TT-SAM (english Version)

An EL84 Guitar Amplifier as an introduction to the world of tube amp construction!

Goals

The goal in the development of this project was to design an extremely low-output amp, which sounds nevertheless full andpowerful. In addition, the amp should be simple to build, so that it is interesting for people who have little amp-building experience, but who wish to try it without investing huge sums of money.
Furthermore, the amp should be interesting for pros who place value on high-quality components, reliable performance, and good tone. As the amp is built on a terminal board, it can easily be modified, whethter changing the
value of a single capacitor or changing the entire tone stack. Of course, because the amp is so simple, it can also be built with true, direct point-to-point wiring.

The result is the TT-SAM, a 3-watt EL84 tube guitar amplifier with a single 12AX7 preamp. The tone stack can be easily switched to taste, and the basic design includes both typical Marshall and Fender tone stacks. Other stacks can easily be integrated.

The Circuit

The preamp is simple and is designed to meet the requirements of the EL84 output stage.

(Click the image for a larger view)

The input signal from the guitar is amplified by the first half of the 12AX7 (V1) and sent (via the coupling capacitor C2) to the gain control. This pot is used to reduce the signal level as desired, so that either the full signal or only a part of it is sent to the second half of the 12AX7 for further amplification. Next, the preamplified signal is
sent through the tone stack and the volume control to the EL84 output tube, in a classic signal-ended configuration.

Because a good, overdriven rock tone may be wanted, even at lower volume levels, a diode clipping circuit has been included between the second preamp stage and the output stage.

Regarding Diode Clipping

The use of diodes to generate distorted sounds has fallen into disrepute, primarily because it has been used by the large manufacturers - but not correctly. As a result, the sound and the technique have fallen out of use; however, it is possible to achieve far better results with properly designed diode clipping.

Any diode can be used to clip a signal - the upper and/or lower peaks of the signal are cut off. The amount of clipping is dependent on the voltage drop over the diode. The result is a distorted signal. Because diodes switch very quickly when the threshold voltage is reached, the distortion can be very sterile and hard. A small capacitor is placed in series in front of the diode circuit, producing a rounder, softer tone. That's the whole trick.

A further advantage of diode clipping in this case is that one can combine a variety of different diode types to produce whatever tone desired. In addition to the normal LEDs, other diodes can be used, such as the 1N4001 or 1N4007. The diodes can be wired symmetrically or asymmetrically, in series or parallel and so on, all with inexpensive,
widely available components.

Construction

The construction of the amplifier proves to be simple und problem-free, if the guidelines in the layout diagram are followed.

(Click the image for a larger view)

The basic position of the components should be first be taken from the photograph of the amp itself. This layout has been been tested in practice and has proven to be functional and problem-free. No hum from heater wiring or interference from other sources is introduced.

The amp fits well in a Hammond 22 aluminum chassis, which is also very easy to cut and drill.

The transformers should be placed as shown in the picture. The boards for the power supply and preamp components should also be placed as shown.

Note: a socket is shown in the pictures which is unconnected and unused. This is included for future modifications, if desired. The components are also mounted on an eyelet board; while this is not a standard item; however, good eyelet boards can be built easily and cheaply - if one places value on appearance.

It is critical to twist the heater wires together, and to use shielded cable from the input to the first preamp stage and to the middle lug of the gain pot, so that hum is minimized.

Resistors R2 and R5 can be soldered direct to the socket lugs of the the grids of the 12AX7 to stabilize the circuit, or they can be installed on the terminal board and soldered to the grid lugs of the socket with short leads.

It is also important that the distance between R11 and the grid of the EL84 output tube be kept as short as possible. This is achieved by soldering it directly to the socket lug.

Resistors R17 and R18 are used to create an artifical ground for the heaters, which can drastically reduce the hum produced by the amplifier.

All grounds run together to a single point (star ground), which is electrically connected to the chassis.

That's it takes! Now, go get those soldering irons!

Components

We have assembled several part kits for the construction of this amp, so that you don't have to order every part individually. These are available in the online shop. The following are required:

• Basic parts set: kit-sam01

• Tone stack (Fender or Marshall): ki-sam05 or kit-sam06

• Tubes: 12AX7 and EL84 (recommended: TT12AX7 and JJ EL84)

• Knobs: it doesn't matter what we include, someone won't like them ;-)

• Diodes: again, this depends on personal taste and how complex the circuit will be. The 3mm red LEDs are a good starting point.