LED's in the cathode..

[Tony]

the best lessons learned are those that were made in error.....

[Palustris]

I have a couple of questions regarding using LEDs to replace a cathode resistor.

What is the lowest practical current that can be used with the standard red LED? For instance would a 6SL7 with 2mA plate current be practical or would the current be too low for the LED to be in its linear range.

What is a practical way to inject GNFB into a first triode stage of a PP amp with LED bias? If we assume a bias of the first stage is 2V then it doesn't seem practical to inject feedback but if there is say 4V bias then it would seem that a resistor could be interposed between the cathode and the LED and FB injected directly to the cathode.

[SY]

Originally Posted by Palustris

What is the lowest practical current that can be used with the standard red LED? For instance would a 6SL7 with 2mA plate current be practical or would the current be too low for the LED to be in its linear range.

What is a practical way to inject GNFB into a first triode stage of a PP amp with LED bias? If we assume a bias of the first stage is 2V then it doesn't seem practical to inject feedback but if there is say 4V bias then it would seem that a resistor could be interposed between the cathode and the LED and FB injected directly to the cathode.

In order:

For best operation, you want at least 5mA flowing through the LED. If you have a plate CCS or a very high value plate resistor, then lower currents are no problem since the nonlinear Z vs I is removed from the picture. If the tube is loaded by a resistor where the current swing becomes significant (1mA or more), you'll want to increase the current through the LED.

You got it- the way to use the LED in a feedback situation like you describe is to put a small resistor in series. For my latest RLD iteration, I used a 47 ohm resistor, with the resistor being grounded, the LED connected between the resistor and the tube cathode, and the feedback returned to the top of the 47 ohm resistor.

[M Gregg]

Just for interest,

I tried the resistor from B+ to the cathode to increase the current through the LED and it works well in my case 100K..I haven't looked but there must be a cut off point, ie how low can you go before the LED won't conduct and shuts the tube down..

Regards
M. Gregg

[TheGimp]

I've tested some LEDs and seen them begin emitting light in the 50uA range. This is most likely the knee of the curve, so one would want to go 10 times that at a minimum.

Personally, if I wanted to operate that low I would swamp the current with a resistor to B+ to insure lower impedance.

[SY]

Remember, the current is whatever the tube is drawing, at a minimum. That's unlikely to be less than 500-1000uA and usually more.

[M Gregg]

I find this interesting,

Just a suck it and see test..

Ok, this is the scenario.
Triode gain stage with standard resistor anode load..<< sounds good..
Resistor in the cathode replaced with LED string..<< sound is OK current a bit low so..
Resistor put from B+ to cathode to increase current<< sound very good, but lower treble and HF detail..
CCS put in anode load sound is OK treble rolled off…still have extra current resistor B+ to Cathode..
Resistor removed from B+ to cathode << extremely high treble and more detail..
Are we hearing a varicap diode change?
If this is the case will frequency response change with signal current?
If this is possible then do we have to be smack in the middle of the linear zone of the LED?
If this is so then the above seems to suggest that high current = low treble/ low current =high treble?
Or is this some instability because of the resistor providing the extra current?
Interesting its linked to the CCS..
Just empirical tests… Just thought I would drop this in here and see if anyone had similar experience.

Regards
M. Gregg

[Merlinb]

Originally Posted by M Gregg

I find this interesting,

If this is so then the above seems to suggest that high current = low treble/ low current =high treble?

Plug in your signal generator and measure the frequency response of the stage with and without the feeder resistor, and possibly at different levels of input signal. Better than empirical tests!

I wouldn't expect the frequency response to show any difference in treble. It's possible that what you're hearing as 'more treble' is a brighter sound resulting from more harmonic distortion, because the LED is dropping into its more non-linear region when the feeder resistor is removed.

[M Gregg]

Originally Posted by Merlinb

Plug in your signal generator and measure the frequency response of the stage with and without the feeder resistor, and possibly at different levels of input signal. Better than empirical tests!

I wouldn't expect the frequency response to show any difference in treble. It's possible that what you're hearing as 'more treble' is a brighter sound resulting from more harmonic distortion, because the LED is dropping into its more non-linear region when the feeder resistor is removed.

I'll have a look next time I strip the unit..Its a bit difficult to get at..However it is very interesting..

I didn't have much time to test ..hence the empirical tests..You may be correct..However there is a definate change like day and night..

Its interesting with the feeder resistor that the HF detail is definatly less..
And the higher the current the more rolled off it seemed..
It was more pronounced with the CCS anode load..Current from the feeder resistor is 5mA..

Regards
M. Gregg

[DF96]

I second post 68. Distortion masquerading as frequency response.