Tubelab Simple PP – Description
♦ Back ♦ Description ♦ Construction ♦ Performance ♦ Pictures ♦
This amplifier was built as a “beta test” of George Anderson’s latest “Simple PP” tube amplifier design. Like the Simple SE, the project is geared toward novices and utilizes a simple design and detailed instructions to help achieve a high probability of success. Unlike the Simple SE, this one is available as a kit with the PCB and all the parts needed to build it. The prospective builder need only supply transformers, tubes, a chassis, and the various bits that tie it all together (jacks, wire, etc).
The amplifier design is based on the tried-and-true classic simplified Williamson. A single 12AT7 per channel implements the common cathode input gain stage and a split-load “concertina” phase splitter. No driver stage is needed because of the very light drive requirements of the output tubes: the venerable EL84/6BQ5 setup as pentodes with cathode bias. The result is 12-18W of output power, depending on the B+ voltage provided by the power transformer and the primary impedance of the output transformer.
This amp doesn’t have the output tube flexibility of the Simple SE simply because there are few other tubes that swapped for the EL84. The only other choices are direct substitutes, such as the 7189 and the Russian 6P14P variants. Some of these types can handle higher B+ voltages than others. I found that the 6P14P and other made-in-Russia “EL84” brands from New Sensor will start to glow with much more than 300V across them, which limits the B+ voltage to about 315V or less. Current production JJ tubes seem to be able to handle the higher voltages without issue.
The amp can also be built to use an entirely different type of output tube: the EL86/6CW5. However the operating requirements of these tubes are very different from the EL84 such that entirely different power and output transformers are required. There are no current production versions of these, but with them 16-20W of output or more can be achieved with the right transformer and tube combination. Not all 6CW5s are created equal.
