New Power Amp Build Matchless Clubman

dunner84

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Hello all,

I have been planning a new dedicated power amp project for a while now, and I am finally ready to start cutting the chassis. The power amp will be a relatively faithful copy of the Matchless Clubman with a PPIMV to replace the stock MV, and a switchable added gain stage similar to how the Mesa Boogie 50 50 is setup.

I have several preamps that I will be running through it ranging from simple matchless/vox type circuits, Dumbles, Soldano SLO, and many more. Most of my tube preamps are faithful clones right up to the PI input, so having the option of plugging direct to the PI of the power amp is important. The added gain stage will be for everything else. I think the 3PDT On-On switch will get it done.

I made a quick and dirty layout as the build will mostly be P2P with a small turret strip.

I would love some fresh eyes to take a quick look, and make sure there are no glaring errors.

A few things I am most concerned/unsure about:

- The preamp switch for going between the added gain stage and PI input. I am pretty sure I got it right, but you never know.
- The Matchless schematic has some oddball (to me) resistor node values. I will definitely be tweaking those to get the plate voltages correct.
- The EL34 1K screens seem odd to me. The wattage values are quite high. Maybe someone could school me quickly on that.
- The EL34 cathode resistors seem high. Most amps I looked at go with 270s. The schematic didn't state power handling. I assume 5 - 10W is about right.
- Is grounding the input and output of the gain stage ok as I have done it?

In a previous thread I inquired about adding switchable, variable negative feedback with Presence and Depth controls to this circuit to make some of my higher gain preamps more useable, but the Matchless PI runs with different values than most other EL34 amps I looked at, and I couldn't get any traction on the idea without changing the stock PI. I would like to keep it stock. I will maybe come back to this idea once I have the mp built and working.

I attached the quick and dirty layout, and patched together schematic. I appreciate any feedback!

Cheers!
 

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dunner84

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I had some time to move things along this week. The choke and PT bell covers were a little rough, so I hit them with some steel wool and spray paint.

I just need to decide on a transformer orientation before I mill the chassis. With the PT wires pointing towards the back of the chassis I gain some room inside for my board, but things will be tight where I want to stick the fuse, and on/off switch. With the wires leading into the centre of the chassis I gain room for the switches, but lose some interior space, and the OT leads don't fall right onto the tube sockets like I want them too..
 

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dunner84

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Slow progress, but I had some time to mark the PT placement, and cut it out today. I am a little frustrated I didn't punch two of the holes quite hard enough, and the step bit slipped. It would have been much cleaner. Oh well, things fit nicely.
 

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Kevin Wolfe

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That’s pretty cool 😎. I’d be interested in seeing those pre-amp designs too. I played a rack mounted bi-amped bass rig back in the day so I dig the versatility.
 

Paulg

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I used the Clubman power amp in this recent build. I mated it to a preamp in Merlin Belcove’s book (recommended reading!). Im very pleased with it. The power amp really has that swirling tweedish sound. Simple and effective! Good luck
55E8C907-D16D-48A9-8741-5A5713C0C7C6.jpeg
172D1448-302B-42BC-B529-3A97C96E2756.jpeg
 

dunner84

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That looks great! I love the cabinet! Thank you for mentioning the tonal flavour. That is exactly what I am hoping for. It will be quite different from all of my other big bottle amps. Do you happen to have your voltages? I would love to compare when I am finished.

My project didn't move much this week, but I have the next 10 days off, so I am hoping to be able to get it built and functioning before I go. I also have plans for a faceplate, grill, and head cab later this fall when I have more time off.
 

Paulg

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Thanks for the nice comments. I rechecked my log book. The B+ was 380v and final cathode resistors were 680 ohms for 40/42 ma per tube. I would, absolutely, use this output stage design again. It’s way loud enough with excellent tone. Also makes it easy on the end user, just plug in some new EL34’s and go!
Just a side note on the project. I built this in early Covid times ( can you remember that long ago?). I wanted to clear out some parts I had on hand. Hence, the tweed twin chassis and a JCM 800 OT.
 

dunner84

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I made some more progress today. Chassis is cut and drilled out, and started roughing things in to see how it will fit. I ended up choosing an eyelet board from a Hammond organ tone box. I like that it mounts upright. I like the extra room, and wire routing will be a bit easier due to the fact I am not rack mounting this anymore, and I want the tubes towards the front, so I can watch them glow through the metal screen.

The different board layout/tube configuration means I need to redo my layout. I will post a revised layout once it is finished. I just need to figure out the best way to rotate the tube sockets. In the meantime, here are some pictures of the progress so far.

Feel free to comment/critique!

