Building Amps

Tony Eyres

During the 2020 COVID lockdown in Australia, I formed a Rockabilly/Blues busking trio. This meant returning to an amplified blues harmonica sound, after a break of some decades. I have a fine Lone Wolf pedal which approximates this classic sound; however, nothing beats a harmonica-friendly valve amp. Time to roll up my sleeves and make one. Not entirely silly - I worked as an Electronics Engineer in the 1980s before moving on. I also taught introductory electronics for some years. I figured this past experience would keep me from stupid mistakes. It did not. More on this later.

There is a large community, entirely male as far as I can tell, who build classic 1950s/60s guitar amplifiers. The relatively simplicity of these now ancient circuits, mostly from Leo Fender, makes these enterprises feasible. The glorious sound which results makes the effort (and considerable expense) worthwhile. As I am finding, the projects become addictive. Amp builders like myself gather online at the “Shock Brothers DIY Forum”, on TDPRI.com (Telecaster Discussion Page Reissue). Here folk display their projects (some of which are exquisitely well done), and seek advice when things go wrong. Valve amplifiers routinely produce more than 400 volts DC, waiting to pounce on the inexperienced or careless operator. I treat these voltages with great respect, and hence still have ten digits to type with. In particular, I plan and rehearse measurements before turning the amp on, and unplug it once they are done. Capacitors will happily retain lethal voltages after shutdown, so I install discharge resistors to eliminate this hazard.

I warmed up, so to speak, on a 1957 Fender Deluxe guitar amp. The circuit is called a 5e3, the kit which I built came from China. The story of this build (and the failures along the way) are for another day. However, it sounds just mighty with my 1950s Gibson archtop, which I play in my new band, along with my harmonicas, the latter in a Seydel Gecko Rack with the excellent Greg Heumann Rackit! featuring his Bulletini mic. The 5e3 build behind me (until it breaks down), I set on a valve harmonica amp to complement my busking rig. My guitar and vocals go through an AER Alpha, 40 watts of incomparable German engineering. This runs from a 500 watt inverter, connected to a 12 amp hour Lithium Ion battery. I’ve only once run the battery down while playing, the day I brought a Fender Champ out for

my harmonica. So, my valve amp had to draw less power than the Champ. Enter the Champ Micro, a 1 watt version of the 5 watt Fender Champ. The circuit comes from Rob Robinette - Champ Micro builds feature regularly on the Shock Brothers forum. This low powered amp would suit my battery - I figured (correctly as it turned out) that 1 watt would meet my modest busking volume needs. Plus I would be able to drive the amp flat out without the feedback issues which come when a regular Champ is driven too hard.

I settled on a 10 inch speaker, a Jensen P10R Alnico, a remake of the classic Jensen speakers which powered the 1950s Fender “Tweed” amplifiers. My build also drew upon my shaky woodwork skills, which were put to use building a cabinet. Blackboard paint gave the project a suitably scuffed look and hid my dodgy woodwork. Reproduction grill cloth and a Fender handle rounded out the look, as the picture shows. But back to the amplifier build. I eventually got the parts together (the Australian branch of Evatco are basing their retirements on my purchases), and soldered and screwed everything into place. When switching on a new amplifier for the first time, a wise safety move is to include a 100 watt light globe in the circuit. Any faults will cause the globe to glow bright and, more importantly, will limit current into the amplifier, thereby protecting the parts. I duly engaged this globe, put safety glasses on, then switched on the amp. The globe went bright for a second, then the circuit breaker at the front of our house went off.

Strike one.

I soon realised that major stupidity on my part had caused this mishap and rebuilt the offending part of the amplifier. I turned it on again, the safety globe stayed off, and the house power stayed on. My joy was short lived: smoke rose from near the output valve. I did some calculations, and concluded that a short circuit on the output valve was to blame. Heat-shrink tubing went onto the valve pins, the smoke disappeared, and the amp worked. With a guitar, the Champ Micro produces a bedroom level volume. The harmonica sound is just wonderful - the stage debut was to be my gig last Friday. Then Sydney went into another lockdown…