After experimenting with several current-production tubes, I discovered that the 12BH7 and JJ ECCC99 worked surprisingly well in an Aikido line-stage amplifier with a B+ of only 48Vdc. I planned on making a new PCB production run on the 24V 48V Aikido PCB, but I realized a problem. The 24V Aikido board also held a solid-state unity-gain power buffer for driving headphones. By doubling the B+ voltage from 24Vdc to 48Vdc, the buffer's dissipation would also double. I knew some users were already at the end of the heatsink's limits and different and more expensive solid-state devices would be needed. So I set out laying out a much larger PCB, large enough to hold bigger heatasinks and a power supply rectification circuit and regulated power supply. The board was getting big, much bigger, too big. I halted this design effort and decided to go in the other direction: smaller. I jettisoned the solid-state buffer and the idea of incorporating a full power supply on the board. The result was an astonishingly small PCB, only 3.6in by 5.4in big.
The key advantage of the LV Aikido is that it can operate under relatively low B+ voltage. Where the typical tube-based line-stage amplifier requires a B+ of 200V to 400V, the LV Aikido can get by with only 24Vdc with 6GM8/6N27P/ECC86 tubes; or 48Vdc with 12BH7 or ECC99. The heaters are all placed in series and this heater string is placed in parallel with the B+ and ground. Thus, a single power supply voltage is all that is required.
The power supply is external to the LV Aikido PCB and can be mounted in, or outside, the chassis that houses the PCB. The optimal power supply voltage depends on the tubes used. Four 6GM8s (6N27P/ECC86) can be used with a low 24V power supply, with 12BH7 or ECC99, 48Vdc—either a switch-mode or a linear power supply. After dealing with 400-volt power supplies, it is a joyful relief to work with relatively low voltages. We must address, however, a few important issues. For example, although we do not need much voltage, the heaters add a heavy current burden on the power supply. With 6GM8, the heater string requires 330mA and the four tubes require a total of 8mA, for a grand total of 338mA or (rounding up) 350mA. So, 0.35A against 24V equals 8.4W of dissipation. With the ECC99, the heater string requires 400mA and the four tubes require a total of about 10mA, for a grand total of 410mA or (rounding up) 450mA. So, 0.45A against 48V equals 21.6W of dissipation. (In other words, expect a good deal of heat and provide plenty of ventilation, in spite of the low B+ voltage.)
The LV Aikido is a perfect candidate for a wall-wart power supply. Both linear and switch-mode wall-warts are readily available in a 24V or 48V output voltage.
Includes 16-page user guide.