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100.2.133.74
in a tube LTP design to replace the cathode resistor. The specs says it is good up to 100kHz. But I am not sure I can use it in a high end design.
Edits: 04/08/25Follow Ups:
Would the Morgan Jones bipolar design be appropriate here? I would never use the LM317; even SPICE simulations indicate large increases in distortion for applications like this.
Yes you can. I used a modified version as a tail impedance for a 6SN7 LTP in the input stage of my mono-blocks. However, if you are passing more than 10mA a simple cascode 8N100D2 will work very well and is easier to implement. The change I made to the MJ's BJT CCS was to replace the resistor that supplied current to the bias string for the BJTs with a DN2530/LM334 (temp. com. @2mA) CCS. Replacing the resistor with a current source was suggested by Walt Jung in an article as a way to enhance performance for this CCS.
Leo, for some reason wants to use a -100V neg. PS. With BJTs the top transistor will be required to handle more voltage than the 30V BC549C. The higher voltage device will have lower current gain which will decrease the impedance and performance of the current source.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I've tried numerous active loads and devices in the cathode. In the end nothing sounded as natural and organic as a good resistor.
Don't ignore a simple resistor. Configure it appropriately for a LTP, maybe with a negative voltage if you prefer that.
I've been going back and forth with this guy and he's reluctant to use a neg.PS and only has 7V to work with. That would be the "shortest" tailed pair ever and wouldn't work.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
In Reply to: RE: Pros and Cons of 300B PSE and PP posted by JKT on April 13, 2025 at 06:18:17:
Another thought. I can do a -100V negative regulated PS. 100V on CCS. 350V on 6SN7 for inverter stage. 200V on 6J5 for gain stage. It has no issue to output 10V peak to peak to drive 6SN7.
A cascode MOSFET CCS using 8N100D2s only requires 30V across it. With 100V a resistor could work (not as well); but you would have to calculate the difference in plate loads to achieve equal amplitude outputs. With a very high impedance CCS plate load resistors are matched.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
CCS + resistor = problem solved.
Configured as a CCS, they will limit the current inrush into a cold heater and you can set them up so that the tube gets the 6.3 volts it needs.
Agree with all of the below. The late Allen Wright originally used an LM317 in his CCS sold as a kit. He later became completely disenchanted with the sound, and he attributed the negative results to the LM317. He informed owners of his kit to remove the part or bypass it. Obviously, this is hearsay.
LM317 is a 1.5A device. Its performance at 15mA is quite poor. See the above figure. LM317L is a 100mA device. It should be better t 15mA. It has a 65dB ripple rejection at 100kHz which is similar to 136k Ohm impedance.
A cascode made from 8N100D2 MOSFETs will have an impedance in the audio band greater than 50Mohm. The impedance does decrease at 3dB/octave due to device capacitance.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
As an alternative you could use a 2 terminal JFET CCS rated for your combined bias current. It would be quieter than a LM-317.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
For low minimum voltage drop, LM134/234/334 is an interesting option. Minimum voltage drop ca. 0.6v. Not the greatest high-frequency performance, but still > 1meg dynamic impedance at 10kHz. One resistor makes a 2-terminal current source, no need to adjust the current-set resistor. I have even cascoded an IXCP10M45S with it - worked fine.
After I created the thread, I found this. My original thought was to use 20V bias.
Edits: 04/09/25
With just 7V to work with, a cascode MOSFET CCS would require a negative PS. What's the bias current? For current less than 10mA I would recommend a cascode NPN BJT CCS. That too would entail a negative PS. My system has 16 long tail pairs; so, I have some experience with CCS tails. IME the higher impedance cascode CCSs are sonically superior.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
LM317L should work well with 6SN7.
It will work; just not as well. If you don't want to deal with a negative PS go for it. I have used cascode MOSFET CCS with 6SN7 diff. pairs to good effect.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Walt Jung published an article in AudioXpress on Current Sources. One of the circuits was a cascode based on the LM-317. Dave Davenport of Raleigh Audio implemented it in his Line Stage kit as a plate load. At the time I had one of his Line Stages and I tried it. It caused a noticeable amount of White Noise. This was due to the LM-317. I don't know how much this would effect its use as a tail impedance; but, as a plate load, unacceptable.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
I think you would be better off with depletion mode mosfets.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I would recommend IXYS 8N100D2 in cascode (if you can source them).
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
The 500V variant works quite well also. I survived long enough with 400V 2540N5's to think the extra 100V d-s tolerance is a fine improvement. Still like 3545N3 for the low voltage element.
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Mouser has them back in stock, about 850. Get them before the tariffs hit.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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