Marshall Lead 12 Head tube conversion

[RogerWilco99]

With my Bogen CHB14 project well under way, I've begun thinking about a project I've wanted to do for some time.

I have two Marshall Lead 12 amps. One is a combo which I have modified by putting output jacks on the back of the chassis and by adding an IEC power inlet and removing the stock power cord.

What I want to do, though is to turn both of these into tube amps. Ideally, I'd like to use the same amp design for both. Converting the combo isn't really a problem. There's plenty of room for transformers and tubes to hang down into the speaker chamber.

But the head is another kettle of fish entirely. The entire vertical space in the head is only 5.25 inches. The chassis that's in there is a "dish" which means it's laid out exactly like the one in the combo. All the electronics are on the inside of the "dish" which has an open top for cooling.

The vertical depth of the chassis is 2.25 inches. It measures 12.75 by 6.25 inches.

By subtracting the chassis height from the vertical space you see that that leaves exactly 3 inches. Really not enough clearance even if there was something to on top to mount the tubes, etc. to.

Yesterday, while installing the cliff jacks on my CBH14 chassis, I had an idea that I thought I would run by you guys and see what issues I may have missed as to why it's a daft idea.

Basically, the idea is to not try to modify the existing chassis but still use it. To do this, I would use a .125" aluminum plate that's 4"x11.5" This will leave enough room to clear the cliff jacks, fuse holder and IEC power in the rear as well as clear the cliff jacks, power switch and pots in the front.

By putting the plate on 1.25" standoffs, I can then mount a circuit board and power supply board underneath it, and mount the transformers (Hammond 270EX and Hammond 125E) as well as four noval sockets for the preamp tubes and the two EL84s.

What I want to build into this amp chassis is a Marshall 20W PA but with lower voltages to get less power output and less heat.

I am concerned about trying to stick too much into this chassis and having heat problems. I can use quick disconnects/screw mounts to make the plate easier to get in and out. I would also have to put some aluminum foil on the top inside of the cabinet to keep from cooking the wood.

By using a .125 aluminum plate and 1.25" standoffs, that will leave me with 3.875 (3 and 7/8) inches of clearance for the trannies and the tubes. The height of the EL84s is about 2.9 inches (3.10 on the socket) and the PT is 3.06 inches tall. I could raise the plate height a little (maybe .125) to give more clearance to the components underneath. But I don't think that would leave enough clearance for the tubes. I'm looking for "flush" mounting tube sockets to lower the tubes by the .25" normally taken by the sockets. I know these exist because my Bogen amp uses them.

Again, too much heat is my main concern, followed by the cramped quarters for all the components. Arcing is not a good thing.

Does this sound practical at all or would there be heat/hum problems. I am concerned about air circulation building this in a "tub". I would be replacing the piece of wood that normally closes the back with heavy wire mesh that is all I have left over from a JCM800 amp that I bought for parts long ago.

Also if anyone has a better idea for transformers that might have enough power (250-0-250 HT 100ma, 6.3v 2A minimum) and a 15W PP OT, I would welcome your suggestions. I haven't bought the trannies, yet, since this is still in the planning stage. A Hammond 270DX fits the requirements above (and has 5v as well that I don't need). But it's just as tall and only slightly smaller.

I was considering using an EZ81 instead of a bridge rectifier, but that would require another amp of heater current (3A). The 270EX has that (and 275 HT as well), but I'm trying to minimize the components and get as clean power as I can. I can use a bigger initial cap in the PS if I go with the bridge.

I pretty much have to go with a standup PT since I will have to mount the PS and circuit boards on the bottom of the plate.

You may ask why I want to do this. I just think it would be cool to have a small Marshall head with a Marshall tube amp inside it instead of a SS amp. And building it this way, I can put all the original components back in the chassis and have only a few small holes in the bottom of the chassis if I decide to switch it back or it doesn't work.

I'm wide open to suggestions at this point since I have only done some preliminary work in measuring things and doing some drawings in AutoSketch to check clearances as well looking at the dimensions of lots of tubes and transformers.

I could do this in a larger head cabinet but then it wouldn't be a Marshall (design) amp in a Marshall cabinet. It would sound like a Marshall but it wouldn't look like one.

And the big kicker. I could have this Marshall tube amp head on top of my Marshall Lead 12 Micro Stack. Quite a novelty, there, I think.

I could have a taller custom cabinet built that looks like a Marshall JCM800 cabinet cut down in length, but without the Marshall badge. It wouldn't be as "cool". I'm also on a fixed income now that I'm disabled.

