Handbook Amp Project – Part 1 – Chassis

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Guitar Amp Handbook Amp Project – Part 1

I bought this book by Dave Hunter, months ago, and it is a really good read if you are into valve amps, as it is more or less a historical tree of the key amp designs starting with the 1956 Fender 5E1 Champ. There is also a Project build for readers, but the info is now outdated and the publisher’s kit offering unavailable as far as I could find, so I had to buy the bits myself from the component list.

The original 2 Stroke design kit offered at:

Priced at $1000 dollar as this includes the cab and speakers I believe…
In comparison the UK parts I bought cost about 160 pounds for 1 amps chassis parts as I don’t need the cab or speakers.

There are a couple of features I like the idea of – like an cathode EQ boost switch rather than just a Tone control (thinking I could add a footswitch jack here also?), and options for combining or changing different output tubes in single or dual mode, variation of transformer voltage options to drop the tube voltage for crunch at lower bedroom levels etc.

As I didn’t want to do another stock kit build like the Aston this time, but a bit of a hybrid possibly, with options for changes if I want, I bought the parts from Barry at www.ampmaker.com as his site allows single component purchase as well as full kits, so I bought two of each component with the idea of using a set mounted on a wood board as a test bed and experimentation. That didn’t happen (not yet anyway!).

I only got one chassis because of this – a mistake I won’t make again as the slightly cheaper, bargain bin “prototype” drilled chassis had loads of holes in the wrong places, no lid and no transformer cut outs so was not worth the time and effort of doing all this myself to save 10 quid on a properly punched chassis from a stock kit.

This meant a front panel needed to be cut and added to cover all the extra switch and volume pot holes – Doh!

Still, it got done and it’s more personal already, and I had a few hours with my Big Bro, his compressor and tiny air drill, so learned what Aviation Engineers use for aluminium work.

I had marked the squares and he “chain” drilled the holes for me. Just quicker than me doing it with cheaper gear. Also, I wanted to check with him, the options of filling the 10 unwanted small drill holes, and 2 larger transformer wiring holes, using rivets maybe. I’ve decided self tapping screws and 2 cage nuts and screws will solve the problem and still look OK. I’m not the fussy type when it comes to looks – just interested in function rather than form, but two 8mm holes in the top above the circuit is not a good idea if something fell in when turned on…

I spent time actually putting some components in the chassis to decide the best layout for space, and trying to utilise and cover as many drill holes with the transformer bodies as possible. This is a large chassis with plenty of room, so I decided I would do the soldering to the mounted components though this is NOT the best way normally. Also, I thought I would be able to take more time (but a lot more effort) to build it in stages electrically. This means taking the board in and out a lot to add the components, but that way I can research, log and explain the components and their working details with the oscilloscope as I go, and show the changes made by each circuit section. This is a good way to understand electrical circuits – as separate modules that are joined together – like all engineering or computer programming etc.

So what’s in the circuit? Not much really – which is good – as I don’t have a circuit diagram in the book!

It is described as a simple but versatile circuit.

There are only 18 or so components to be mounted compared to the Aston’s 27 board parts:

The only info in the book is the parts list and board layout:

I plan to use the Aston and Barry’s 18W amp circuit as references to build and check this one as I go.

The transformers I bought are used in Barry’s 18W Plexi here:

The build info is here:

Starting in the next Post, I will cover the theory of the AC mains power input components, to the first stage of DC rectification using (in this case) the 5Y3 rectifier tube which is new to me.