Monthly Archives: October 2014

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Spirit of 86: D.I.Y. Tube Guitar Amp Head

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Here are two videos of the Spirit of 86. The first one is done with a Fender Telecaster and the other is with a Gibson SG. Scroll down for photos and an erudite philosophical discussion.

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Spirit of 86 schematic

This is the Spirit of 86… or as I like to call it, the Buttrock Express. It’s like a little shrine built around an EF86 tube (http://en.wikipedia.org/wiki/EF86). A couple of years ago, I started wondering why I knew of a number of amps that use an EF86 as a first preamp stage, but I’d never heard of any amps that use a preceding tube to drive an EF86 (of course, that doesn’t mean there aren’t any). I thought it might have been because it would just sound horrible or because resonance and microphonics would get the best of the EF86, but I really wanted to find out. So after dawdling around for several months, I put one together last year, with parts and components I happened to have handy, and I tweaked it every few weeks until a few days ago when I decided to immortalize it in a blog post and stop fiddling around with it.

Spirit of 86 layoutTo the right are a schematic (above) and a diagram for a possible layout which is close to the one I used for the amp. Of course it’s not the only possible layout, and the positioning of the elevated heater supply (upper right) is arbitrary – I wound up kludging it onto the end of the turret board.

In the layout, the ground connections (thick blue lines) are an approximation, but most of the points that connect to a specific location such as the from the Gain control to the turret board are drawn that way for a reason – to reduce noise – because this project was probably the most noise-prone one project I’ve done to date. The biggest problem was AC hum. Randall Aiken has a good article about grounding in tube amps and why it’s not always good to perceive the ground as a sort of uniform field:

http://www.aikenamps.com/index.php/grounding

… and I try to do whatever the voices in Randall Aiken’s head tell me to because it almost always pays off. Side note: I tried grounding the EF86 tube to a couple of different points on the turret board instead of running the ground back to the star. I really thought it would make a difference in noise level but didn’t, to my ears anyway.

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This project has been interesting and educational for me, but I still can’t decide if I like the way it sounds or not. The overdrive is aggressive and “gnarly” for lack of a better term (which makes sense because the EF86 is a pentode), but it doesn’t have a lot of sustain or the “squishiness” of cascaded triode stages. I suppose that might be a good thing for articulation and clarity if this amp were being played by someone with better guitar skills than I possess.

The Buttrock Express might serve as a template for an overdrive channel at a future date. Getting a nice loud clear clean tone out of it is an exercise in futility.

The Thomas the Tank Engine sticker on the front face is there to cover a drill hole that I would up not using. The TH Audio badge is there because I had some extras – this is just an experiment, not something I’ll be offering through TH Audio (my side business for effect pedals and, eventually, amplifiers).

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Controls are, from left: Bass, Mid, Treble, Gain, Bright, and Master Volume. The Bass/Mid/Treble controls are a fairly standard Marshall-style 3-band EQ positioned after a cathode follower. The EQ stage uses a couple of unusual values: 68k for R7 (see schematic) and 250k log for the Bass pot (instead of 500k or 1M, to counteract the mounds of low frequencies that pile up). The Bright and Master controls are connected near the input to the power tubes, like one might see in a Vox AC15.

The amp dishes about 20 Watts, with a 6.6 kOhm output transformer and a 6L6 push-pull pair. At 20 Watts, the 6L6’s aren’t doing much, but I keep them there because a) the OT I’m using is a nice match for them (better than the 4k OT I was using previously, b) a 6V6 pair sounded glassy and midrange-y, and c) I wanted to hear the EF86 overdrive without an extra layer of power-tube breakup.

I used a tube rectifier instead of some diodes because… well, because I could, I guess… although the tube rectifier might be introducing a little ‘sag’ that mellows the general gnarliness of the pentode overdrive. To be honest, I don’t know if it does or not.

For this project, I used Tung-Sol tubes because… well, because I already had some. I’ve swapped out the Tung-Sol set with an Electro-Harmonix set, and I don’t hear much difference at all. In either case, it wasn’t difficult to get something close to 90V at the plate of the EF86, once I figured out the voltage drops. Normally, I like JJ tubes and have used them in a number of projects, but to my ears, the JJ EF86 sounds weak in this particular project (I tried two different ones).

One last thing: this amp is *not* a clone of the Dr. Z Route 66. The Route 66 uses a single EF86 into a passive EQ section, to a phase inverter and a fixed-biased KT66 push-pull pair. The Spirit of 86 uses 12AX7 -> passive EQ section -> EF86, into a phase inverter and a cathode-biased 6L6 pair (which is hardly doing any work).

-T. Hallenbeck, Oct. 2014

th@thallenbeck.com