The whole project in the head cabinet might be impractical. But I'm still going to do it in the combo. I want to build the same amp in both, though, using all the same components. And have two variations of the same amp.

I'd love it if someone actually made a full stack like the JCM800 series but with lower (less than 15 watts) for home players and even for studio work. I don't play anymore (I can't) but I know several people who would love to have something like this.

Again, I'm open to suggestions, criticisms, questions, ideas, etc.

Cheers,
Bruce Lewis

[Deftone]

Any pics to share? I am interested in doing the same thing with my Lead 12, in fact that is why I purchased it.

The later made Marshall 15w mini stack might be another option. I believe the head is larger.

[olddawg]

At the moment I am in the process of putting an 18 watt Marshall clone chassis into an old Marshall Lead 100 Mosfet head cab. It has plenty of room and I have the matching 1965 A 4X10 speaker cab. The old 80s Lead 100 Mosfets look a lot like a JCM800. You can find them cheap if you look around, especially if they're broken.

[olddawg]

Come to think of it there are a couple of techniques that might help you save space. In old Heatkits, Eicos, etc they used to cut and bend a flap out of the chassis so that they could mount the tubes at a 45 degree angle which saved verticle clearance. Another thing you might consider is using pcb mounted or integrated tube sockets. That way you could mount the board and the tubes deep in the chassis and just have some of the glass sticking up through the holes in the main chassis. A lot of modern amps do this. You can even recess the transformers. You might want to consider a whisper fan if you do this. Any of this is cheaply and easily done.

[RogerWilco99]

Any pics to share? I am interested in doing the same thing with my Lead 12, in fact that is why I purchased it.

Is your Lead 12 the stack/head or the combo? I have one of each. The combo would be easier to do. But I'd prefer the head with the two cabs. I already have my combo setup to be able to run my two stack cabs. It sounds better that way.

The later made Marshall 15w mini stack might be another option. I believe the head is larger.

I have some pictures of my Lead 12 stack but I don't know if that's what you're talking about.

I've looked at the MG15MSII head and not only is it the same height, it's pretty cheezy construction. Made in China flakeboard junk for the amp and cab cases.

I have looked at possibly the MOSFET type stack. But those are pretty rare these day (most have died and been thrown away, I think) so that the ones you do find can be pretty expensive.

I don't know anywhere else to look for used stuff (I can get around easily since I'm not supposed to drive) so eBay is about my only mode along with Craig's list.

I really like the idea of a 10/12 watt tube head for the Marshall stack, though.

Marshall just sort of stole my thunder on this, too. They just introduced a 15 watt tube stack with built-in digital effects.

I could buy one but where's the fun in that, though. At least Marshall has acknowledged the appeal of a low powered tube amp (very late to the party I might add).

[RogerWilco99]

Come to think of it there are a couple of techniques that might help you save space. In old Heatkits, Eicos, etc they used to cut and bend a flap out of the chassis so that they could mount the tubes at a 45 degree angle which saved verticle clearance. Another thing you might consider is using pcb mounted or integrated tube sockets. That way you could mount the board and the tubes deep in the chassis and just have some of the glass sticking up through the holes in the main chassis. A lot of modern amps do this. You can even recess the transformers. You might want to consider a whisper fan if you do this. Any of this is cheaply and easily done.

Yeah, I even though of mounting the tubes horizontally above the circuit board. But I just couldn't arrange everything inside the amp. Not enough "floor" space for the trannies, the board and the tubes that way. The best solution I've come up with (in AutoSketch) is a mounting plate that sits about 1.25 inches recessed into the chassis.

Another possibility is to make a new chassis with a "facade" front panel that is actually only about 1.75" tall and havnig the facade continue up to the full 2.5" height. That would get me another .75" for my components which should be enough and I could put the circuit board, jacks and pots mounted the "traditional" way for a bottom mounted head chassis.

I'd have to use mini pots for that (16mm instead of 24mm) and probably couldn't use the traditional big square switch(es) as the JCM800s. Part of the visual appeal for me and one reason for trying to cram all of this into the Lead 12 head.

I "could" have a custom head made that looks like the Lead 12 head only about 1.5" taller. Then have a custom chassis made as well so that all the components could be mounted in the normal fashion using the Marshall knobs, cliff jacks and big square switches, too.

That's an _expensive_ route from what I've found so far.

Still I keep working on it.

One thing I'm thinking is to scale the head up to fit a set of 1x12 cabs (like the new Marshall 15 watt tube amp) and have it like the Lead 12 head. You could even use the head with the 1x12 cabs from Marshall and make your own stack that way.

