12AX7 Guitar Amp

Dave Ewing’s 12AX7 Tube Guitar Amplifier

IMG_0129sIn this article Dave Ewing takes us through the build of his 50watt tube amplifier.  The amplifier itself is one designed for use as a Guitar amplifier, not the kind of audio projects normally featured on Justblairs Audio and Electronics Pages, however for anyone interested in building a Tube Amplifier the build process is essentially similar regardless whether it is to be used for playback or guitar amplification.

This article originally appeared on Dave’s own Geocities site, as you may be aware Yahoo has pulled the plug on Geocities hostings and many sites will be lost in the process.  Dave agreed to publish his articles here on order to preserve them for the DIY Audio community.  This article reads as a blog.

Here are a few pictures of my newest project, a 50 watt custom guitar amp. The goal of this project is to create the kind of amp I like. It blends the best of the old and new. I want the maximum tonal flexibility, and no compromise on the dynamic range, and audio clarity. Classic look and feel is also one of my primary goals, I like the look of a classic Marshall amp, and I am going to try to put some of that into the amp but with my own flavour.

As far as the circuit is concerned, the power transformer is from a Fender Twin Reverb amp, plenty of power to drive high dynamic range. The power supply is all solid state, and all regulated for the tightest reproduction of sound. The preamp board will consist of both a clean and an overdrive channel, and will use a couple of 12AX7 tubes, and the Overdrive will have an additional 12AT7 for the fuzz. Switching of the channels will be done using a bunch of Vactrols, and a PIC microprocessor, so I get to do some easy embedded programming as well.

Using the PIC will allow me to use buttons on the front panel or foot pedal operation to change the audio flavours, and will let me define a couple of different voices for both the clean and dirty channel. The design is reminiscent of a Mesa Boogie, lots of audio stages and lots of control over audio tone.

All of the audio path is vacuum tubes except the effects loop which is fed by a couple of high performance OP-275, or LF-412 op amps. I have taken care to be sure that the effects loop doesn't colour the sound of the amp, it only faithfully reproduces the sound of the pre-amp.

The output is a standard set of 6L6 tubes into a hammond 1650r transformer. I am using 12vdc from the regulated supply on all of the 12 volt tube filaments to eliminate hum, and I am using a regulated screen voltage supply, and regulated bias supply for stability and a very sonically transparent output section.

This is the first project I have done which will be fully CAD enabled. I will create the chassis, the circuit boards, and the front panel on the computer first. I have found a few cost effective sites on the internet that now let you download a their own version of a CAD program for free, then you create a chassis, front panel or circuit boards, press a button, and in a few weeks they deliver your master piece to your front door, what a deal. So I want to see if I can build an amp from scratch that is better than what can be bought from the local music stores. I have been able to build better circuits in the past that sound better than store bought, but I have never been able to give my projects the polish that you get from the store. Now I think I can.

Check back from time to time since I keep adding to the blog, although somewhat slowly.

Click on any image to see the high res version...
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8/2003 The amp is nothing but a bunch of parts bags and an idea at this point

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1/2004 Well, the circuit boards have arrived, and I am finally ready to put some real live components together instead of just a bunch of bits in the computer.

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Here are several pictures of the circuit board test for the power supply and power amp boards. It puts out about 50 watts. In fact, I didn't realize my test speakers which you can see mounted in the far left and right walls were not stout enough to handle the power, I fried one of them during the test. Subsequent testing was done using a Black Widow speaker in an old Peavey cabinet that I had lying around... oops...

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In this close up you can see the 6l6 tubes, and 12ax7 driver on my makeshift aluminium test box with the audio output transformer. I later ordered a different audio output transformer, I wasn't happy with the sound of the one in this picture. The computer is great, but there really isn't a substitute yet for good old fashioned prototyping...

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Here is the test of the Pre-Amp board. It didn't work very well when I first turned it on, I had to fix a couple of trace errors on my PCB. Nobody's perfect. If I decide to build more, at least the next boards I order will be better.

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3/2004 I will have to stack the Pre-amp and Power amp boards one on top of the other in order to get them to fit in to a chassis of any reasonable size, so here is the stack

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5/2004 OK, now I'm getting somewhere. Finally after months of testing, and computer time, I received the chassis in the mail, my cat can't figure out what the heck this thing is. I have fitted several of the parts, and they all seem to fit except that pesky AC plug in the back. Its a good thing the square hole won't be seen because its going to take some grinding for that.

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I have started designing the front panel of the amp. I have my CAD program fired up again, and this is the paper representation I have come up with. Now all I need is to place the order for the aluminium version.

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6/2004 I finally got to fire up the amp today. I am still waiting on my front panel, but I put everything together anyway because I got tired of waiting. I can easily disassemble enough to put the panel on when it comes. After my jam session with it today, I can tell you the amp sounds great. It is every bit the sound I was looking for. Crystal clear on the clean channels, and nasty on the overdrive channels. Not even the smallest hint of hum. It seems to be equally happy running at a good solo listening level, or all out LOUD.

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You can see from the front view that I haven't installed the pushbuttons to change channels, or the LED's. Those would have been a little more difficult to disassemble when the front panel arrives. I chose a gold colour for the front panel, and blue LED's.

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Same on the back side, I didn't put the jacks in for the foot pedals, or the effects loop for my test today.

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Too bad this chassis will be hidden in a cabinet. It turned out nice. Oh well, That’s it for my blog this week, I'll be getting the Front Panel in about two weeks, I can't wait to put it on, and start building the cabinet.

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8/2004 Yes!!!, the chassis is finally complete. The amp sounds great, the clean channel has the finest definition, and the ability to define tone character in a myriad of ways. The overdrive channel is just nasty.

You can see from the bottom side photos that the teflon coated silver plated wire I had was just a few different colors, but I needed to connect about 80 wires to the mother and daughter boards, so I just painted the ends with testor's model paint to be able to easily connect and disconnect the right wires.

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After trying several different LED's on the front panel, I finally decided to go with these green ones because they had just the right amount of output, and made the gold front panel glow with style.

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On the back panel, I had to use dry transfer letters, and then paint it with clear enamel, I couldn't find a better way to do it, and ordering another expensive aluminium panel wasn't part of the project budget.

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Next I get to tackle the cabinet. I think I am going to cover it with the standard black tolex, and put some handles on either side on the top, and put a cane grill above the font panel. Hopefully I'll be able to put that together in a month or two. The pace of this project has been a little slower than I expected because of my real job. Oh well, it pays the bills, and lets me build amps.

You can contact Dave Ewing with comments and questions by clicking this link