introducing the McGrath Mini guitar amplifier

The perfect small amp?

This is the new prototype guitar amp that I've built: basically, a Pandemic Project.  It's a small, all-tube amplifier, featuring a single-ended EL84 driving a 6-inch Jensen speaker.  Power output is probably around 3 Watts (haven't made an accurate measurement yet).

My model for this amp -- or more accurately, my design reference -- was the Fender Champ.  I have a Champ, and it's a great little amp in many ways, but it has a few limitations and issues that keep it from being "the perfect small amp", at least for me.  After experimenting with making modifications to the Champ, I gradually came to realize that I'd be better off designing exactly the amp I wanted, from scratch.  This, then, has become my answer to how to make a better Champ.

The first and most-pressing issue with the Champ, for me and for many rock guitarists, is that it lacks much gain.  It is a "relentlessly clean" amp, with many aspects of its circuitry being specifically designed to avoid distortion.  For those who *seek* distortion, the only option is to turn the Champ up all the way.  This results in a brash "crunch" tone, which can sound good in a classic-rock context (Eric Clapton used a Champ on Layla, and Joe Walsh was known to use the Champ on many of his hit songs).  However, to quote youtuber Ola Englund, the Champ "does not chug".  And even the crunch tone is not available except near the maximum volume of the amp.  The Champ is low in wattage compared to many other amps, and it probably can't compete on stage with a full band and drummer, but at home or in a small venue like a restaurant, turning it up all the way is not likely to be popular with other people in the same building: the Champ can get surprisingly loud.  That's not always convenient.  Anyone seeking heavier rock or metal tones with a Champ will inevitably find themselves running overdrive pedals or other gain devices ahead of the amp; and they will then probably have to carefully dial the EQ ahead of the amp, to prevent the Champ's generous bass response (which sounds so good on clean tones) from flubbing out and ruining the overdrive tones.

Another problem with the Champ is that it lacks an effects loop.  Of course, this isn't normally a big problem when the amp is running clean: reverb and delay effects sound perfectly adequate running into the input of the Champ.  However, if one makes changes to the circuitry to produce distortion at low volume settings, as many guitarists would desire, then time-based effects at the input will be distorted along with the dry guitar signal, producing a nasty blur of noise which is seldom musically desirable.  The usual approach with the Champ would be to add the effects later, at the mixing board: this is workable for recording sessions, but it means the Champ is not an all-in-one solution for practice and live situations.  High-gain amps need effects loops.

A final criticism of the Champ is that it is simply too big.  It's the smallest of Fender's traditional offerings, to be sure.  Yet, the Champ is far from a compact design.  It seems that Fender started with a desired size and shape of cabinet, and then fit the simple Champ circuitry in, allowing plenty of space for "breathing room".  Consequently, the Champ is delightfully easy to work on, and there's lots of extra space for additional circuitry or even a larger-diameter speaker.  But it could be much smaller.

So with the McGrath Mini, I set about to build the smallest all-tube amplifier that I could, within certain parameters.  I felt that a 6-inch speaker was the smallest that could reasonably be expected to produce any kind of good tone, and I knew that I wanted a single-ended power tube, like the Champ (recently there have been some "micro" sized amps, using non-traditional tubes for the power amp such as the 12AU7, and generally putting out 1 Watt or less: these are cute, but they can't really fill the all-in-one "perfect small amp" role that I have in mind).  The Champ uses the 6V6 for its power amp; the McGrath Mini uses the EL84, which is (arguably) somewhat better suited to high-gain applications, and is physically smaller than the 6V6.  The desire for lots of available gain and tonal flexibility, means that three 12AX7 preamp tubes are needed.  Finally, it was my desire to use a tube rectifier, rather than solid-state diodes.  The McGrath Mini uses the EZ80.  Given that roster of five 9-pin tubes, and the 6-inch speaker, the outer dimensions of the case were more or less determined.  The width (10.25") is the minimum practical width in which the row of five tubes can fit; the height (11.5", including the stand) is just enough to allow space for the speaker and the chassis (control panel) above it; and the depth (7.25") is the size of the narrowest standard lumber which provides enough room to fit everything cleanly (the next size down, 5.75", would not leave room for the tubes to fit behind the speaker magnet).  

I call this amp a prototype, because I intend to manufacture them in the future.  The circuit design is still a work-in-progress, and there's more work to be done figuring out the details of how to mass-produce these amps (on a small scale).  I have no idea, yet, what the price-point will be, but I'm afraid it won't be cheap.  Just the raw materials for any tube amp are quite expensive these days.  The tubes and the transformers in particular, are line-items totalling well over $100 on this amp.  Then there are dozens of other electronic and mechanical components, the Jensen speaker, circuit boards to fabricate, metalwork and painting/silkscreen, and of course the nice-looking wooden case.  It's hard to picture getting the price below "four figures".  However, as a struggling musician myself, frequently annoyed by the inaccessible pricing on most "boutique" gear, I will do what I can to make some of these amps available to the rest of us.  There will likely be a "beta" phase of development, where I will build a number of amps and get them out to as many musicians and reviewers as possible.  These will likely either be loaners, or available for purchase at barely above materials cost, in return for their review and commentary about the product.

Also, as usual for me, the documentation of this amp design will be fully "open source", made freely available here and other places on the Internet: anyone who wants to is encouraged to build one of these for themselves.  I'd certainly ask that you give credit to me if you do so, and that you share any modifications, fixes, or improvements that you make.  I absolutely *don't* mind if you want to build these amps commercially and sell them, just as I intend to do myself.  I don't see that as competition, rather I see it as assistance in developing product awareness and market penetration.