I'd love to get a look inside the new Marshall 15W tube head. So far there's very little info on them since I don't think you can get them here, yet. I think they've only just been added to Marshall's lineup.

[Deftone]

I'm not doing it myself, but I found an amp builder nearby who is working on mine. He is pulling the SS parts and replacing them with a handwired 5 watt tube amp based on a blackface champ. His name is Jeff Check (he actually led me to this post and this Forum) Here's what his plans are:

"I have been looking at a bunch of Marshall schematics and I believe that for a 5 watt amp it be best to build you a blackface Fender Champ circuit with Volume, Treble, Middle, and Bass controls (no Gain control) and the line out jack. It would use a single 6BQ5 (at least that much would be Marshall) and a solid state rectifier. This would keep the tube count down (just 1-12AX7 and 1-6BQ5) and keep the heat down as well. The blackface amps are known for being clean (up to a point) and the controls are responsive. I would utilize a used Power transformer and a new OT (8 ohm output)"

I asked him for another OT and two outputs so I can use both speakers. I should have the amp back in a couple of weeks.

[RogerWilco99]

I have been looking at the Marshall 18 watt circuit for this. It's a PP design with 6 tubes in it's original configuration. What I'm looking at is putting a JCM800 2204 type pre-amp but with a single tube then a 12AX7 PI tube and the pair of 6BQ5/EL84's.

It requires a litle more iron than the Champ but it's a Marshall all the way through even if it is a FrankenMarshall.

I've thought also about dropping the EZ81 (6CA4) rectifier tube and going with an SS recifier. That would also simpify things. The JCM800's don't use tube rectifiers anyway.

You can drive the two Marshall 1x10 cabs from a single 8 Ohm output. They are 16 ohm cabs. You just need a parallel pair of jacks on that 8 ohm output. The lead 12 head already has that layout. I can post a picture of that if anyone wants it.

Is yours the 3005 head or the 5005 combo Lead 12?

I'm putting together a schematic of the circuit I described above. I'm going to make a turret board for it then layout the tubes, and transformers around it. I'm also going to try to use as small transformers as possible. That might mean using a "lay down" PT instead of the Hammond standup I've got now.

Mounting the tubes on the circuit board is an interesting idea. But I'd use standard tube sockets and not PCB sockets. I don't want a PCB.

I'm also probably going to do a separate PS board that I can mount sideways for more room. I can keep all the power on one end of the chassis that way, too. I'm not at all sure I can make that fit.

One thing I do want on my amp is for all the original features to be there and all the inputs and outputs to work as well. I also want to add a black square standby switch next to the power switch (needed with SS rectification).

If I stay with noval tubes, I don't have much of a clearance problem with those. But by mounting the tubes on the circuit board, I can get more clerance and make way for 6V6 tubes as in the Marshall Studio 15. I've also seen some chatter about building full JCM800 2204 circuits and using 6V6 tubes in them to get about 15 watts output. I'm still exploring that, too. The 2204 circuit board is a big one, though.

So, I'm still going in ever tighter circles. I'm sure I'll finally make a decision and start a build.

[olddawg]

Here's an 18 watt plexi schematic and layout. I'm making something like this for my Marshall Lead 100 Mosfet cab.

[Deftone]

I have the Lead 12 head and 2 x 10" cabs. I got it for $160 + tax at a local pawn shop. I would have liked Marshall architecture just to be authentic but I don't think there is any sub 10 watt Marshall schematics to follow. Plus, the head is very small.

[RogerWilco99]

I have the Lead 12 head and 2 x 10" cabs. I got it for $160 + tax at a local pawn shop. I would have liked Marshall architecture just to be authentic but I don't think there is any sub 10 watt Marshall schematics to follow. Plus, the head is very small.

There was a Marshall 10 watt tube amp called the 1930. I've included the schematic for it. What makes it remarkable is that it's an even bigger departure from the "traditional Marshall" than the 4001 Studio 15 was with its 6V6 tubes.

The 1930 uses a pair of ECL86 power tubes that also act as the PI. The ECL86 is a triode and pentode in the same bottle. The pre-amp tube was an ECC83 so at least that was "normal". I have a Bogen PA amp that had the same power tube/PI arrangement in it with a 6AU6-A pentode as the pre-amp tube. I decided that I didn't want a two channel amp like the 1930 and built that amp as a 13watt version of the 18Watt SuperliteTMB. I'm almost done with that one, now. I'm in the process of moving it into another chassis (a new Hammond 16x8x8 steel). The old Bogen chassis looks pretty bad without the cover and I can't use the cover with the controls, switches and input jacks I have on there.

