I finally got around to finishing my second amp build, and as I promised here are the details.
I’m reasonably happy how the internals ended up given that it was all laid out more or less as I went, though I am a perfectionist at heart and this really is far from good enough. Gives me a lot of room for improvements for my next amp, though 🙂
So this all began several years ago when a colleague lent me a Bad Cat Xtreme-tone preamp effect (the one with tubes inside) which I liked a lot. I got my feet wet converting the same preamp circuit to a JFET variant some time after that with the thought of eventually building a full amplifier for myself. To make a long and boring story shorter and less boring I started reading books on the subject, and ended up adding to and modifying the original idea to what you see above.
I’d like to bring your attention to a book that, more than any other, helped me understand a lot of this stuff. “Designing Tube Preamps for Guitar and Bass” by Merlin Blencowe is a surprisingly well written and downright enjoyable book made to help novices like myself getting into this great hobby/addiction (I can also recommend his book on power supplies). I found a lot of the changes and modifications here.
So the basis for the amp is the Hot Cat 30, which again is , I believe, based on a Matchless, which again is based on a Vox AC30. So it has the AC30 power section lacking the typical negative feedback that many other amps have.
I replaced the clean channel with a gain stage borrowed from the Dr. Z Carmen Ghia (a favorite of my friend). Then following this I added a standard BMP tone control with two added switches giving the option of a mid- scoop or boost (or a frequency shift towards bass when both are engaged) that I arrived at by experimenting with Duncan’s Tone Stack Calculator.
For the gain channel I replaced the tone stack with a Marshall one (bass, mid, treble), and followed it by an active parallel/serial FX-loop I got from Merlin’s book. I also got rid of the “edge” control and consolidated the two gain controls into one dual pot. And lastly I replaced the odd first gain stage with a more standard cathode biased one.
The schematic lacks a proper symbol for the output transformer and the following wiring, as well as the input power socket wiring, the heater wiring, the indicator LED + resistor, and the humdinger trimpot. There are also small tweaks to the power supply section as well as a “tube rectifier emulation” switch, all of which was borrowed from Merlin’s book (as far as I remember).
So to sum up, it’s a two-channel clean/high gain amp with an FX-loop, running 5x 12AX7/ECC83 and a push-pull power section with 2x EL34. It’s very loud and very cool, though being a perfectionist there are things I would like to improve/change.
Initial thoughts/lessons learned:
- It’s too loud for home use (I knew this though). Maybe an attenuator will help. Some kind of power scaling would have been nice.
- The FX-loop, though fully functional, could probably be scaled down a bit/made simpler.
- I didn’t get much out of the “tube rectifier emulation” so will probably just stick with plain silicon rectifiers next time.
- The point-to-point-layout-as-I-go ended up looking less tidy than I would have liked. Maybe this amp was a bit too much for such a small enclosure (8″ x 16″). Bigger enclosure or less components next time maybe.
If there’s requests for it I’ll add parts list and voltages. I also welcome suggestions, advice and criticism on anything from schematic and design to building and what not.
I’ll add more pictures when the amp is fully dressed up in tolex etc.
Update (01.05.2013): Here’s a rough overview of the various sections within the amp.