This project originated from the research I carried out in the article Damping with zero feedback: is it possible?, which explored the possibility of achieving a good damping factor in a tube amplifier without relying on negative feedback. Among the tubes examined in that article, the 6336 is particularly interesting. It is a power dual triode characterized by very low internal resistance, a feature that naturally provides good control over the load and therefore a higher damping factor than is normally achievable with many other power triodes. These considerations led to this SB-LAB premium project: a class A push-pull amplifier using a single 6336A per channel, with the two triode sections inside the tube used separately as the output devices for the two halves of the push-pull stage.

A single 6336A per channel in class A push-pull

The output stage operates with an output transformer specifically designed for this configuration. The use of the 6336 provides an inherently favorable starting point in terms of damping, allowing the overall negative feedback to be limited to just a few dB: only what is needed to fine-tune the overall behavior of the circuit and compensate for the small, unavoidable imbalances between the two halves. The objective is therefore not to heavily correct the amplifier through feedback, but rather to design an output stage that already has good electrical characteristics on its own and use negative feedback only to the extent strictly necessary.

Fully balanced input and driver stages

The input and driver section uses a fully balanced topology. Two ECC88 tubes are used together with a 6SN7 as the final driver for the 6336. The ECC88/ECC88/6SN7 stages are directly DC-coupled, eliminating intermediate coupling capacitors. The only two capacitors in the signal path are those coupling the driver to the two sections of the 6336. This is a deliberate choice: to keep the circuit simple, linear and free from unnecessary components along the audio signal path.

Separate power supplies and CLC filtering

The power supply has also been designed without cutting corners. The output stage and driver section have separate power supplies, with diode rectification and CLC filtering. Separating the power supplies helps prevent the high currents drawn by the output stage from interfering with the small-signal and driver stages. The set of transformers and chokes is specifically manufactured for this project and includes the output transformers, power transformers and chokes required to build the filters specified in the schematic.

The sonic result

Those who have already built this amplifier have particularly appreciated its clarity, control and ability to reproduce a detailed soundstage without losing the naturalness and sense of presence expected from a good triode amplifier. Its character remains warm and engaging without becoming excessively soft, with a well-controlled low-frequency range and good definition of the sonic image. This is precisely the central idea behind the project: to obtain the typical qualities of a triode output stage without having to resort to large amounts of negative feedback to maintain control over the loudspeaker.

SB-LAB Premium Project
The complete circuit schematic is supplied together with the corresponding set of transformers and chokes specifically manufactured for the project. The price is quoted upon request according to the components required and the supply conditions at the time. For information, availability and a quotation for the complete set, you can contact me by clicking here.

Circuit schematic and amplifier built by a customer

The complete circuit schematic for this amplifier is available exclusively together with the corresponding SB-LAB transformer and choke set and is not supplied separately. The photographs below show an example of the amplifier built by a customer based on the SB-LAB design.