luck... but I wouldn't have thought of the headphone mod. That's some cleverness right there.