WARNING: Never attempt to open up your tube amp chassis or repair/modify it in any way, as lethal voltages may still be present for extended periods after the amp has been turned off and unplugged from the AC wall socket. Instead, take your amp to a qualified technician for inspection or servicing.
One of my interests is building class A, single-ended, tube guitar amplifiers. My interest in amp building is not only personal, but a case of carrying on some time-honored traditions. You see, I come from a family where music, woodworking and electronics have been both professional and amateur pursuits for generations. It was only natural that I should eventually want to hand-build/hand-wire my own amps and cabinets in the pursuit of perfect tone.
As a guitarist and owner/fan of the Fender Champ (the Rolling Stones’ recording amp of choice for many moons) this amp seemed the obvious starting point for a homemade design. But, it could be improved upon. The Champ is optimally suited to the recording studio precisely because it can be cranked up into distortion without having to dial in ear-splitting volumes. However, it’s too quiet to be used in a rehearsal context even with a small band. Many devotees of this 5-watt wonder also bemoan its lack of bass response, due to the standard 8 inch, 4 ohm speaker configuration.
Nonetheless, the clear sonic benefits of class A, single-ended amps (thanks to the simplicity of their circuits) warrant not throwing the baby out with the bath water. In this type of circuit, the guitar signal goes as directly as possible from source to speaker resulting in a tone that is incredibly ‘pure.’ These amps do indeed reveal the true tone of your guitar, whatever the make. The other notable benefit is the “touch responsiveness” whereby striking the guitar strings harder or softer results in radically different output tones, ranging form clean and mellow to biting overdrive.
Who says you can’t have it all?
Early 2015, a friend put me onto a circuit design by Dave Hunter called the ‘2 Stroke’ that marries the best of classic 1950s 5F1 Champ and 5F2A Princetons to a larger output transformer and 12inch, 8 ohm speaker. It also features an 3-way adjustable cathode bias on the pre-amp that alters the tone and bass response without adding to the signal path. I’ve made some considerable improvements to Hunter’s design resulting in a far superior amp. Some amp makers (such as Victoria Amps in the US) prefer to stick exactly with the original Fender designs (and wherever possible, materials) in case they somehow mess with their mojo. This is both understandable and prudent.
My approach was to first make a prototype that stayed as close a possible to the Fenders and then spent the next two years fine tuning subsequent prototypes, changing only one aspect at a time and then carefully A-Bing between the original and the new versions (as well as with original Fenders). Victoria are right, even small changes impact both the sound and play-ability of the amp in sometimes subtle, and at other times, quite noticeable ways. Despite this fact, when enough time is devoted to research, development and testing you can indeed make a superior amp. You can have all the of the benefits of vintage technology along with 21st century reliability, improved performance and safety. You just need a couple of years of plugging away at it, until all the factors come together just how you like it.
These improvements include a bleeder circuit for increased safety; a choke (inductor) to remove the annoying hum that was a big problem with the original Fender and Hunter designs, and a ‘switching’ output jack to give some protection if the amp is ever turned on without a load connected to the jack. I’ve also updated the input circuit to dramatically reduce hiss, as well as, add diode protectors to prevent any AC entering the B+ power rail should the rectifier tube short circuit. The grounding of the amp has been also dramatically redesigned (the original Fender grounding systems were haphazard, to say the least) and a grid-stopper has been added to the pentode tube (as is traditional, but omitted in the 2 stroke).
I house the amp in a large Deluxe 5E3 narrow face-style tweed cabinet I also make myself, adhering to the same construction methods and components used by Fender in the 1950s: finger jointed solid pine construction; hardwood reinforcement dowels; a floating Baltic Birch ply baffle; copper front screws; hide-glued real tweed fabric, and nitro-cellulose lacquer finish. Add the finest 50s-style electrical components available today – including carbon comp resistors (used in places where they’ll make an audible difference) and Sprague Atom/Jupiter capacitors – and you have a real winner. I call my combo version of this tube circuit the Mama Boo!