Cathode bias on Crack

[mrgoodsound]

Hi, I have a stupid question.

If one were to replace the LED bias on the Crack's 12AU7 with a traditional RC cathode bias, what would be the ideal cathode resistor and capacitor values? And what would be the technical pros and cons of doing so?

[Paul Birkeland]

The bias voltage is 1.5V and desired plate current is about 3.2mA.  V=I*R, so 1.5V=0.0032A*R, R would be 468.75 ohms (or 470).  You don't need to put a cap across these resistors, as the reduction in gain wouldn't be a bad thing.

The LEDs provide just about a perfect 1.57V of bias with very low dynamic AC impedance and quiet operation, which is why we use them. 

[Drew1d]

The LED is what kinda blew my mind when I saw the kit.  I never encountered it in the Guitar tube amp world. (Guitar amps are more mid-fi, than hi-fi)  So, the current should kinda go both ways, but a diode is a one way deal....crazy.  I guess the cathode is negative, but still, the signal is AC.  It's hard to picture in my mind.  So, I know AC and DC can exist on the same circuit, but I can't picture it in my mind yet.  I mean, the AC can "ride" on top of DC.  And I guess that's what "elevating" the ground is.  I'm still learning.

And then add that it is the bias of the tube and   

I've tried Fixed Bias on power tubes.  (applying voltage/current to the input)  I've tried grid leak bias, where the cathode is ground and the input grid leak resistor is megs.  Also, and most commonly, I've tried cathode bias on preamps where you put a resistor to ground on the cathode. ("bottom" of the tube)  But a diode?  a one way faucet....  I had to buy the Crack after seeing that...  Also my HD650s had no bass, I mean...that too....

I mean, I wouldn't call the Crack 12AU7 a preamp, I know the cathode follower is more a buffer or unity gain.  You still need a good voltage signal in.

With a resistor vs LED , I'm sure you can hook it up to a swtich to "pepsi challenge" it.  And I've thought of doing it.  It's just the Crack sounds good,  And I'd need oodles of time to do it in a classy way.  Good luck and report back....

[Paul Birkeland]

Here is the voltage vs. current curve for the HLMP-6000 (it's on the standard LED line).

If we have 3.2mA of quiescent current flowing through the LED and maybe our AC signal changes that by +/-1mA, you are moving up and down on the portion of that line that's nearly vertical, and so the voltage at the cathode remains nearly constant (I suspect there is some wiggle in the thousands digit if you looked closely enough). 

Remember that a triode is also a one way device too   There's no PNP vacuum tube (yet).

[Drew1d]

I'm not sure if the LED biasing the input is the best representation of the signal.  I can understand wanting to be able to "center bias" the input tube to get some desired effect.  That's the only reason I'd think you'd want to change the mechanism to bias the input tube. 

But the next level of this circuit must be in maintaining voltage on the anode.  I'm sure the speedball has more to do with that voltage.  It would seem keeping that more stable would keep the output more..."true."

But is that what you really want?  I would think that accurate representation of sound is only secondary to pleasing sound. 
The beauty of the Crack is in it's simplicity.  Even if it may distort certain frequencies, it does so in a matter pleasing to the ear.  To change that would take away from it's character.

Paul. I wonder if you mean by a "one way device" if you mean that a tube can only deliver + voltage to the next stage.  It's strange to me.  I'm sure that it's something to do with it's reference.  Like, the center point of a guitar pickup is like, 0, and it gives a positive and negative signal as the string moves back and forth over the pickup.  And then it amplifies and rides on larger B= which makes a greater range of high + voltage and lower + voltage as it goes through the first preamp stage.  The range is still there, it's just "shifted" up higher.  It's hard for me to explain because I don't understand fully.  I feel like I need to prove that I understand that a tube is a one way device or something...

I read this book by Musahi Miyamoto, and he said, you can't teach anyone the sword, they have to learn it for themselves.   I feel the same way about electricity.  Not that it acts in a different way for people, but it "clicks" for everyone differently.  That understanding is somewhat personal. 

Paul, You understand electricity and circuits in a deep way. Obviously, your knowledge is valuable to troubleshoot peoples problems with audio circuits.  If you really want to understand audio circuits, headphones/speakers and training your ear for the differences, how would you do that?  Is it a book or concept that would facilitate that? I think the spirit of trying to change the bias, is trying to shift on how something is listened too.  Perhaps it's Jermaine to question...or not.

In your experience, what does altering the bias do?  I feel that the crack has been transformative in my listening to music.  I pay attention more, perhaps I hear more clearly all the different elements in music.  I don't know, I'm still trying to figure it out for myself.

Is changing the cathode bias of the 12au7 a good way to change the character of the amp?  or is it a fools errand?

[Paul Birkeland]

Indeed current does only flow in one direction in these devices.  Our signal applied to a tube simply modulates that current to be more or less than it is at idle.  The tube vs. transistor comment was more about tubes only being controllable like NPN devices. 

The LED bias is really a lot like using battery bias on the grid. 

Using cathode bias resistor and plate loading resistors will provide a very stable plate voltage on the 12AU7 that will resist changing as the 12AU7 wears down.  The cathode resistors will also throw away some (unwanted) gain if you don't bypass them.  If you do bypass them, then you have to figure out where to put a pair of 220uF+ caps in. 

Accurate representation vs. pleasing sound for us depends a whole lot on what you have to do to get the accurate sound and whether that perturbs the pleasing sound.  There are mechanisms we could apply to our amplifiers to dramatically lower THD that we avoid because we don't like the resulting sound, but active loads certainly aren't on that list.