LED bias the first cathode, direct couple to the second stage CF, use CCS loading of each stage. You'll be able to get by with a much less sophisticated power supply, drop distortion and noise, and lose an RC time constant. I'd use a 10mA CCS for each stage, green (2V) LED.
So it's only to change the green Led for both cathodes, CCS 20mA for both tubes, I have several quesions tha I don't have clear:
1. Output I suppose is for the anode 2nd tube stage 150R in series with 2u2 & 1Meg to gnd right?
2. B+ 1st tube anode 250V, B+ for 2nd tube anode?
3. Interstage is like the cathode follower that I posted so: 0.1uF + 1k in series between anode 1st tube & grid 2nd tube & between cap & resistor one 1Meg resistor to GND?
No. The CCSs used in the second and third stage (those are the ones to emulate) are 10mA each. The green LED substitutes for the red in the second stage (which will now be the first stage of the line amp). A 250V supply for both stages will work fine- I used a pretty simple regulator which is all you need. A fancy reg won't hurt, but it won't bring any advantages to the table; the CCS usage makes the power supply rejection quite good.
Yes, input at junction of R10 and R11. B+ to the same point I show in the schematic. You want to bypass the B+ to ground locally as I've shown- ECC88 have a tendency to oscillate if the bypassing isn't right. The neon NE2 is there just to protect the tube during startup- you can use a reverse-biased 1N4007 if neons are hard to get. All transistors are DN2540 (cheap).
You got it!
In this circuit, you'll see less than 0.03% THD (mostly second harmonic) at 5VRMS out. Output impedance will be under 500 ohms.
Make sure the 1k resistors are tight against the grid pins of the tubes- they are necessary to ensure stability (like I said, ECC88 have a tendency to oscillate if you don't cater to their whims).
In this circuit, you'll see less than 0.03% THD (mostly second harmonic) at 5VRMS out. Output impedance will be under 500 ohms.
Make sure the 1k resistors are tight against the grid pins of the tubes- they are necessary to ensure stability (like I said, ECC88 have a tendency to oscillate if you don't cater to their whims).
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Nope. You left out the CCS on the plate of the first stage again. Take a closer look at my schematic.
I would recommend against a resistor instead of the neon tube (or reverse biased diode). With direct coupling and the CCS, the tube self-biases- why upset that? like I said, the neon (or diode) only function at turn on- once the tube warms up for a few seconds, they're out of the circuit.
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