13 May 2006

Since these boards follow the same rules as the 9-pin mono boards, please read the previous blog entry for more information.

Aikido tubes: results and values
I keep getting requests for more help in selecting tubes and cathode resistor values for the Aikido line stage amplifier. As many of you can guess, I am not about to say that one tube or one set of resistor values is perfect. (What separates me from so many other tube gurus, aside from my not considering myself to be a guru, is my steadfast refusal to believe in one tube, one resistor value, one topology, one capacitor type…) So, instead of one quintessential archetype, I’ll give many design examples. I have compiled a table of tubes and values and results.

The table above lists many triodes suitable for an Aikido amplifier (including a few that won't work with the PCBs, like the 12B4). The same tube is listed with different B+ voltages and cathode resistor values. Two gains are listed: the first is the gain the tube realizes in the input position in the Aikido; the second is the gain of the tube in the output stage. To calculate the final gain we must multiply the two voltage gains together (or add the gain in dBs together). For example, given an Aikido line amplifier with a B+ voltage of 300V, and a 6CG7 input tube with cathode resistors of 1k, and a 6DJ8 output tube with cathode resistors of 481 ohms, the final voltage gain equals 10.1 from the 6CG7 against the 0.96 gain of the 6DJ8, with a product of 9.7. or, working with dB instead, 20.1dB plus -.35dB, for a total of 19.75dB. (Aren’t decibels great?) If you have additional data, send it in and I’ll add to the list.

Aikido and 300B amplifiers
Once again I was overly pessimistic. Many want to build a single-ended, 300B-based amplifier that uses the Aikido amplifier as the front end. My guess was that the 300B would require too much voltage swing from the Aikido stage. Well, I now think that I was wrong. I performed a quick experiment: I threw together an Aikido amplifier (nothing like having a stack of PCBs to play with) that used a 12AX7 as the input tube and a 12AU7 as the output tube. I then fed the Aikido a 1kHz signal. At 1V out, the Aikido is distortionless, at least as far as my eye can tell (I couldn’t use my FFT setup and I believe the eye can see down to about 3% distortion with sinewaves). I then drove the 12AX7 to a 70V output. I expected to see grid conduction and obvious distortion; in fact, I saw neither. It still looked mighty clean and the cathode still had a 0.25V positive advantage over the grid.

Now for some “howevers.” At first I used a B+ voltage of 400V, which made me nervous, so I then lowered the B+ voltage to 300V and I could see a hint of distortion, maybe as much as 1%. I found that the two safety resistors, R5 and R6, were dragging down the gain too much, so I increased their value to 10M. At first I used 1k cathode resistors for the 12AX7, but they had to be upped to 2k to get the best results. I forgot to write down the input signal level, but I remember it being about 1.7Vpk, which is a tad too high for passive line stages, but would be no problem for any active line stage. Below is the experimental circuit.

Now all that is needed is a 300B output stage. I would use two 300Bs in parallel with a B+ voltage of 440Vdc and a one-electron UBT1 output transformer. Why? Using two output tubes means more watts and allows me to use the UBT1 transformer, which I already own. (Actually, the UBT1 is a great transformer, as it has a low winding ratio that allows it to pass an undistorted signal. Usually the higher the winding ratio, the more headaches.) I know that many readers do not approve of parallel output tubes, so here is a single 300B version. 

Next time
I hope to cover unity-gain power buffers for driving speakers.