Category: Build

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 πŸ˜‰

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! πŸ™

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 πŸ™‚ 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 πŸ™‚

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

Request: Marshall ShredMaster

Here’s a quick vero layout for the Marshal ShredMaster. I’m assuming this is a distortion of some kind just from the name alone. Let me know what you think if you build it.


Update (22.02.2013): Built this one tonight and it works. Thanks to Timothy for verifying this one as well. There seems to be an oscillation issue with the drive at max. Placing a resistor in series with the “Drive 3” connection fixed it for me (I used 22k).

Update (23.06.2013): So apparently there’s two schematics of this effect floating around, and they’re slightly different. Since a few people have had trouble building this I decided to make a layout for the second schematic as well. With a bit of luck this is the one that works πŸ˜‰


Both schematics can be found in this thread over at My initial version is based on the “1992” schematic, and this new one is (obviously) based on the other one, dated “4/17/02”.

Update (25.06.2013): Just built this second version and it works as advertised without any modifications. There was a bit of oscillation with all controls dimed, but this is not surprising with long test leads attached to the pots flying everywhere, and I expect this to be gone once properly boxed.

If you’re thinking of building this effect I’d go for the second layout, though both work.

Update (26.12.2013): I’ve had this thing around for some time now, but didn’t put knobs on it until today. Here’s how it came out. I’m quite liking it πŸ™‚

ShredMaster buildShredMaster gut shotShredMaster board

Updated: 26 December, 2013 — 14:44
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