Using diode for biasing

[bocka]

The source impedance of a cascode CCS is likely to be 10M or higher; Jung's measurements of the MOSFET cascode that I use show 100M+. For a 20V swing at the plate, then, delta I = 0.2uA. For a 5R dynamic resistance LED, that's an AC voltage of 0.2uV across the LED. For that 20V swing in a high mu tube like a 12AX7, the input voltage is 200mV. So the impressed AC voltage (not the distortion!) from the LED is 1ppm or 0.0001%. So even if the nonlinearity of the LED's dynamic resistance were 100% (it's nothing even vaguely close to that), the contribution of the LED is -120dB! For a more realistic nonlinearity, we're talking more than 140dB down, or 1/20,000 of the distortion contribution of the tube.

This is true only in a static case. For ac frequencies a CCS or the input of the following state provides some capacitance (as well as internal capacitance Cag) and so you have an impedance at the anode which is much smaller than 100Meg. As none of these capacitors are referenced the the cathode of the tube (anode of the LED) you have some current variations on the LED. Assuming 100pF effective load capacitance you have an impedance of 318kohms at 5kHz. For the given value of 20V ac on the plate this results in 62uA current swing on the LED. Not really much but this shows you do better run the LED on higher currents and not with 1ma or so. In this case LED biasing an ECC83 or so is not such a good idea or the ac voltage on the plate has to be quite low.

 

[SY]

For ac frequencies a CCS or the input of the following state provides some capacitance (as well as internal capacitance Cag) and so you have an impedance at the anode which is much smaller than 100Meg.

Did you see Walt Jung's measurement of impedance vs. frequency (AX 4/09 p40)? It's nothing like 100pF. If your concern is what happens at 10MHz, I suppose you could insert a small inductor, but for sub-1MHz stuff, the MOSFET cascode CCS has an impedance many orders of magnitude higher than the plate resistance.

As an exercise in scaling, what's the change in gain for a 12AX7 loaded with 1M versus 100M?

 

[regal]

As an exercise in scaling, what's the change in gain for a 12AX7 loaded with 1M versus 100M?

Av = (mu * Rp)/(Rp + ra)

Av1= 94
Av2= 99.9

 

[bocka]

Did you see Walt Jung's measurement of impedance vs. frequency (AX 4/09 p40)? It's nothing like 100pF.

When you have an interstage tube which is loaded by another tube the following stage loads the first stage. Wenn you have an ECC83 (u = 100, Cag = 1.6pF) the second stage has an effective input capacitange of (u + 1) * Cag = 160pF unless this tube is not cascoded. As this is somewhat overestimated (u=100 = gain assumption) 100pF load capacitance seems a feasible value to me, as the MOSFET capacitance is unconsidered.

This has nothing to do with the overall gain of the tube but this capacitance leads to some current deviation on the LED on typical audio frequencies.

 

[SY]

Av = (mu * Rp)/(Rp + ra)

Av1= 94
Av2= 99.9

0.5dB. That's livable.😀

 

[SY]

When you have an interstage tube which is loaded by another tube the following stage loads the first stage. Wenn you have an ECC83 (u = 100, Cag = 1.6pF) the second stage has an effective input capacitange of (u + 1) * Cag = 160pF unless this tube is not cascoded. As this is somewhat overestimated (u=100 = gain assumption) 100pF load capacitance seems a feasible value to me, as the MOSFET capacitance is unconsidered.

This has nothing to do with the overall gain of the tube but this capacitance leads to some current deviation on the LED on typical audio frequencies.

That's true if the following tube is a 12AX7 configured as an undegenerated common cathode stage, which is not exactly common nor sound design practice. By dropping the load on a 12AX7, the LED nonlinearity is the least of your worries- the variation in rp with current is very, very large. Look at the slope of rp versus Vg (or ip) on the datasheet.

You've put your finger on the reason (IMO) that the 12AX7 has gotten a bad rap for linearity- it absolutely MUST be buffered. The high and variable rp makes it a tricky tube for extracting high performance.