Alembic F-2B

Here’s some build info I’ve put together on the Alembic F-2B. I’m planning to build myself one of these sometime real soon.

The original…


My redrawn schematic is primarily based on R.G. Keen’s, except for the selectable hi cutoff switch SW1 + C2, which was not part of the original design. But, I saw it in someone else’s F-2B project and decided to include it.


The power supply shown in the GEOFEX scheme is just a tad cryptic. It has a voltage doubler, but no indications about the transformer except for 6.3VAC on the secondary that’s used for the filament supply. But, on the Alembic page, it indicates that the F-2B uses a 300 volt supply. So, I went with that and just started from scratch. I also wanted to be able to fit everything into a 1U rack enclosure, like the original. But, I had no luck finding a single low profile transformer with dual secondaries for both plate and fillament that would fit. So, the PSU I came up with uses a back-to-back pair of identical compact toriods with rectifiers. The low voltage side for the filaments is pretty much right out of the LM317 datasheet and supplies DC voltage to the filaments, instead of AC like the original. I also included the 5K trimmer to make it variable, so I’ll be able to see what different filament voltages might do to the sound. For the high voltage side I used PSUD2 and shot for around 300VDC.


Here’s a PCB layout for the audio section…


The PCB layout was made to utilize these cool horizontal tube sockets that I picked up from Angela Instruments.


And, here’s a PCB layout for the PSU…


That’s all for now…