Category: Amplifier

Amp #4 – Something JCM800-like

I’m going to be short and to the point since I’m poor at writing. It was finally time to build amplifier #4! This time I wanted to have another go at something high-gain (because I think that’s pretty cool).

I started out with the JCM800 #2204 schematic and used that as the general structure for the gain channel. I wanted a 30W-ish EL34 push-pull output stage, so I reused (with appropriate modifications) the output stage I put together for amp #3. I also had the output and power transformers ready as I scrapped amp #2 (it didn’t work properly, go figure…); a Hammond 1750Q and 372FX, which gave me some design directions.

After prototyping and going through several iterations I ended up with a two channel amp, high-gain and clean, with a blending phase inverter because I thought that would be fun. But since blending both channels in the P.I. would prevent the use of negative feedback, I also added, again for fun, a passive switching scheme that could take the amp between blending both channels, and muting one channel while giving the active channel the option of having negative feedback applied.

I had great fun and success building the previous amp on fiberboard, so I wanted to do the same this time, foregoing the point-to-point disaster that was amp #2 😉

I always forget how much work goes into building an amp from scratch. I must have been working on this six months straight, not counting the idea and drawing phase. And the shear price of all the parts had to be divided across many months anyway. This is certainly not a cheap hobby…

Lots of more or less interesting images of the amp taking form.

Spent some time designing nice face- and back plates, and got them etched in acrylic plastic from . Came out really great at a reasonable price. And of course I ended up bothering the circuit and ditching the “Mid” control in favour of an “Output” not long after. Typical! 🙁

No big, complicated build without figuring out what’s wrong and doing some changes along the way. Notice the fire extinguisher.

Had to build a house for it as well. Not the most exiting part of the project, but it’s nice seeing everything coming together.

And finally, done.

I’m pretty happy how it ended up. Here’s a quick, off-the-top-of-my-head feature list:

  • High-gain channel in the spirit of the JCM800.
    • James tonestack with shift control (based on idea from Merlin Blencowe’s books).
    • Less bright than the JCM800, because I don’t like too bright.
    • Gain into- and out of the gain channel for some added possibilities (turned out essential for being able to blend the two channels).
    • Parallel FX-loop
  • Clean channel in the spirit of the DC30.
    • “Big muff”-style tilt control for single adjust of both bass and treble.
    • Not enough amplitude compared to the gain channel. Next time…
  • Master volume and presence control in the output stage.
  • 30-ish watts of cathode biased EL34 push-pull output. I like cathode bias for the additional fail-safe. And it sounds good 🙂 Oh, and no manual bias adjustments required.
  • Blending of gain and clean channels possible (no NFB).
  • Single channel output (with or without NFB).

The schematic isn’t 100% when it comes to drawings of transformers, impedance switch etc. I didn’t want to spend time drawing more components in EagleCAD, so I just went with something close.


Updated: 29 November, 2017 — 00:06

Amp #3

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!



Updated: 18 May, 2014 — 13:56

Amp #2

I finally got around to finishing my second amp build, and as I promised here are the details.

Amp2 outsideAmp2 inside 1Amp2 inside 2Amp2 inside 3Amp2 inside 4

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 🙂

Amp2 Schematic

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.


Updated: 2 May, 2013 — 00:44

Amp #2 – Parts

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.

Updated: 21 December, 2012 — 22:40

Bad Cat Mini II clone

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 and also discovered a thread on 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.

Bad Cat Mini II parts

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.

Bad Cat Mini II chassis buildBad Cat Mini II gut shot overviewBad Cat Mini II gut shot detail 1Bad Cat Mini II gut shot detail 2

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…

Updated: 21 December, 2012 — 20:43
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