Monkeymatic Penta (Matchless Clubman inspired)

[Darkbluemurder]

@SilverFox: I'm not sold on this design. My original goal was to experiment with EF86 pentode in the preamp (but not in V1) to learn what color it could give to the circuit. Instead, as I found with this amp, it does not distort at all, but pushes the PI into clipping very easily. It's a good sounding amp, for sure, but not for the reasons I was after.

The first gain stage in the original has very low gain, about 4. If you raise that considerably, e.g. by increasing the 10k plate resistor to say 56k and adding a cathode bypass cap you can push the pentode harder, which in turn allows you to decrease the pentode gain.

Not my idea BTW - it's all in ValveWizard's preamp design book.

Cheers Stephan

[turbofeedus]

Sorry to resurrect a 4 year zombie, but I recently finished my clubman, my first "from scratch" build.
Heavily inspired by xtian's layout, and all the good info in this thread. Big thanks to all of you!
I made a few changes, moved some things around. Both the transformers needed a cutout so that needed addressing.

Anywho, I remade my layout in DIYLC and created a BoM, and edited the schematic to match the build so far.

turbofeedus's Matchless Clubman EF86 Bill of Materials

turbofeedus's Matchless Clubman EF86 Layout - DIYLC File

Notes:

• I had the chassis custom made by Synaptic Amplification on Reverb. They were very helpful with design, were able to do custom dimensions in a heavier gauge alu than others, and did the difficult transformer cutouts. Thanks Steven.

• The transformers are from Transformer Design and Supply in CA. They are the same transformers Matchless currently uses. Bill was very helpful, you can reach them by phone at (951) 654-9199. Price was $400 for the full set (power, output, and choke). Note: The PT from TDS is 290V-0V-290V, not 345V-0V-345V as shown on schematic.

• I've added fuses on both sides of the HT winding as per valvewizard's recommendation.

• I've added diode protection to the rectifier as per basically everyone's recommendation.



Questions and Improvements:

• I've increased V1 plate resistor to 100K from the 18K in the schematic, with the hopes of driving the EF86 into saturation more readily. I'm still confused as to why Matchless chose 18K in the first place.

• The treble control works "backwards" as shown, and I see this is a problem xtian sorted here. I will probably do the same.

• Because the voltages from this PT are lower, the EL34 are sitting at ~50% plate dissipation with the 470 ohm cathode resistors I thought I needed. Will be trying lower values soon, 240R should put me right under 100% dissipation.

[xtian]

Wow, great work! And thanks for sharing your files with us. How does your amp SOUND?!?

The owner of my build is very happy, though he came back twice to try to lower the overall gain and increase clean headroom. I put an AT in the preamp, and ...I forget what else, but he's happy.

[turbofeedus]

Wow, great work! And thanks for sharing your files with us. How does your amp SOUND?!?

The owner of my build is very happy, though he came back twice to try to lower the overall gain and increase clean headroom. I put an AT in the preamp, and ...I forget what else, but he's happy.

You know, I like how it sounds, and all the voltages match up (correcting for the different HT voltage). I think I had a similar issue you had though, where the tone controls don't seem to be doing much for most of the sweep of the pot. Reading back, it looks like you changed that 220K to ground after the volume pot to 470K, and that helped? Also fixing the treble control made a difference as well? I know this was a while ago.

I'm also waiting on a mouser order for the lower resistance cathode resistors, I don't want to jump to any conclusions about sound yet.

[xtian]

I know this was a while ago.

I know! I don't remember.

[turbofeedus]

So I was going to swap cathode resistors today, get up closer to 100% plate dissipation, and noticed some issues with my voltages.
Firstly, I have 40Vdc dropping over my choke? DC resistance is ~460 ohm, that's about 86mA? Can't be right for three preamp tubes and two EL34?
Why do I have such a huge voltage drop across the choke?

I've updated the voltage chart in the schematic i've attached.
My transformer is measured at 303-0-303, and 124VAC at the wall.

Any thoughts?

[Tony Bones]

Your meter is probably confused by the presence of large AC ripple on one side. Your DC measurements of plate and screen voltages show a difference of only 14V. I wouldn't worry about it.

[turbofeedus]

Your meter is probably confused by the presence of large AC ripple on one side. Your DC measurements of plate and screen voltages show a difference of only 14V. I wouldn't worry about it.

Turns out I actually had the OT center tap on B node instead of A. My man sluckey sorted me out.

[Tony Bones]

So how did it sound like that? As close to class A as those things run, you can probably get away with it if the choke can handle the current.

[turbofeedus]

So how did it sound like that? As close to class A as those things run, you can probably get away with it if the choke can handle the current.

I actually made a few changes, so I can't say that any change in sound I heard was from that specifically.
Here's a new voltage chart. I changed the cathode resistors to give me 97% dissipation (according to robrob's calc).
I also swapped in a 12AT7 for the PI.
Bass control still isn't working right. It doesn't do anything audible for 90% of the range, then the last 10% is a huge boost in bass and gain. Not sure what that's about...

[turbofeedus]

Made another round of changes.
1) 12AT7 in the PI. I had a NOS one in the parts bin, figured I'd give it a go, I like the change.
2) Swapped the 220K dropping resistor at E node to 22K, kept V1 plate resistor at 100K
3) Decided to add a 180pf bright cap on a switch.
3) Still not 100% on the bass control, but I think it's just a taper issue. I have some linear pots on order at mouser.

Really starting to enjoy the sound now. Changes to V1 are definitely fitting my preference. I was looking at Vox schematics, and even the chieftain, and decided to try the 22K dropper and 100K plate resistor. It's a change I think is a step in the right direction.
Definitely has a powerful low end, maybe too much. Hard to tell with the bass control not working correctly. Hopefully it's just a taper issue, which will be solved with the linear 1M pots I have on order.

Schematic with changes and new voltages attached.