Benson Monarch Reverb

[Hoodnight]

So I've been doing a pretty deep dive on the Benson Monarch Reverb for the past month or so, largely as an exercise in reverse-engineering. I've found the process extremely gratifying, and I thought I'd share my results. I also thought I'd start a thread dedicated solely to the Reverb model, as that's the one I've been researching.

I've looked at all images I could source from across the internet, listened to interviews with Chris Benson, Bryan Sours, read Merlin Blencowe's website about 5000 times. I've read through the entire OG thread here multiple times as well, so a massive shoutout to anyone who contributed there as well.

Some assumptions and substitutions have been made, largely around transformers. I believe that the PT is equivalent to a Mercury FTDP-DUAL-U, however in my LTSpice model I have used a Hammond 370DX which should give voltages within 2% AFAIK. Similarly, Benson uses Soursound custom transformers which I believe are more-or less equivalent to a Tweed Deluxe 8KΩ:8Ω but I'm using Hammond's 1750E. Additionally, I believe that the filter capacitor network uses a parallel 10uf with half a 32/32μF can for the screens. Chris Benson said on the previous thread that it was 25->47->25->25μF, however a couple of Monarch owners confirmed 32/32μF cans, which makes the 32+10μF seem more likely. This is also a very similar arrangement to the old Supro Thunderbolt, and Chris has expressed his fondness for these old designs. Additionally, on the Reverb, there is an extra filter cap that feeds the reverb tube, which I believe is a 47μF.

I've created a GitHub repository for those technologically and collaboratively inclined: https://github.com/ctimlock/monarch-reverb

I'll also add the files below, and will try and keep them updated as best I can. There'll probably be mistakes, as I've never built an amp before, and I'm going off of grainy-ass internet pictures.

I'm also working on a modified layout that uses radial electrolytics rather that cap-cans among other things, which I will post here when I'm done.

Just because, here's the preamp's EQ with various tone and UKUS switch position combinations.

monarch-reverb-eq.png

[brewdude]

Nice work.
I look forward to watching this project evolve.

[CraigGa]

Nice work,
On my build I found that the US/UK switch popped when switched, a 10M resistor across it cured it.

Craig

[mhat]

How goes the build?

[Hoodnight]

How goes the build?

Still in the theoretical stage! I'm a bit of a perfectionist and don't have the space for hand tools, so currently designing a chassis to get it laser cut and fabricated.