Cheers
 

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dunner84

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Screen Shot 2022-08-13 at 8.40.35 PM.png


The screen resistor wattages listed on the schematic seem quite high. This got me thinking about the OT impedance. The Trainwreck Express also uses a 25W resistor, and those amps use an OT with a 6k primary. I remember reading once that increasing the primary impedance on a pentode increases screen current. I wonder if Matchless does the same? I did some digging, and I couldn't find an explicit answer to this question.
I have a 35W OT with a 6k tap that may be better for this build, OR I can stick with the 4k tranny, and use the selector switch to try 8k to get that high impedance el34 thing.
 

2L man

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Those screen resistor power values must have lots of safety margin :)

Using a multimeter and measuring AC voltage over screen resistor when playing long loud notes give good estimate for screen peak current calculatipn. Also DC voltage drop can be used. Multimerers are not best here because Screen operstion can be very dynamuc. Using an oscilloscope it is possible to see detailed operation better.
 

dunner84

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Im finally back home, and had a chance to continue this project. Truthfully, I kind of shot from the hip on this one in the planning department, and things ended up being more P2P than I originally intended. All in all I am still happy with it. I ended up breaking the turret board I wanted to use. I got a little cute and thought about drilling holes for some radial caps to sit in. Well the old board was much more brittle than I thought, so I was stuck using a smaller board I had on hand.

Good news is it lives! I started with 100k for the first dropping resistor, and this resulted in pretty low voltages on the switchable gain stage. Things were pretty crispy, so I dropped it down to 10k, and things are much better.

A quick calculation on the bias shows it to be around 110%. I may up the cathode resistors from 330 to 360 to see if that settles things down a bit.

Voltages:
B+
A - 403v
B - 399v

El34 (only showing 1. Voltages were within 0.5v)
Pin 1/8 - 25v
Pin 3 - 397
Pin 4 - 372

Plate to Cathode - 371

with 100k dropping resistor
B+ Node
C - 213v
D - 208v

PI 12ax7
1 - 207
2 - 52
3 - 63
6 - 206
8 - 63

Gain stage 12ax7
6 - 144
7 - 0
8 - 1.4


With 10k dropping resistor
PI 12ax7
B+ Node
C - 368v
D - 359v

1 - 356
2 - 83
3 - 105
6 - 355
7 - 84
8 - 105

Gain stage 12ax7
6 - 252
7 -
8 - 2.35

Things I still want to try:
- Add light negative feed back. Just a fixed amount on a switch
- Parallel the added gain stage. I think as a quick trial ill keep the plate and cathode as-is. Matchless does this, so I am curious to note the affect.
- OT secondary is 4K I believe the matchless is 6.8K. I can get to 8k quickly by changing the impedance selector, and I will experiment from there.

Next step is to clean the flux/grime out of the chassis and build a little cabinet/faceplate. I'm off the the hardwood store now, so hopefully I can whip it together this weekend.
 

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dunner84

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Well I made some progress on the head cabinet. I was a little skeptical of the cheap mahogany ply, but it turned out well enough to not start all over with something more appropriate. I added a few pics of the forms, initial fit, and first coat of stain/poly.

I haven't decided what I want to do with the faceplate/grill. I have some ideas, but I need to draw a few up. If I had easy access to a CNC machine I could produce my idea quickly. Without one I either need to cut it all by hand (which is doable), or shop around for some other materials. Either way, it will take me a month or so to finish it off.

On the circuit side I made some further modifications. I will try to remember to update the schematic.

The 330r Cathode resistors had the bias around 110% The next easily available size was 390R, and they put the plate dissipation right at 25W for 100%.

The added gain stage wasn't doing it for me. It was too crispy, and the distortion wasn't the most pleasing. I lifted the bypass cap, and that made an instant improvement, but still want quite right. I thought about it for a while, and realized I really love the low input of my jcm800. For anyone unfamiliar the low inout on a jcm800 bypasses the first gain stage, and goes direct into the cold clipper. I tried a 10k un-bypassed cathode on the added gain stage, and voila. It was nearly perfect. I added 4uf bypass cap, and things are right where I want them.

To summarize mods from the original schematic:

- First power supply dropping resistor dropped from 100k to 10k
- Mesa 50/50 gain stage cathode cap/resistor changed to 4uf/10k
- EL34 cathode resistors changed to 390R

I know it is not exactly a mainstream project, but I figured I would document it as most of my other projects have gone undocumented, and I am sure someone else out there might want to try one!

Sonically it is exactly what I want. It offers a different flavour from everything else in my collection, and serves as a better power section for a handful of my preamps.

I will post a few more pics once I am completely finished.
 

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