LEDs or RC?

[no gas]

I'm about to build a Kt88 SE and am wondering if I should use LEDs instead of the RC on the input 6dj8. What would you suggest? I'm shooting for 4v.

[tomchr]

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

[DF96]

RC will compensate for valve ageing and production spreads. LED will not.

[Doz]

Originally Posted by DF96

RC will compensate for valve ageing and production spreads. LED will not.

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

[SY]

Originally Posted by DF96

RC will compensate for valve ageing and production spreads. LED will not.

...which makes the use of CCS plate loads even more compelling.

[DF96]

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.

[SY]

Originally Posted by DF96

Maybe I am old-fashioned...

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.

[Doz]

Originally Posted by DF96

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.

Thank you. I don't like to sprinkle too many capacitors around mine

[Doz]

Originally Posted by SY

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)

[DF96]

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?