All-Pentode Guitar Preamp from Metasonix

(Click for a close-up)

Metasonix introduced the TM-5 guitar preamp at Winter NAMM, which contains three tubes in the signal path: First in the path are two pentode type tubes for gain. Metasonix uses NOS pentodes only. Pentodes have a nice, clean sound, especially at clipping. They have lots of voltage gain. Their screen voltage may be varied to change the tube's gain. And pentodes don't need huge plate voltages to sound good. The pentodes in the TM-5 operate on a 70-volt DC supply--they don't need 250v or more, which most triode tubes need. Pentodes were used in the first generation of guitar amps and professional audio equipment, almost exclusively. 12AX7s replaced them in the 1950s, generally for economic reasons.

Next, the signal passes through a special tube. The "BN6" was originally developed by Zenith in 1948, to act as an FM detector in table radios. It is a special "beam modulator" device, roughly similar to a pentode with a very weird structure. It gives asymmetrical clipping, which may be adjusted by varying the voltage on its screen grid. The result is a unique "buzzy" sound which is suitable for heavy rock. BN6es have never been used in audio amplifiers before, so far as we can tell. The TM-5 can accept any of the usual three versions of this tube--it comes in different filament voltages, 3BN6, 4BN6 or 6BN6--without changing anything in the TM-5 circuit.

Specifications:

• Voltage gain is approximately 5 with all level controls set to maximum.
• Hum and noise may be minimized with DRIVE/HUM BAL control.
• Power input 10v AC to 12v AC, 50/60 Hz, 1.2 amp at idle--derived from an appropriate plug-in AC adapter.
• Inputs: phone jacks for audio input; optional input for PREAMP CLIPPING CV.
• Output: phone jack, audio, mono, 1k ohm impedance.
• Envelope output, 1k ohm impedance, mono phone jack, DC voltage which tracks input signal amplitude, maximum output 1.5 volts DC.
• Dimensions: 7 1/2" (192mm) x 4 3/8" (112mm) x 3" (77mm).
• Weight not including accessories approx. 5 pounds (2.2 kg).

The Metasonix TM-5 is hand-made in the USA and carries a 90-day unlimited warranty.