gemini://gemini.hitchhiker-linux.org/gemlog/re

I tend to collect hobbies, and I love diy. I've been playing guitar since I was about ten years old myself, and have often combined my love of music with other interests. In the spirit of diy, I thought I might share one of my guitar amplifiers.

I've built four all vacuum tube (or valve, depending on which side of the pond you're from) based guitar amps, all designed from scratch. The first one was an experiment in cheapness, reusing a lot of parts from a cheap old record player, including the output transformer and 25L6 output tube. It was an ugly rats nest inside, and never really finished, but sounded way better than it had a right to.

The second amp I called the "Miniplex". It was based around the transformers taken from a Carvin Vintage 16, which never really satisfied, but had a preamp topology very much like an early Marshall. This one was neater inside and out, but still not "finished" to a great standard. It sounded good enough that I took it to gigs occasionally. Probably about 18 watts of unrefined power that could pull off a great early rock vibe.

Amp number three was an experiment in excess, seeing if I could pull off a modern high gain metal amp. It had a five tube preamp section and a single ecc99 output with the twin triodes in push pull mode. This one was a bit of a flop. It did the job if you dialed the gain way back, but there was just too much gain to control overall. I learned a lot though.

The last one, and the actual subject of this post, is my still current daily driver. I completed this amp in 2010 and have been using it ever since. I wanted to strip it back to just the essence of what makes a great rock and roll tube amp, but with a couple of little tricks that I'd been wanting to try. It has a two stage 12ax7 preamp with a gain control between the two triodes, feeding into a 12ay7 long tailed pair phase inverter. There is an additional DC coupling path between the second stage and the phase inverter, allowing for the phase inverter to be smoothly overdriven without any blocking distortion. That's custom feature number one.

The output is a pretty simple 6v6 push pull stage with cathode bias, very much like an early Fender Deluxe minus the negative feedback loop. It's got a 12ax7 tube based tremolo oscillator which bias modulates the output stage for some nice, swampy tremolo that sounds a lot like the old brown tolex era Fender amps.

Trick number two is the post phase inverter master volume, allowing for the preamp and phase inverter to be fully overdriven at any volume and retaining a lot of the character of a fully cranked amp. This was the tricky part, because the tremolo modulation had to be applied in a way that did not interact with the master volume. What I came up with in the end, and which is to my knowledge something never done on a production amp (or any other amp I've ever seen) is to place a 50r resistor in between the normally grounded lug of the volume control and ground, and apply the tremolo at that junction. This only works because the oscillator tube uses it's second triode as a cathode follower buffer, which can easily drive that low impedence without much loss. It's a cool circuit.

The chassis was custom fabricated from sheet aluminum by a fellow enthusiast out of Florida, and the cabinet is dovetail jointed pine. The speaker is a ten inch Alnico magnet Weber gifted to me by a good friend. This was the first amp I built that I consider professional quality, minus the fact that I never quite got around to having nice control panels made for it. But with only four knobs, a footswitch for the tremolo, and in and out jacks it's pretty easy to keep it all straight. I also never got around to spraying the finish on the back panels of the speaker cab, but that could be easily done still. The cab also has a split back panel and can be used as either a fully enclosed or open back cab.

I've used this amp for both at home practice and small gigs. It actually matched up really nicely with an old leslie cab that a friend of mine had, and it drives a 4x12 cab to painful volume levels. Since there's no tone stack, you're pretty much getting just the raw tone from your guitar. It's different than what most of us are used to hearing, as most guitar amps have a tone stack that automatically creates a scoop in the mids even with all of the controls set flat. But it's a more honest tone, and paired with a hot bridge pickup it's definitely not lacking in high end.

I don't have any shots of the inside right now and the schematic only exists now on an old hard drive from an old laptop, which may or may not work now. I fully intended to follow up with more projects, but my personal life was blowing up around this time and money has been a problem ever since. I do have a number of schematics and layout drawings ready to go, and one of these days I'll be getting back to it. The next one up is a revisit of the 25L6 concept, but this time with DC coupling going to the output stage using a number of my own ideas including a twin voltage power supply.

RE Jazzmaster - my amplifiers

2022-04-22

Tags for this page