The next step in the “not quite so Simple P-P” is a P-P Driver board. The design goals are simple, it needs to be capable of driving a pair of tubes to full power in P-P. It should be capable of driving anything from a pair of 6V6's or 6L6 type tubes in conventional grid drive up or cranking a pair of sweep tubes to near meltdown in screen drive or even cathode follower mode. This board must be able to drop in to the original 300Beast amplifier, replacing its driver board while offering equal or better sound quality.
OK, writing down some lofty goals on paper (or in the computer) is easy. Building the circuit and getting it to work is not. This project has been alive since before there was a Tubelab web site. In many ways it has evolved from my 300Beast amplifier. It incorporates a lot of the 300Beast design and many techniques that I have learned since the 300Beast was built.
I laid out this prototype PC board in a hospital waiting room in Pittsburgh during the Christmas (2008) holidays. I started building it when I got back to Florida. When I completed it and powered it up I found that I had swapped the top and bottom layers when making the board, resulting in a mirror image of what I laid out. After nearly tossing it in the trash, I realized that all I needed to do was solder a set of tube sockets on the back of the board. The board is alive and working now. More details are on the Universal Driver Board page.
I have received numerous requests for an amplifier using all octal tubes. Octal tubes use up more board space than miniature tubes and are usually more expensive, so I have not done a completely octal amplifier yet. I decided that a complete P-P amplifier would not fit on a single PC board if it used octal tubes, so an octal tube amp must be broken up into blocks. While working on the Universal Driver Board a thread developed on the diyAudio forum dealing with designing an amplifier using 6L6GC's in AB2. The thread originator desired to use 6SN7's for the driver tubes. I was assisting in a "paper design" effort by modifying a Universal Driver Board for this project. I got a little carried away and 350 forum posts later I had a working a prototype amplifier that used all octal tubes.