If you want 4 V, I suggest looking at LED data sheets and find an LED that gives you either 4 V or 2 V at the cathode current you plan to run in the 6DJ8. The second selection criteria would be the dynamic impedance of the LED at the chosen operating point. This can be deduced from a graph of forward voltage vs forward current. You want the lowest possible dynamic resistance if you want the highest gain in the input stage.
Red LEDs tend to run at 1.7 V; green 2.0 V; blue somewhere around 4.0 V. I think IR LEDs are about 4 V also. Morgan Jones has a table in his book of the forward voltages of different LED types.
You can find plenty of LEDs and their datasheets at digikey.com or the various LED manufacturers.
~Tom
Red LEDs tend to run at 1.7 V; green 2.0 V; blue somewhere around 4.0 V. I think IR LEDs are about 4 V also. Morgan Jones has a table in his book of the forward voltages of different LED types.
You can find plenty of LEDs and their datasheets at digikey.com or the various LED manufacturers.
~Tom
Hi DF ... this has made me think ...
Surely it can only compensate for production spread if we curve trace the actual tube we are using.
Also, as the tube ages, we would have to again alter the value of the resistor, requiring another curve trace, so as to place the tube's operating point back to where we desire it?
Am I right here?
No different really, than swapping our LED's round, although the resistor cap option allows us much more fine adjustment than a volt here or there with an led
Doz:
Normal RC cathode bias can be designed using the nominal valve described in the datasheet. If your particular valve takes more current, then the cathode voltage will increase and reduce the current. Vice versa. Most valves are not too fussy about exact bias, given a good circuit. A poor circuit might be more fussy, but that is a sign of poor engineering rather than the 'fine discrimination' which people sometimes claim.
SY:
Maybe I am old-fashioned, but I have always been averse to sprinkling too much sand around my valves (apart from diodes). I accept that it can work extremely well, though.
Normal RC cathode bias can be designed using the nominal valve described in the datasheet. If your particular valve takes more current, then the cathode voltage will increase and reduce the current. Vice versa. Most valves are not too fussy about exact bias, given a good circuit. A poor circuit might be more fussy, but that is a sign of poor engineering rather than the 'fine discrimination' which people sometimes claim.
SY:
Maybe I am old-fashioned, but I have always been averse to sprinkling too much sand around my valves (apart from diodes). I accept that it can work extremely well, though.
Got it in one!
A CCS doesn't have to be silicon, but for the same reasons one might favor silicon in constant voltage source/sinks (we call those things "power supplies"), one might also favor it for current source/sinks. This allows the Queen Tube to sit on an appropriately comfortable and well-tailored throne, a metaphor that an Englishman will understand.
As a side note, my phono stage was roundly and loudly criticized by Tim de Paravicini for exactly that reason. He maintained (and I'm quite serious) that one should not use any technology in a tube design that wasn't available in the 1940s.
When I started playing with valves (about 10 years ago), I did so because I wanted to learn about a technology I wasn't taught when I went to college, and I wanted to "hear" what all the fuss was about. I promised myself my first amplifier would not have a solid state device from the mains plug through to the speaker sockets. It didn't
It was an enjoyable learning curve (and continues to be) ... but sometimes a nice solid state device does have it's place, the LED being one such device (nearly went off-topic there - phew saved it)BTW I am even less keen on sprinkling vacuum onto a solid-state circuit, but some people seem to like combining the worst of both worlds while others do it because they know it sells well.
I accept that logically a CCS is no different from a silicon rectifier in a PSU. Maybe one day I will use one. I guess a CCS on top and an LED below could pull the poor valve in opposite directions, so limiting headroom for samples which don't match the datasheet?
I accept that logically a CCS is no different from a silicon rectifier in a PSU. Maybe one day I will use one. I guess a CCS on top and an LED below could pull the poor valve in opposite directions, so limiting headroom for samples which don't match the datasheet?
Do it all the time, no SS articfacts that I can detect, no edge, compression, blandness, etc. Just great sound.
I detest the sound of what a wise man once called (paraphrasing) a huge stinking pile of dielectric [Thank you SY]. Rather avoid those than a few semis.
Sometimes battery bias at the grid is an option, if a cap is unavoidable at the grid, then connect the cathode to ground.
Stuart
I never mentioned SS artifacts. I like blandness, which I call accuracy. My concern was quiescent anode voltage.
Are you wary of all caps, or just electrolytic? This aversion always puzzles me, because it seems to force people to use even less desirable techniques to get round a self-imposed problem.
Are you wary of all caps, or just electrolytic? This aversion always puzzles me, because it seems to force people to use even less desirable techniques to get round a self-imposed problem.
with a CCS you want a PSU 100V higher than anode voltage (as a minimum, it depends on the application) ... so it might not be a simple drop-in replacement. in any case you can fix only 2 of the 3 variables (K-G voltage, current, plate voltage) whatever biasing you use so I really do not see why you worry about anode voltage. PLUS the CCS will give you a much much higher PSRR
Maybe I don't keep my designs together long enough for tube aging to be a problem, but after a few years' constant use, the anode voltages still seem dead nuts. one element takes a little getting used to when adjusting a CCS fed diode biased circuit (SiC Schottkys work well also): higher plate voltage means higher tube current. SY suggested isolating the plate from the CCS, so using a resistor here provides a way to measure anode current.
As for caps, they all have a sound. Better to avoid the "sound" when practical. Minimize the effect by minimizing the number (and yes, they are all "in the circuit" to one degree or another). Adding a cathode resistor bypass cap (talk about a "less desirable technique to get (a)round a self-imposed problem"!) adds another phase shift and LF rolloff. The notion of adding, say, a 220uF electrolytic, then 1uF and 0.01uF film bypass caps is most definitely undesirable. I'd much rather have lower gain, a bit of local NFB and add another gain stage or switch tubes for more gain, if need be.
Try this: build yourself a simple grounded cathode circuit, line stage level, on plywood. Medium or low gain tube, nice and linear (12B4, 6SN7, etc.) Make it as simple as possible. For the output cap, rig up a Jones barrier strip so you can switch out caps.Go through the cap selection you have and use a well known piece of music. Listen for differences. See what you think. If you don't hear any deleterious effect on the source material from different caps, you're quite fortunate. you can save lots of money.
Stuart
True, with the further refinement that CCS minimum voltage is dependent on type of CCS and as you point out, how much voltage do you need to swing?
In phono stages, when swing is always <2V, I use as little at 30V across a 10M45S, or a little more across a cascoded pair of DN2540s. But a Pimm style or C4S will need more voltage impressed. Best to provide headroom.
I'm in agreement with you also that there's no need to worry about exact plate voltage; to a degree it doesn't matter much (within limits, of course).
Stuart
In almost any reasonable circuit the non-linearities of active devices far exceeds that of passive components. In addition, almost all our music is delivered to us by equipment designed by people who believe this so even if they are wrong there is little we can do about it. What we can do is add some pleasant distortion, by using poor circuits or choosing components on the basis of their 'sound'. By all means do this if it pleases you.
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