My Amp Building Journey, Its a kind of blog ... Options

[Andrew Cockburn]

Hi All,

Some of you will probably have seen my posts pestering MickeM and Tonymirro endlessly about the finer points of Amplifier knowledge and terminology. Yes, I am on a quest for tube tone - I have been convinced that there is more than Line 6 and I am setting out to find it.

I was pondering Carvin and Bogner, Marshall and Vox when I saw this post on the forum - AdamB put up a picture of a tube Amp he had made ... then it hit me, why couldn't I do the same? (Thanks for the inspiration Adam, and your Amp looks incredibly cool!).

So with that goal in mind, I am going to build an amp, maybe several, until I get the sound I want. I have a lot of experience in electronics (built all sorts of gadgets as a kid) but I have absolutely no experience with tubes and tube circuits, althouthg I do understand the physics of them.

Research

I have been looking around the net and have found a lot of websites that sell kits, but I wanted a bit more than that - I wanted to understand how it all worked and maybe tweak things a little to get the sound I have in my head. Adam pointed me to 18watt.com - they have a number of projects on the go, their first design was an 18 watt arshall like tube amp. I had a look around but found the website a little hard to navigate, didn't really want a Marshall sound and was generally uninspired by it all. After some more searching, I happened upon http://AX84.com. A really cool website with a very active community. Over the last 10 years they have designed a prototype tube amp (the P1), along with many variants, and are now on a grander work of prividing a library of reference designs consisting of Pre and Power amps you can mix and match. I liked the site and decided to build one of their simple designs - the simplest have just a couple of tubes and a few tens of components, but building a tube circuit can be tricky and I wanted an easy place to start.

The Amp For Me

Part of what I wanted was to explore the different sounds out there. I have zero tube amp experience. None, NADA. I don't know Marshall Brown from HiGain from Vintage from a Vox - so this is a huge learning curve for me. I have had some good advice from Mike and Tony and know that:

1) Tube Amps are loud
2) they have to be played fairly hard to get all of the tube benefits.

A 35 watt amp with an efficient speaker at 10 feet can rival a jet plane landing for decibels, so something that you can play high enough to get the right sound yet record and practice with needs to be pretty small indeed. The AX84 P1 is a 5 watt design based on a 12ax7 double triode for the preamp and an EL84 for the power amp stage. I loooked at it initially, but although it was simple, it was basically a crunch type of amp, and I am looking for something higher gain, although not monstrously high.

The P1 has a couple of variants, the Extreme, and the High Octane. High Octane is a more complex build but has an EL34 or 6V6 power tube, a 3 stage preamp with cathode follower which makes it a high gain amp. The Extreme still only has 2 preamp stages like the P1 but does benefit from an EL34 or 6V6 power tube. This was appealing as I like the voicing of an EL34 based amp but would also like to experiment with 6V6 - normally hard to do as to exchange tubes you need to re-bias which needs knowledge of the workings of the circuits. The Extreme also keeps the simplicity of the P1, but can be upgraded to be more like the Hi Octane if I need more gain.

So, my choice was made, and I ordered the parts this weekend. I got a kit and a pre-drilled chassis, since my metalwork is dire. I also upgraded to a better output transformer and a variable bias option so I can easily swap power tubes. It has cost me around $300 so far, and counting ...

The Speaker

What little research I did on the internet seemed to point to the Celestion Alnico Blue speaker being the holy grail - they were used extensively in Vox amps for a while where they gained their reputation. Not cheap at $250 a pop, but I am doing this to get tone, not necessarily to save money so it seemed worth it.

Other Bits and Pieces

I'll need something to put all of this in, and I am looking into building a combo cabinet - unfortunately my woodworking skills also totally suck, but maybe this is a chance to learn some other stuff and do it right, I'll think about that some more. I could get a nice tweed style combo cab ready made for about $300 - could probably build it for 75 - 100. Still thinking about this.

Some Links

AX84.com

Schematics for the P1 Extreme

A couple of soundclips, here and here (next sound clips will be from the one I built!)

That's the end of my first update - if there is interest I'll keep the story running

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This post has been edited by Andrew Cockburn: Sep 25 2007, 08:56 AM