18Watt.com

Hi all.
My mate blew up his Marshall JTM30 (apparently problems due to overheating are common with these amps), so he has agreed to let me build something similarly powered in the chassis. I'm using a Vox AC30 reissue transformer (Hammond 290MX) and a Fender 35 watt output transformer with 4, 8 and 16 ohm taps and a 4000ohm primary (Hammond 1760J). The output section is pretty standard 36 watt as far as I can tell, but the preamp is a mash up of 18 watt, Plexi, JCM800, Mesa and Soldano.

I've attached the first draft schematic and layout drawings. I've never tried to design a 'high gain' preamp before, and whilst I've done a fair bit of research, I'd appreciate any tips if people spot glaring issues or mistakes, or perhaps something that just straight up won't work.

I'm also curious about adding a tube/SS rectification switch. The circuitry/wiring isn't too hard, but I'm wondering if I'll end up running the tubes too hard in SS mode. What are peoples thoughts? It seems like it would make sense to have the option of that firm response with this higher gain style amp.

One more question... Does anyone see any issues with my 'Saturation' control, which basically just varies the cathode resistor value on the cold clipper stage? It's designed to run from roughly 5k through to 40k or so, so warmer than Marshall, all the way through to SLO100 and Mesa Dual Rec territory. Also, the 'Gain' control, which is basically just a variable resistor between V2A and V2B that controls how much signal gets dumped to ground. Will it work?

Thanks as always.

Possibly i am very wrong, but from what i read AC 30 shall have lower voltages/higher current to get them closer to pure class A operation. So more likely such PT will be ok even with SS rectifier. Depends how you bias them off course.
As for preamp i would go for a pure 2204. It is easy to dial in good tone and nice sound

Thanks for the response.
I thought about going for a straight up clone of a 2203 or 2204 preamp. I may do that in the future too, but part of this project for me is to better understand high gain preamps, and how to build cascading stages that work without over-overloading the next stage. How to use different value coupling and bypass caps to shape tone etc. I've got a really long way to go, but I've already learned heaps by doing this. Once I've arrived at a design I think will work, I'll spend an afternoon plotting load lines etc for each stage and see what transpires.

I think you're right with the SS rectifier. I'm going to try it and see. I can always bias for SS mode, which I think will leave a slightly cooler bias when in tube rec mode.

I've attached an updated schematic too. A few changes from revision one. I haven't worked up the layout for this revision yet. That's tomorrow or Wednesdays job.