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Triophoni: Triode Triumph
Article By Dan Schmalle From VALVE Issue 8, August 1994

  This month I will take the opportunity to show off a little (Oh brother, not again...). I have today been putting the finishing touches on my latest amp project, a pair of triode output power amps called Triophoni. Since I spent a fair amount of time writing up a blurb to advertise them I will use excerpts from said blurb to describe them: 6CK4 cathode type triode valves operate class AB1 push-pull as the output tube. This tube exhibits low drive and plate voltage requirements, allowing for fewer drive stages than the typical triode amplifier, helping to reduce phase shift.

A triode's transfer curve is more linear than that of a pentode. This helps minimize amplitude, harmonic and phase distortion, resulting in the incomparable triode sound.

The triode also exhibits low effective plate resistance, which minimizes:

1) Frequency response variations due to small inconsistencies in inductive reactance and distributed capacity in the output transformer and...

 2) Transient distortion due to loudspeaker impedance variations. The low plate resistance to load resistance ratio of the triode in conjunction with the high turns ratio of the output transformer allows connection of speaker loads as low as 2 Ohms.

A 6CG7 dual triode (like 6SN7 in a miniature envelope) operates as a long tailed pair phase inverter. The cathode circuit of the inverter is connected to the bias supply to give increased output voltage. The end result is an extremely stable amplifier with few amplification stages, requiring a low 12dB of negative feedback.

A 6267 / EF86 pentode serves as the input stage. It is of course the low noise, low distortion pentode used in the classic Mullard circuit. The input and phase splitter are regulated by two 6626 / OA2WA gaseous regulators supplying 300VDC +-1%.

The choke input power supply incorporates a 5U4GB vacuum rectifier, yielding regulation of raw high voltage of + - 15VDC for a gentle ramp up at turn on. All power supply components are overrated for the puny 50 mA drawn by the 6CK4 tube.

The AC line input incorporates a neon pilot light, which flashes when G-d forbid a fuse blows, and an MOV across the mains. The bias supply uses a separate transformer, an ultra-fast diode rectifier and low ESR filter capacitors.

Bias and balance are adjustable and a socket is provided for connection of a voltmeter. Construction is point to point, using star grounding, with AC wiring (in twisted pairs) and ground wiring running close to the chassis.

Resistors are metal film where prudent, and coupling and bypass caps are polypropylene. All tube sockets are ceramic. Finishes are commercial grade paints.

Power output is 15 Watts at the onset of clipping (about 0.7V input) and peak power is around 35 Watts. That sounds pretty wimpy, but these amps even drive my Magnepans well. I think the reason is the gentle distortion characteristic of the triodes. I found myself turning the volume control up a notch or two from my usual setting with my Dyna amps, and distortion was not much of a problem, even though input sensitivity was greater. Treble is clean as a whistle. No transistor grit, no pentode edge, as Mike would say, "triodes are sweet".

Midrange is present and well blended on both the high and low end. Female voice is in phase and realistic. Cellos show their marvelous buzzy idiosyncrasies. Bass is bloody marvelous! I can't believe the clarity, detail, attack, punch and all those other adjectives. My basses really growl and groan, and my Tyrnpanis hit my beer belly.

I thought this would be a nice cool jazz and chamber music amp, but I find myself re-listening to Count Basic's big band and Beethoven's Ninth.

These are going to be hard to part with. I wonder what four 6CK4's in push pull parallel would sound like?

Come by and hear 'em before they get sold.

-- Dan

Click here to download the schematic (Adobe Acrobat file).