LED's in the cathode..

TonyTecson · 2012-04-13 2:41 pm

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

Palustris · 2012-04-13 3:05 pm

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 · 2012-04-13 3:41 pm

Palustris said:
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 · 2012-04-27 11:27 am

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 · 2012-04-27 2:04 pm

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 · 2012-04-27 2:08 pm

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 · 2012-05-06 10:06 am

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 · 2012-05-06 10:27 am

M Gregg said:
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 · 2012-05-06 10:43 am

Merlinb said:
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 · 2012-05-06 1:55 pm

I second post 68. Distortion masquerading as frequency response.

dgta · 2012-05-06 2:57 pm

M Gregg said:
If this is so then the above seems to suggest that high current = low treble/ low
M. Gregg

No it doesn't. It suggests that we have no data to base any opinion on.

My guess is that in at least one of the configurations the circuit is improperly biased and you are hearing gross distortion.

Post the circuit with values, etc. and the frequency response and distortion data and then we can talk.

DavesNotHere · 2012-05-06 3:34 pm

leds are interesting fun in the cathode. I use them in fuzz/od effects section of guitar ampness.

but I am set in my ways with audiophile circuits and a really nice resistor is all you need. (no solid state junk in my tube audiophile)

and caps stink in the cathode.

Palustris · 2012-05-06 5:21 pm

"really nice resistor is all you need"

It isn't the resistor that we are trying to eliminate; it is the bypass cap. I know many go cap-less, but I have found that the bass suffers when I do.

DF96 · 2012-05-06 5:27 pm

Omitting the bypass cap raises anode impedance, which generally degrades PSRR. Like everything, it is a matter of careful design.

tim614 · 2012-05-07 2:40 pm

ok, so ive been following this thread and i experimenting on LED on my own amps.
so the amps are using 6n6p as driver, before the led i had 150v on the plate and 150ohms on the catode with bypass cap, 3.5v cathode to ground so thats around 23ma.
after reading the thread i put 2 RED led in series and measured 4v cathode to ground.

now what im wanting to know is did i raise the current with the LED?

what i notice is the sound are more detail not mush together and i think allitle less gain but im not sure, also they sounded clean when i turn the amps all the way up, with the resister in cathode i can hear the amp distord when volume is max.

so what does this mean?
am i on the right track?

BTW, amps are PP kt120 with IT phase splitter.

thanks
Tim

DF96 · 2012-05-07 2:50 pm

4V at the cathode vs. 3.5V means you have slightly less quiescent current and so less small-signal gain. On the other hand for stronger signals the bias point will shift less with the LED so this might give less distortion.

tim614 · 2012-05-07 3:16 pm

so if i want more current i need to eliminate one of the led?

DF96 · 2012-05-07 4:13 pm

Yes, but you could get too much current and increased distortion. What does the load line show?

Palustris · 2012-05-07 4:57 pm

If you omit one LED you will need to draw new load lines and re-engineer the front end.

tomchr · 2012-05-07 7:53 pm

tim614 said:
so if i want more current i need to eliminate one of the led?

Or try an LED of a different color. Many blue LEDs run 3.5~4 V. The HLMP-D101 red LEDs that I use tend to be 1.75 V - so two of those in series would give you 3.5 V. Yellow LEDs tend to be around 1.75 V, green around 2 V.

I suggest looking at Digikey's LED selection. Download datasheets for different LED types (IR, different colors, etc) and look at their forward drop for the current you want. It's tedious work, but I'm sure you can find an LED that'll give you 3.5 V either as a single LED or with two in series.

~Tom

LED's in the cathode..

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