LED cathode distortion?

[tubenit]

I tried a "search" to no avail on this.  I did remember Sluckey experimenting with LED's in tremolo circuits.   (see post #13)

http://el34world.com/Forum/index.php?topic=604.0

And Valve Wizard had some info also:
http://www.valvewizard.co.uk/trem1.html

I also remember Geezer built an overdrive pedal with different LED's in it and liked it.

So I am wondering if an LED in an amp gain stage would give a really smooth distortion?

AND I am wondering IF anyone has done something like a relay switched LED in the cathode of a gain stage? 

And more importantly, what tonal difference is there in overdrive with and LED vs. a resistor with cap in the cathode?

I know zip about LED's but I do like to experiment. 
Keep in mind that more of my experiments don't work than do.


With respect, Tubenit

[tubenit]

And just to further show my lack of knowledge .............  what about using relay switching to parallel an LED with a regular cathode resistor and cap?

With respect, Tubenit

[sluckey]

I bet you're only 5 minutes away from trying these ideas. BTW, different color LEDs have different voltage drops, so get a variety pack and play. Let us know what you find out.

[chocopower]

Just one question.
Will that LED bright when signal applied like in the tremolo option?

[tubenit]

IF I have time, I'll give it a try this wkend and report.

I found some more info about LED's by Valve Wizard.

He does acknowledge some tonal differences used in a cathode of a triode, but doesn't clarify what those are?

http://www.valvewizard.co.uk/OtherStuff.html

On an LED used in a cathode setting, which leg goes to ground, please?  I am thinking maybe an LED paralleling a cathode resistor and cap would sound good?

Thanks, Tubenit

[HotBluePlates]

So I am wondering if an LED in an amp gain stage would give a really smooth distortion?

Your examples show LEDs in the cathode circuit of a preamp tube. Ordinarily, you have a cathode resistor and bypass cap there. Each is doing the exact same thing, which is to provide a d.c. bias for the tube.

What is different about these, and why the mention of using an LED for a trem oscillator? You already know you can adjust the bass response of the gain stage by changing the value of the bypass cap. In other words, depending on the cap's value (and that of the resistor) it can approach a short-circuit at some high frequency while being a large impedance at some low frequency. At frequencies where the bypass cap is not a good-quality short-circuit, there is local negative feedback across the cathode resistor and reduced stage gain.

To get that good-quality short-circuit at very low frequencies, you need R*C to be large. The value of R is fixed by the needed d.c. bias of the tube, and that means you need a very large C. But even a 330uF cap across a 1.5kΩ resistor is -3dB at ~2Hz, and trem oscillator frequencies are around 3-7Hz. So we still might have insufficient gain from our oscillator with a 330uF bypass cap, and gain is critical to getting the oscillator started and running.

The solution is to remove the dependence on cap value by changing the bias method. You could ground the cathode and apply fixed-bias to the grid, and get full gain down to 0Hz (because there's no frequency-dependent feedback). You might even supply that bias with a hearing aid battery or a AAA battery, if your desired bias voltage is ~1.5vdc.

Or you could use an LED. LEDs have a specific On-voltage, which is the voltage across the LED while it is conducting and lit up. The LED will draw as much current as needed to reach that on-voltage, and once lit will maintain that voltage even down to a fairly low current (a wide enough range for preamp tube use). You can take advantage of this characteristic, and use it as a free d.c. voltage pegged to the on-voltage of the LED you choose (different LEDs and different colors can have different on-voltages).

Like a battery, this bias method will maintain a solid bias voltage down to 0Hz, and give full gain from a trem oscillator.

[HotBluePlates]

I found some more info about LED's by Valve Wizard.

He does acknowledge some tonal differences used in a cathode of a triode, but doesn't clarify what those are?

"LED bias. Obviously this has the same effect as a bias resistor with a perfect bypass capacitor, so carries tonal considerations."

If you previously used a bypass cap to trim/set the bass response of the gain stage, that tone shaping is out the window. The stage will have full gain down to 0Hz. So you'll need to trim bass some other way (like a coupling cap adjustment, or some other tone circuit).

On an LED used in a cathode setting, which leg goes to ground, please? 

It is the side with the "-" on the schematic, which is usually the leg next to a a flat side on many LEDs (look at the packaging and/or data sheet to be sure).

This is the same as a rectifier diode: The "arrow" side is at the most-positive voltage, and points towards the "-" and the most-negative voltage.

I am thinking maybe an LED paralleling a cathode resistor and cap would sound good?

Try it for yourself to know for sure. I'm thinking the LED will swamp the action of the resistor/cap, and the stage will sound exactly the same as with the LED alone.

[Fresh_Start]

The Ax-84 Blues Preamp uses parallel triodes with red and green LEDs for tone stack recovery:

http://ax84.com/static/corepreamps/Blues/AX84_Blues_Preamp_Schematic.pdf

Note that this stage comes after all tone shaping.

There was a discussion on Ax-84 about the effects of various LED colors ( read "voltage drop"), but it was a long time ago.

One technical point which may or may not have a tonal influence is that an LED on the cathode establishes a fixed bias for that triode. With a resistor, the bias varies depending on how much current is passing through the cathode resistor. That variation may not be a big enough swing to make any difference we can hear though. I just wonder if there might be some effect on touch response and/or compression compared to a typical resistor biased triode.

