Just to throw a monkey wrench into the discussion, a LED's internal resistance isn't always a bad thing. It results in a bit of cathode feedback and though I don't understand the mechanism it's my experience a small amount of feedback has a much more significant and worthwhile impact on higher-order distortion components than on the dominant 2nd. The benefits are more obvious to a spectral analysis than a straight THD reading. To use an example, in one test with a CCS loaded 6C45 (I think) biased with two green LEDs, bypasssing the cathode with a 1000 uF capacitor raised the third harmonic distortion component 15-20 dB while having almost no effect on the 2nd.
Mine is too. I use a cheap red LED from RS $1.49 for 2. They bias a 6N23P at exactly 2V and about 15mA with 105V on the plate. I use this stage direct coupled to a 2A3. Sound is clean and clear.
The led cannot be applied to any vacuum tube stage.
The ac impedance of led can create problems because increase the output impedance of the stage so if on anode there is a choke or a transformer this cut the low frequency.
In my GM70 amplififier I have used this solution (led/optocoupled) with good result because the choke is 70H but in my 813 SE with the same input tube but an interstage transformer with 50H if I use the led I have a cut of at 20Hz at -1.5dB instead of 10 at -1dB.
The ac impedance of led can create problems because increase the output impedance of the stage so if on anode there is a choke or a transformer this cut the low frequency.
In my GM70 amplififier I have used this solution (led/optocoupled) with good result because the choke is 70H but in my 813 SE with the same input tube but an interstage transformer with 50H if I use the led I have a cut of at 20Hz at -1.5dB instead of 10 at -1dB.
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I am using green led and the vaccum tube is a D3a in triode connection with a voltage of 70x.
In the original circuit there is a RC on cathode 470uF and 100ohm.
Now my problem is find a capacitor with the sound of led.
1) Sanyo OS-CON
2) ELNA Cerafine
3) ELNA Silmic II
There are not very good.
In the original circuit there is a RC on cathode 470uF and 100ohm.
Now my problem is find a capacitor with the sound of led.
1) Sanyo OS-CON
2) ELNA Cerafine
3) ELNA Silmic II
There are not very good.
You might want to try this with a red, or possibly a different green. At (say) 10mA idle current for the D3a, an HLMP6000 (a tiny, inexpensive unit) will cause a rise of about 150R in the plate resistance, well within the tube tolerance. I haven't measured the green from the HLMP series, but you might give that a shot.
Let's look at the numbers. 70H plate choke? At 10 Hz, the inductive reactance is 4k4. For the 20mA you chose as plate current, plate resistance is about 2k for a grounded cathode. Mu is about 75. Gain = (4.4/6.4)*75 = 52. With a 5R resistance in the cathode circuit (typical for surplus LED, higher than HLMP), the plate resistance is 2k + (76*5) = 2380. So gain is then (4400/6780)*75 = 48.6. 20 log (48.6/52) = -0.6dB difference at 10 Hz between an LED and the cathode tied directly to ground. If you're seeing more difference than that, you've got something else going on besides the LED.
Let's look at this a different way: your original circuit had a 470u bypass cap. At 10 Hz, the cap has a reactance of 33 ohms. That's in parallel with a 100 ohm resistor and the looking-in impedance at the cathode (4k4/75 = 59R) for a total cathode impedance of 17.5 ohms, a higher number than the LEDs. Hmmmm.
Let's look at this a different way: your original circuit had a 470u bypass cap. At 10 Hz, the cap has a reactance of 33 ohms. That's in parallel with a 100 ohm resistor and the looking-in impedance at the cathode (4k4/75 = 59R) for a total cathode impedance of 17.5 ohms, a higher number than the LEDs. Hmmmm.
Like I said, it might be worthwhile to figure out why you see this; it may be enlightening (bad pun, sorry). You've got something odd going on and getting to the bottom of it will likely improve the design and give all of us a better predictive tool. Many of us have used LEDs for cathode biasing with great success and no frequency response or distortion anomalies.
It's also possible that you picked some really odd and poor LEDs- a measurement of AC impedance is easy (I give some details in the Red Light District article on my website) to confirm or refute this possibility. One fun thing to try is to put a series resistance in the cathode to simulate the higher impedance of the RC bias and see what happens to the frequency response. The 1dB or so midband gain difference is suggestive. Could it be that you used LEDs with a built-in series resistor?
It's also possible that you picked some really odd and poor LEDs- a measurement of AC impedance is easy (I give some details in the Red Light District article on my website) to confirm or refute this possibility. One fun thing to try is to put a series resistance in the cathode to simulate the higher impedance of the RC bias and see what happens to the frequency response. The 1dB or so midband gain difference is suggestive. Could it be that you used LEDs with a built-in series resistor?
In looking at the measured frequency responses it looks like you are getting peaking from the 470uf cathode cap.
The smooth LF roll off response with the LED shows that you don't have a resonance between the power supply final cap and the interstage transformer. When you swap from the LED to the RC cathode circuit the frequency response actually rises a bit before rolling off, and the roll off does not look like a simple single pole response. To verify the theory that the RC cathode circuit is causing a LF resonance you can move the resonance around by changing the value of the cathode capacitor.
The smooth LF roll off response with the LED shows that you don't have a resonance between the power supply final cap and the interstage transformer. When you swap from the LED to the RC cathode circuit the frequency response actually rises a bit before rolling off, and the roll off does not look like a simple single pole response. To verify the theory that the RC cathode circuit is causing a LF resonance you can move the resonance around by changing the value of the cathode capacitor.
The sharp corner of the roloff and 12dB slope sure does look like resonance as Gary suggested. My problem is that assuming a 2500 ohm Rp and 70 hy's the rolloff corner with the LED is still off by an octave. This suggests either a 5K Rp or only 35hy's of inductance (or possibly a combination of higher than expected Rp and lower than expected inductance.)
what was the signal level of the tests?
dave
what was the signal level of the tests?
dave
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