That is right. The cathode of the tubes were biased with a resistor. The circuit board really has nothing to do with biasing a vacuum tube. Essentially there are 2 types of bias, cathode bias and voltage or fixed bias. Cathode bias uses a resistor or resistance to establish the voltage drop between cathode and anode which the tube needs to operate.I'm just wondering if this is a thing, because it doesn't sound like a thing. But a friend was like "well, old tube radios didn't need to be biased, and they didn't have circuit boards".
Educate me people.
This may simply be using an IC like the LM317 which is a voltage regulator as a constant current source. Apart from the IC and a resistor for setting the voltage, it has the same number of components as cathode bias, which is a resistor and a bypass cap. There may also be some over voltage protection of the lm317 via a diode.The Primaluna gear markets their stuff as adaptive autobias or some such thing. I don't know how it works but the marketing writeups I've seen about it do lead me to believe it is monitoring the output stage somehow and correcting bias as needed not just at idle but all throughout the power curve to offer lowest distortion output at any power level. The pictures I've seen of their amps do show circuit boards that implement that circuit.
What has been termed here as "self bias" or "cathode bias" is orders of magnitude simpler than that with just a single resistor and bypass cap in the cathode circuit of the output stage.
"self biasing" is another term, along with auto bias, for having a the tube biased by a resistor. Although this statement seems to be at odds from Kev's assessment.A guy was stating the Primaluna Dialog has no circuit boards and is self biasing. I had just never thought about it before and assumed it had to be a function that would require some sort of monitoring circuit."
Right I think this is the article I have seen in the past.Art Dudley covers this feature of Primaluna design in the fifth paragraph of this review. No schematic, but the PCB has some kind of active device on it.
It might be like that but I would guess the PrimaLuna circuit is much more sophisticated. For one thing, the board in the link you provided doesn't appear to have any active parts (no transistors). But the real give away is that it says it manages bias independent of drive signal (as if this is a benefit, wherein actually I think it is a detriment--unless you're doing full blown regulation of plate, bias, and screen). So it seems this board is simply looking at an average of DC conditions so that as the tube ages you don't need to readjust quiescent DC conditions. I guess that's partially useful.
But the real advantage here I believe is that PrimaLuna's circuit does not do full blown regulation and IS dependent (indirectly) on drive signal so that it automatically adjusts bias not only under DC conditions but also under AC conditions (meaning under the conditions of music playing and at ALL power levels the amp is capable of delivering).
But what do I know. I'm just speculating given the marketing writeup and not having seen the schematic. I do agree with Gadget and expect it's some sort of servo monitoring "output conditions" of the tube in real time. Still wonder if it's analog or digitally sampled though. Looks like there might be an ASIC or custom programmed processor on that board.
There is nothing new about using servos to bias output tubes. There was 'how to' article in Glass Audio back in the '80s and conrad Johnson design has been doing it since probably the early 1980s with their MV 45 and MV 75.
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As interesting as this discussion has become, we seem to have strayed way off the OP query. I don't think any of the "old radio" mentioned in the first post had something close to any solid state device.Thanks for sharing that link, but this doesn't appear to be a description of the Primaluna circuit.
I do not think so. For nearly twenty years, Tentlabs has been selling similarAs interesting as this discussion has become, we seem to have strayed way off the OP query. I don't think any of the "old radio" mentioned in the first post had something close to any solid state device.
Anyway, i am finding that that type of bias circuits which have LED or ICs become problematic after about 10 years or so. That is my experience from repairing those type of amps. I am currently working on a Sonic Frontiers amp with a bad either led or pot in the bias circuit.