Cheers,

Chip

[Willabe]

One technical point which may or may not have a tonal influence is that an LED on the cathode establishes a fixed bias for that triode. With a resistor, the bias varies depending on how much current is passing through the cathode resistor. That variation may not be a big enough swing to make any difference we can hear though. I just wonder if there might be some effect on touch response and/or compression compared to a typical resistor biased triode.

Very good question.


                 Brad     

[sluckey]

Are you describing zener diode type effects?

No he's not. Zeners are most commonly used in a reverse bias situation. He's talking about the constant forward bias voltage drop of a LED, which is typically 1.5 to 3.5 volts, depending on color for one thing. All diodes have a constant forward voltage that must be met or exceeded before they will conduct. Silicon diodes are typically .6 or .7, depending on which book you reference.

[sluckey]

With a resistor, the bias varies depending on how much current is passing through the cathode resistor. That variation may not be a big enough swing to make any difference we can hear though.

That's true. The effect is local negative feedback, AKA degenerative feedback and generally provides less distortion, higher fidelity, etc. But as soon as you put a big bypass cap across that resistor, the ac signal component is filtered out, leaving a pretty clean and constant dc voltage (just like the B+ filter caps).

I'm gonna wait to get tubenit's take on sound differences. I trust his ears and he's pretty good at describing what he hears.

[HotBluePlates]

The Ax-84 Blues Preamp uses parallel triodes with red and green LEDs for tone stack recovery:
...
There was a discussion on Ax-84 about the effects of various LED colors ( read "voltage drop"), but it was a long time ago.

One technical point ... is that an LED on the cathode establishes a fixed bias for that triode. ...

If you take two parallel 12AX7 stages, bias one to 0.5v and the other to 1.5v (with essentially the same plate-to-cathode voltage), will they sound different? Yes, because they are biased to different operating points, and therefore amplify differently and run into a different set of limitations.

You might land on those different bias points by choosing different resistor values (and providing a suitable bypass cap), deriving a fixed d.c. voltage from some source, or you could use LEDs with differing forward voltage drops.

The magic then is in the selection of the bias point, not the method of biasing the tube to that point. I can only say that because there won't be a signal-dependent change in bias when using the LED (it's truly "fixed") the way there can be with local feedback across a cathode resistor.

[HotBluePlates]

... I also remember Geezer built an overdrive pedal with different LED's in it and liked it. ...

To make sure I don't assume something not true, please point to Geezer's description/schematic of his pedal. I am thinking there is a very different way LEDs are being used in it to create distortion (and differing distortions for differing LEDs).

[tubenit]

To make sure I don't assume something not true, please point to Geezer's description/schematic of his pedal. I am thinking there is a very different way LEDs are being used in it to create distortion (and differing distortions for differing LEDs).

Sorry, it was in a personal email that was deleted some time ago. 

Guys, I appreciate all the responses.  Will probably try this out soon and let you know what I think. 

With respect, Tubenit

[Geezer]

... I also remember Geezer built an overdrive pedal with different LED's in it and liked it. ...

To make sure I don't assume something not true, please point to Geezer's description/schematic of his pedal. I am thinking there is a very different way LEDs are being used in it to create distortion (and differing distortions for differing LEDs).

Very different.....used as clipping diodes in a stompbox in place of silicon types (for different favors of clipping distortion)

G

[HotBluePlates]

...used as clipping diodes in a stompbox in place of silicon types (for different favors of clipping distortion)

In the feedback loop of an opamp, like a tubescreamer? Or as anti-phase (i.e., opposite-direction) diodes to clip the top & bottom of a signal?

[Geezer]

Both.....of course, most designs are in the opamp feedback loop, but some clip the signal later in the circuit, and some use both methods

I have steered away from opamps and diode clipping lately, opting for the more pleasing JFET generated overdrive....MUCH more tube-like, both in tone and in circuit operation & "tweakability". The Himmelstrutz Fetto is my absolute favorite (I've built two of them, and have plans for a third)

G

[jojokeo]

Both.....of course, most designs are in the opamp feedback loop, but some clip the signal later in the circuit, and some use both methods

I have steered away from opamps and diode clipping lately, opting for the more pleasing JFET generated overdrive....MUCH more tube-like, both in tone and in circuit operation & "tweakability".

I also like mosfets... germ trannies have been fun too but that's a horse of a different color. :)

The magic then is in the selection of the bias point, not the method of biasing the tube to that point. I can only say that because there won't be a signal-dependent change in bias when using the LED (it's truly "fixed") the way there can be with local feedback across a cathode resistor.

This pretty much sums it all up in practical application and in reality.

You see in a blues amp circuit from a ways back that utilizes an LED or two in place of standard resistor/cap situations and they provide a saturated & fully bypassed type of flavour to your signal, hence nice for higher gain/blues-amp situations like what Fresh_Start mentions. But, you won't get Fender cleans from an amp or part of the circuit that uses this method. Therefore limitations but that's why a switching option is a good idea.

The "magic" in the Japanese DS-1's of old has a lot to do with the biasing point of the clipping mentioned above that gives them their characteristic feel that's so desired by many and why they go for so much money. There's also a few other things to do to modify modern/current pedals but when done you pretty can't tell the difference or you can tailor it to your own desires.

I'm getting off subject, but when using LED's or diodes (or both, *hint*), you can get a nice payoff where you otherwise wouldn't by going with only one or the other. I've used this (clipping/anti-phase) approach in a smaller practice-type of amp with great results but in another and not so much and was removed but I ended up going in a different direction entirely and had VVR so it wasn't needed anyway. As they say YMMV & I think it's one of those things where you definitely need to experiment to find your own sweet spot for your own situation.