Update (18.05.2014): I’ve made a few changes, mainly removing the FX-loop, and also put together a basic layout since I’ve had several requests for that as well. But to keep in mind this isn’t a simple color by numbers project; you need to know how to wire all of this properly, and you also have to keep safety in mind working with high voltages!
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.
Just a small update on something else I’ve been tinkering with lately, namely my second amp build. I’ve been planning to do this for at least a year now, but parts are really expensive to import and I’ve had to source a little at a time over a course of many months. This has of course also allowed me to draw, think, redraw, rethink and redraw everything a few times to where I think I’m happy with it.
The parts count is quite intimidating and I’m not even sure I’ll be able to fit it all in this 16″ x 8″ x 2″ Hammond chassis. At the very least it’ll be a great learning experience, and hopefully I’ll get it working without too much debugging and redoing. Fingers crossed!
More to come soon, and I’ll also post schematics etc. when I have something I know actually works.
Having completed the power supply I felt confident and decided I’d try a tube amplifier. Since this is my first attempt I’m sticking to something relatively easy, like a single ended 5 watt thing. I found a schematic for the Bad Cat Mini II amplifier on schematicheaven.com and also discovered a thread on freestompboxes.org where a couple of members had actually built this thing.
It took me a long time, but I managed to source all the parts eventually.
This is very much a learning experience and gut shots of an actual Bad Cat Mini II amplifier helped me figure out approximately how to lay out the various parts in a practical manner. Turns out the chassis I ordered is a lot bigger than it had to be, but that’s OK. Would have been a lot worse the other way around.
I’ve spent some time reading up on amplifier specific issues and considerations. I tried to implement star grounding, but none of the switchcraft jacks I’ve used are isolated. Once I’ve got the speaker cabinet ready I’ll have to evaluate whether I need to isolate these to reduce hum. Gave the amp a test run with an old speaker I had lying around and it worked quite well on first attempt, albeit at a very low volume. After some investigation it turned out I had the filament wiring on the 12AX7’s slightly wrong, but having fixed this the amp now works like a wonder. Time to get started on the cabinet…
Update (27.10.2010): Here’s my parts list for this project.
After many a month of sitting on a shelf my 5 watt Mini II clone finally received a proper house.
I’m quite happy with it, being my first attempt at an amp, but there’s lots of room for improvement. Building the housing turned out to be a lot more challenging than I first imagined, but this might come down to a complete lack of experience. I expect the next amp project to go a lot smoother in this regard.
In retrospect the housing should have extended a bit further backwards to help protect the power switch and speaker selector. Oh, and I might also place a board across the back to prevent my kid from touching anything back there.
Now I need to build a speaker cabinet…