The 2tube/3tube Option

The biggest change in my more recent builds revolves around the output circuits. Many of my earlier preamps offered two balanced outputs per channel. This came about by experimenting with alternate circuits. I discovered that I could add an output transformer to part of the circuit that sounded better than the original. This yielded a preamp with a 2 tube preamp circuit and a 3 tube preamp circuit (the original.) Each output had its own XLR output jack and its own transformer. I always recommended the 2 tube circuit with the Altec 15095 transformer as the most transparent sounding circuit. The 3 tube circuit was for more colored sounds, up to fuzzy distortion.

Over time, I continued experimenting and made a discovery. It turns out that the sound quality difference had more to do with the transformers than the difference in the circuit design. The Altec 15095 is a better sounding transformer than the original Ampex output transformer, no matter which circuit you put it on. The original Ampex output transformer is on the right.

With this knowlegde, I worked on a new design which I call the 2T/3T option. This design only has one output XLR per channel, which simplifies hookups. There is a selector switch to select either the 2 tube plate driven output or the 3 tube cathode follower circuit. Both transformers can still be used with a selector switch. In a unit with one output transformer, it should be the Altec. Customer response has shown both circuits to have value so this is now my standard design for the output. I will still do the old style, but I recommend the 2T/3T option. The old design put the Altec on the 2 tube circuit and the original Ampex on the 3 tube circuit, with no option to use the Altec on the 3 tube circuit.

The 2 tube circuit has the simplest and shortest signal path. It isn't as stable as the 3 tube circuit and it doesn't have as much gain. It does have a less colored, more transparent sound. The 3 tube circuit incorporates the master gain control. You can attenuate the preamp section and drive the tubes harder, even into total fuzz. The 3 tube circuit has more gain (for ribbons) and is very stable driving any sort of device you plug it into. The 2 tube circuit won't drive a device with a 600 Ohm transformer input without distorting. It does great into modern bridging inputs. (mixer, AD converter, etc.) This is the way I intend for the 2T circuit to be used.