TT SAM MKII

TT SAM MKII

The MkII version of the TT-SAM is an enhancement of the version released a few years ago. The MKII is based on the first version (see the history), but has a few importantant changes and additions.

Power amp

A second power tube has been added to the output stage, so that the SAM can now drive two power tubes in parallel. Each tube has its own volume control, so that it can be individually adjusted. The SAM can still be used with a single output tube, but in this case, the impedance of the output transformer is adjusted accordingly.

When driving two tubes, the yellow OT lead connects to J3 (the 4-ohm tap), while the white lead connects to J4 (the 8-ohm tap). If one tube is removed, or if it is only built with a single tube, the yellow lead becomes the 8-ohm tap, while the white is now a 16-ohm tap. When connecting a speaker, it is very important to keep this in mind!

The SAM's power transformer provides 1.5A heater current. This is enough for a 12AX7 (or similar), one EL84, and one 6V6. If other tubes are to be used which draw higher heater current, the power transformer must be replaced with a more powerful model.

Diode clipping

The diode clipping from the first version is included, although it now follows the first stage. This has the advantage that the diode clipping doesn't cause as sharp a level drop as in the first version. The clipping is also turned on and off with a switch included in the gain pot.

Regarding diode clipping

Any diode can be used to clip the signal. The upper and/or the lower peaks of the sine wave, depending on the voltage drop of the diodes, are clipped, which results in a distorted signal. Since diodes switch quickly once the threshold voltage is reached, the effect can be sterile and hard. A capacitor is wired in front of the diode to minimize the effect, and to smooth out the distortion.

In addition to normal 3 mm or 5 mm LEDs, other diodes such as the 1N4001 or 1N4007 may be used. The diodes can be wired asymetrically or symetrically, in serires or in parallel, and so on. The effect is large, and different combinations of diodes sound very different. The SAM MKII uses two 1N914 diodes wired antiparallel, which produces a great deal of clipping. If instead two 3 mm red LEDs are used (again, antiparallel) the clipping is reduced, and the overall sound is 'dirtier.'

Tubes

The SAM MKII is conceived for a 12AX7/ECC83 preamp tube and one each EL84 and 6V6. The two different power tubes, whose level can also be individually adjusted, lets the overall sound be easily adjusted. No huge difference in sound is to be expected, but depending on the relative volumes, the sound can be made a bit thinner or a bit fuller. The effect is more noticeable with better speakers. The difference will be slight through a V30, whereas through a Jensen Blackbird the difference will be much greater.

• A 12AT7, 5751, 12AU7 or other similar tubes can be substituted for the 12AX7. These tubes have varying amplification factors, which will have a direct effect on the tone and behavior of the amp. However, even different 12AX7s will produce different sounds.
• The EH12AX7 tends to produce an ugly distortion in the amp, when it is turned all the way up.
• The JJ12AX7 is round and warm, but the bass is somewhat undefined.
• The TT12AX7 sounds tight and warm and is recommended for heavier distortion.
• The TT12AT7, like other 12AT7s, less gain, but sounds extremely warm and balanced, similar to a 5751.
• The TT12AU7 has an amplification factor of 20 – like all 12AU7s – so the amp stays 'clean' high up the dial. Highly recommended when clean tones are required.

Construction

WARNING! HIGH VOLTAGE!
The SAM works with high voltages, which can be fatal, and should only be built by people who have experience working with high voltage, who are capable of reading a schematic, and who can search for errors which can easily occur during construction.

Since, in addition to the kit, a pre-drilled chassis is available, the SAM can be built quickly and without great problems.

The voltages listed in the schematic were measured with a wall voltage of 225 V and an 8 ohm load, with all tubes installed, and without an input signal. All pots are set to 0 (turned all the way down).