getting ready to order parts for an Aikido linestage. This one will use two mono 9-pin Aikido boards from John Broskie. I plan to use 12BH7A's in both input and output positions simply because they are, far and away, the best soundiing tube in my scratch-built foreplay. Plan to use a cLCLC power supply, either single or dual mono, more on that later, plus a regulated 12 Volt filament supply. PSUD tells me to expect about 320 Volts B+. Now, a few questions:
First, Just JB recommends a cathode resistor of 523 Ohms for the 12BH7A in output position with a B+ of 300 Volts. However, he also recommends that this resistor be in the range of 200-470 Ohms with a B+ of 300 volts. Seems to me that this must be a typo and he really means 323 Ohms. With 15 ma/300 Volts, his table shows 267 Ohms. This value puts the bias at about 4 volts which seems reasonable for approximately 150 Volts across the tube. The 523 Ohm resistor would put the bias at 8 Volts for 15 ma, which is more consistent with 230 Volts across the tube, which isn't going to happen with a 300 Volt B+. Anyone else checked this?
Secondly, everything I read says that the 12BH7A sounds best run hot, about 15 ma. Any reason (other than tube life) not to run the Voltage Amplifier tube at 15 ma?
Thirdly, any recommendations for the "High Quality" cathode and grid stopper resistors? Has anyone tried the miniature Ohmite 1 watt non-inductive wirewound resistors that Digi-Key sells? At about a buck each, the price is right. Has anyone tried LED bias in the Aikido? It certainly is an improvement in my Foreplay notwithstanding the fact that many say LED bias sounds worse than a good resistor.
Fourthly, I have a beautiful pair of Freed potted 540 Volt ct 70 ma plus 6.3 volt 6 amp power transformers, including electrostatic shield between primary and secondaries. I think that one of these would handle a B+ draw of about 60 ma, but will I have better headroom, dynamics or generally good JuJu if I go fully dual mono and use one per channel?
Thanks in advance for your help, advice and suggestions,
Roger Hill
First, Just JB recommends a cathode resistor of 523 Ohms for the 12BH7A in output position with a B+ of 300 Volts. However, he also recommends that this resistor be in the range of 200-470 Ohms with a B+ of 300 volts. Seems to me that this must be a typo and he really means 323 Ohms. With 15 ma/300 Volts, his table shows 267 Ohms. This value puts the bias at about 4 volts which seems reasonable for approximately 150 Volts across the tube. The 523 Ohm resistor would put the bias at 8 Volts for 15 ma, which is more consistent with 230 Volts across the tube, which isn't going to happen with a 300 Volt B+. Anyone else checked this?
Secondly, everything I read says that the 12BH7A sounds best run hot, about 15 ma. Any reason (other than tube life) not to run the Voltage Amplifier tube at 15 ma?
Thirdly, any recommendations for the "High Quality" cathode and grid stopper resistors? Has anyone tried the miniature Ohmite 1 watt non-inductive wirewound resistors that Digi-Key sells? At about a buck each, the price is right. Has anyone tried LED bias in the Aikido? It certainly is an improvement in my Foreplay notwithstanding the fact that many say LED bias sounds worse than a good resistor.
Fourthly, I have a beautiful pair of Freed potted 540 Volt ct 70 ma plus 6.3 volt 6 amp power transformers, including electrostatic shield between primary and secondaries. I think that one of these would handle a B+ draw of about 60 ma, but will I have better headroom, dynamics or generally good JuJu if I go fully dual mono and use one per channel?
Thanks in advance for your help, advice and suggestions,
Roger Hill
You should just accept everything I say without question. 
Actually, I think JB may have discussed this in one of his articles, though he probably talked about bypass caps rather than LEDs. Basically, the circuit relies on the symmetry between top and bottom tubes. So if you were to bias the bottom with an LED, you'd need to do the same for the top. Now... think about where the top tube gets its signal. That's right, from the voltage induced across its cathode resistor. If there's an LED there, the AC voltage between grid and cathode is zero and the top tube doesn't get any signal.
Actually, I think JB may have discussed this in one of his articles, though he probably talked about bypass caps rather than LEDs. Basically, the circuit relies on the symmetry between top and bottom tubes. So if you were to bias the bottom with an LED, you'd need to do the same for the top. Now... think about where the top tube gets its signal. That's right, from the voltage induced across its cathode resistor. If there's an LED there, the AC voltage between grid and cathode is zero and the top tube doesn't get any signal.
Swapping an unbypassed cathode resistor for an LED makes that part of the stage less like a CCS. The top cathode resistor in an active load stage should normally be unbypassed and not be an LED, unless you want a somewhat unconventional circuit which in most circumstances will have poorer performance.
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