the McGrath Mini amplifier

The prototype McGrath Mini amp which I've built for myself, the one you see in all the photos, may just end up being the only one built with that configuration: three 12AX7s and an EL84.  I am now starting to sketch out what version 2 will look like, and it will probably become the first version that I manufacture and sell. The main impetus for a new version is to improve the effects loop, by adding another 12AX7 tube.  This will allow the loop return amplifier to be non-inverting, and to easily have enough gain for "lossless" operation.  The send circuit will remain a passive resistor-divider, as it is now. Given the heater current capacity of the power transformer that I'm now using (and there seem to be no other close alternatives: beyond this one, current ratings, size, weight, and price all seem to jump dramatically higher), there is not enough power to add a fourth 12AX7, unless current is reduced somewhere else.  Fortunately, there's a perfect solution!  ...

In the view from the back, you can see how the chassis is split up into the upper part, with the tubes and most of the important stuff, and the lower "power bar" where the 120vac wiring goes in.  Also, the low-voltage solid-state circuitry for the footswitch (the relay system) is located inside this lower bar.  The power transformer is mounted on the floor of the case, behind the power bar.  A bundle of cables, visible on the left side of the view, connects the upper and lower chasses.  (There's enough slack in this cable that you can slide out the upper chassis and rest it on the bench next to the amp, with the speaker and power still connected.  This makes it easy to work on and modify.)  Also on the floor of the case, partially visible behind the power bar, there's a little white USB charger plugged into a utility outlet: this is the 5v power supply for the relay system. That rough-hewn piece of aluminum sheet is a heat-shield; I'll design a nicer curved...

I have a number of videos of this amplifier, including some sound samples, on my youtube channel, Industrial Baroque. Examples: https://youtu.be/0UiUABcsrUo https://youtu.be/txAPPNxujPU https://youtu.be/4QJ_X9_iS_w https://youtu.be/TBB-hMDraAs (Keep in mind that many of these videos were made when I first got the prototype amp working, in the winter of 2022; but since then, the circuit has been continually evolving and the sound of the amp is rather different by now -- I hope, better, but certainly at least, different.  Hopefully I'll be able to post some fresh videos soon, perhaps including some use of this amp in real musical recordings, not just my raunchy test-the-amp playing!  I built it to use it.)

Supporting the audio circuitry are two other subsystems, the power supply and the relay system . The power supply is designed with intentional "sag", and most of the voltages and currents in the amp are relatively low, because these factors lead to desirable sonic characteristics.  The sound and responsiveness of this amp shares something in common with Eddie Van Halen's "brown sound", which he obtained by dialing back the AC input power to his Marshall amp, with a variac.  Of course, in EVH's case, he was also reducing the heater temperatures, whereas the McGrath Mini runs the heaters at full power: the "sag" is only for the high-voltage supply. The power supply circuit has morphed a bit since I made the schematic.  E.g., the resistor between B+2 and the B+3 nodes is now 10k instead of 1k.  The various changes have just been things I've tried, in seeking to improve performance or address perceived problems. However, so far, nothing has made an...

  Be aware that this schematic and the accompanying text refer to the first version of the amp, McGrath Mini v1.  I built a single prototype unit of this version, which is still working fine.  However, development has moved on.  The current version is version 2 (McGrath Mini v2), embodying some significant changes and improvements over v1, though still in the same small form factor.  Schematics and discussion of the v2 design are located later in this same blog, starting in 2024. The McGrath Mini audio circuit consists of a collection of five functional blocks, arranged in series from the guitar input to the speaker output.  Several of the blocks are bypassable, through footswitches or toggle switches on the back of the amp.  This provides a great deal of tonal flexibility to the amp.  The bypassable blocks can be used much as one would use a "multichannel" guitar amp, although the Mini is technically just a very flexible single-channel amp. The ...

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 be...