Group: Pushpull    Built: Mar 2005    For: Michael
6BX7 PP
The Story

This amp was commissioned to provide tubed power for a pair of Mission speakers used as the satelites of a bi-amped system. The pushpull configuration was chosen based on the needs of the speaker i.e. more power than can be conveniently obtained from any reasonably priced SE amp. The 6BX7 was chosen to provide true Triode pushpull operation as opposed to a triode connected Pentode.

Design & Build

As all my amps, this one is also designed from the ground up. After discussing various circuit schemes, the owner and I decided to go with a conventional phase spliiter based design. Always a solid choice. This amp uses a 5965 front end followed by a 6C4 (one for each channel) as the phase splitter. The 6C4's are run hard enough to allow a low impedance drive to the 6BX7 outputs.

Vintage Hammond output transformers were chosen for this amp. I've used these OPTs before with excellent results, and I was expecting no less here.

For the power needs, I chose a transformer that had enough heater current to provide the the demands of the four 6BX7s which is nearly double that of four 6L6's. In addition, this tranny had an auxillary 6.3V winding, allowing for an easy heater bias setup.

A choke loaded 5AR4 based power supply provides excellent regulation for the demanding 6BX7 output section. The 5AR4 rectifier was chosen for its efficiency and low impedance. A 5U4 could also be used with a slight reduction in output power.

The amp sounded fine on its own, but a touch of global feedback (3db) was added to really lock it in and lower the natural third harmonic produced by such a design. The amp delivers just the right amount of second harmonic due to the inherent non-linearity of the 5965 grid to cathode.

The chassis for this project had to be heavier than my usual .063 anodized aluminum. So .080 was selected to provide adequate strength for the heavy transformers (mostly the power transformer). We could have used braced .063 as I have done in the past. The .080 comes to me as pre-painted only. Since pre-painted aluminum has already had its acid etch and painting prep, all it needed was a light sanding and it was ready for new paint. I liked using the .080, but I see I will have to improve on my painting techniques.

Hand selected components were used throughout. All resistors were matched by hand. The phase splitter to output coupling is made by some special NOS foil caps I accidentally fell into browsing an old electronics shop. I was surprised how well these measured and performed. Coupling from the input stage to the phase splitter is made using some vintage paper in oil types. The output tubes were selected as a quad matched set from a group of about 30 tubes.

How does it sound?

This amp turned out to be a project where the sum of the parts doesn't represent the experience of listening to it. Deborah and I spent many hours listening to this amp during all phases of its construction. It was one of a select few that got us into the music and away from the speakers and the amp itself. That's what WE like to hear, and we hated to see it go.