Anyway, back to the Lead 12. I think I've finally figured out how to make it all work in that chassis as a "proper" JCM800 with two ECC83s and two 6V6 tubes. I'm mounting the transformers directly onto the chassis, putting the power supply board in on it's side vertically, then having a small 5"x2.5" circuit board and an aluminium 3"x4" plate on 1" high standoffs to mount the tubes on. The design should be open enough to allow for cooling. If not, I can stick a small heat activated fan down by the power switch.

I have a picture of the gross layout that I'll attach. The only thing missing is the standby switch and the presence control. I may have to mount those on the back somewhere. I'm thinking my circuit board may be a bit small and I may have to mount a lot of components directly on the tubes and pots.

If money was no object, I'd have a custom head cab and chassis made with faceplate and back plates that would still fit the Marshall 1x10 cabs. But then I'd probably want them to be 1x12s and make new larger (16 inch) speaker cabs, too. Then I could also make the head wider (16 inches instead of current 13) that would make things easier. But I have to work within my practically non-existent budget, now (I'm disabled).

Of course, if I did that I'd almost be duplicating Marshall's new 15 watt tube amp stack with 2 1x12 speaker cabs. Meh.

I have looked at using the "Valve Junior" approach and use a "Marshallized" VJ board that Ken Watts sells (Watts Tube Audio) with an octal tube socket, mount a 6L6 on it and hook up a gain circuit and the TMB in addition to the volume control. That would probably be pretty close to what you're doing deftone. It seems a simpler route with only two tubes.

You can get 10 watts out of a 6L6-GT/STR if you run it high enough voltage. That's what Epiphone Galaxie 10's use for a power tube. I'm looking at a PP configuration because I think that will generate a tone closer to the "Marshall" sound than an SE amp.

I really want to get a look at the innards of that new Marshall 15 watt amp. I'm betting that it's powered by a pair of EL84's. But I could be wrong. It might be a single EL34 running a lower voltage.

Please post some pictures of yours when you get it back. I'd love to see it. Mine's in pieces right now. I may have to put it back together and sell it. In which case I'd do the tube conversion on the Lead 12 combo I've got. That would be much easier since I could hang the iron and the tubes under the chassis leaving the innards free for the circuit board and the wiring.

Cheers,
Bruce

[lampaculos]

Hello, can anyone send me the measures of the Marshall 5005 Combo?. I used to own one of those little beasts, but I had to sold it, now I can't get one, so I decided to build a replica of my old loved amp.

Thanks in advance.

[RogerWilco99]

Hello, can anyone send me the measures of the Marshall 5005 Combo?. I used to own one of those little beasts, but I had to sold it, now I can't get one, so I decided to build a replica of my old loved amp.

Thanks in advance.

I'll see what I can get for you from mine. I'll put it into Autosketch and produce a measured drawing if I can.

Can you handle dxf/dwg files? Or just a jpg of the drawing?

[lampaculos]

Thank you so much RogerWilco99!!, it is not necesary that you make a sketch of the combo, if you want, just post the numbers, however if you want to draw a sketch, jpg will be fine.

Thank you again!!

PD: Abusing your gentleness, I ask you for some pic of the PCB so it can fit in the cabinet.

[RogerWilco99]

Thank you so much RogerWilco99!!, it is not necesary that you make a sketch of the combo, if you want, just post the numbers, however if you want to draw a sketch, jpg will be fine.

Thank you again!!

PD: Abusing your gentleness, I ask you for some pic of the PCB so it can fit in the cabinet.

Okay, here's two measured drawings. One with fractional and one with decimal measurements. It's all SAE (not metric).

What exactly are you looking for as far as the PCB is concerned? I can give you the measurements of all the inputs, output, knobs, switches, etc. The actual chassis is 12.75" (12 3/4) wide, 2.5" (2 1/2) high, and 6.5" (6 1/2) deep if that's what you need.

Do you need the dimensions of the PCB? The knob spacing? There's not a lot of room in there to build a tube amp if you have to put any of the trannies or tubes inside the chassis. For the combo, you can let those hang into the speaker section but you'll have to watch the spacing with the speaker and keep the preamp tubes away from the speaker magnet and voice coil.

I've modified my 3005 chassis a bit by putting an IEC power inlet on it instead of having the power cord come in through the bottom. I've also put two output jacks on the back of the chassis (similar to the 5005 head chassis) so it can drive the internal 8 ohm speaker or two 16 ohm speakers (like the ones in my 5005 Micro Stack). The circuit boards in the two amps are the same. Originally, the speaker wire went through the bottom down to the speaker with no speaker jacks at all.

Let me know what else you might need. I've got a 3005 and a 5005.