The SEplex


I have never been much of a fan of single ended amps. I started modding amps with a Valve Junior and no matter what I did to it it always sounded pretty bad. My first build was the Two Stroke single ended amp from Dave Hunter’s book and that was way better than the Valve Junior but it eventually evolved into the Sangria.

I had never done a big bottle single ended amp because that seemed to make no sense. The iron would cost a lot and it wouldn’t even sound as good as a push pull 6v6 amp. However, after building a ton of smallish push pull amps and despite all my misgivings I decided to try a big bottle single ended amp anyway.

I decided that one of the problems with many single ended amps was just the concept of taking a random preamp and putting it in front of a power tube without regard for the phase inverter and, usually, negative feedback (more on that later). For the basic circuit I went with a single ended plexi design. A Marshall Plexi based single ended amp has obviously been done before but I wanted to do something about what is normally missing as far as phase inverter gain and negative feed back.

I had built other Marshall Plexi based amps like the 6F4Plexi and decided, based on that experience, that the normal channel was not essential. So instead of using a triode for the normal channel I decided to use that triode for a gain stage after the tone stack where the phase inverter would normally be. I also though this stage could be used to return NFB to. So that’s basically what I had in mind building this.

Here are some progress pics:

early guts

chassis1

I’m using a bud box for the chassis and it worked out pretty well.

Overall the build came together pretty well. I’m using a Weber WSE15 for the OT and the PT is a Edcor XPWR021 which has a 290v output and gets me around 355vdc rectified at the first node. The tag board I’m using allowed the use of radial filter caps which I’ve found are a much better deal than axial caps. I’m using some 22uF
and some 47uF.

Here is the finished product:
SEplex_chassis1

seplex_guts

I had done an early schematic and pretty much stuck with it after firing it up and trying it out some. Probably the biggest change I made was decreasing the the first coupling cap from .02 to .0047. The fact that it was better with NFB (like a real Marshall) indicated to me that I was on the right track design-wise. BTW, The negative feedback circuit came from the 5E6 Bassman but I’m using a slightly bigger feedback resistor.

Here is the schematic:

Here is a layout I have been working on. It’s basically the layout that I used and it’s based on using a universal board like this. There is no switching in this layout.

So despite all the questions I had about building a single ended amp I’m really happy with the way this one came out. Getting good EL34 tones at a reasonable volume is tough and this one sounds pretty good. I think having the triode after the tone stack does allow the power tube to be driven better than many designs that have the tone stack and a master volume directly in front of the power tube. It’s probably about 10 watts and it can get loud. I haven’t played it with a band but I would guess that it could keep up with a sane drummer.

Here is a clip:

Since doing this clip I have added a small 100pF cap on the first triode going from the plate to the cathode. This trims the very high frequencies a bit. It’s on a bunch of different Marshall amps.

Older Version (v1)
Since I posted this design I’ve had a few people build this amp and I’ve heard the original version had too much gain. I can respect that. The current schematic above includes a low gain switch that adds a voltage divider after the tone stack. So in the current version the boost mode is what used to be the normal and there is a new low gain mode. For reference, this is the original schematic.
The gain setting of the clip is what originally was considered the normal setting but is now the boosted mode.

8 Comments

  1. Like the SE Plexi. Very cool. Wonder if I could pull something similar off using 6SL7’s in the preamp. Gain would be lower but it might be cool. Not as well versed as you with this stuff though. But it would be a learning experience either way. Love your site.

  2. I’ve been thinking of design like this, but using a KT88 and hopefully putting out 20w or so.
    Only thing is I’m considering the use of 3 12AX7s to get the normal channel and a cascade switch. I also came up with something more like half a phase a inverter and less like a simple stage, just so the NFB can work as it was intended to.
    Will probably end up using a 50w JMP/JCM PT clone to power everything the right way and don’t come short on that KT punch.
    Any advices?

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