This amp is pretty much a result of having the transformers and a chassis laying around. I figured a pair of 6V6s would work well with the MPS PT275.2 and I had a 8k OT. I also wanted to do another amp with the 5725 pentode but in an amp that included a tone stack.
The first version was a sort of Matchless Clubman clone more or less (where the “Club” in the name came from). This amp worked well and had a great clean tone. My problem with it was the distorted tones. It didn’t sound very smooth to me. If a clean sound was what I was after this would have been a good option.
I tried changing the tone stack and other things but I eventually decided to try a different topology so I moved the 5725 to the input. This setup was the 5725 going into a coupling cap and volume pot then another gain stage followed by a cathode follower with a TMB tone stack before the PI. This was sort of like a Vox top boost configuration but with the 5725 pentode as the first gain stage. This worked pretty well but it seemed to have almost too much gain. Having the pentode going right into the next gain stage with just a coupling cap and volume pot didn’t seem to leave much option for cleans, especially with a Les Paul. I tried adding a voltage divider and it was better but I was still not totally satisfied.
At some point I decided to try something different again. I still have the 5725 as the input but then it goes to a 6N2P cathode follower to drive the tone stack. Then there is the other half of the 6N2P, a voltage divider then the PI and output section. It’s kind of like a Fender Blackface but with a pentode and CF instead a normal gain stage and plate driven tone stack. I don’t think I’ve seen a production amp with this topology before but I’m sure it has been done somewhere. The Fender Bassman 6G6 is somewhat close in that it has a cathode follower and tone stack following the first gain stage.
This configuration had a lot of gain and bass so I had to do a fair amount of tweaking to the cathode bypass caps and coupling caps to dial it in. I also added a PPIMV and a switch before the PI to control the gain there.
I think I’m sticking with it now. I used the Duncan tone stack calculator to tweak the tone stack and the one I’m using now sounds good and is nice and versatile. By using the mid pot you can get a blackface/top boost scoop or turn the mids up for a more Marshall sort of frequency range. By using the volume controls and the mid pot I can get a pretty wide variety of tones. It can get clean blackface all the way to heavily overdriven.
Here is the current schematic:
And here is a clip. It starts out clean and ends up really overdriven towards the end.