Here Ya Are.
Simulation shows -3 db is at ~180 KHz for 12B4 CC Loading
When R = Xc, if we know f we can solve for R
f = 1 / 2*PI*RC……………………….f ~ 0.16 / RC
F ~ 180 KHz and C is 200 pF
R ~ 0.15 / fC………………R ~ 0.16 * ( 0.2*10^9 ) / (0.18*10^6)
R ~ 0.16*1.25 (10^3……………….1.25K
The output impedance largely dependent of "lower" FET.
Samples:
https://www.diyaudio.com/community/threads/26-pre-amp.151421/page-221#post-5117183
Samples:
https://www.diyaudio.com/community/threads/26-pre-amp.151421/page-221#post-5117183
I guess you discuss source follower mode, not at the anode of the driver tube used, where the ccs should have an extremely high impedance and Zout of the circuit being close to the tubes Rp.
As you can see, the simulation of gyrator shows 109R impedance at 1kHz (105-109R at 10Hz..100kHz).
Read here too
https://www.diyaudio.com/community/threads/26-pre-amp.151421/page-211#post-5033415
https://www.diyaudio.com/community/threads/45-amp-build-direct-coupled.320418/page-6
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Thanks for the links Bela, you are right the lower fet used is 2SK3557 not BSH111BK (I was confused because I own four PCBs sorry), I do my maths:
Ale's formula:
Zout = (Rp+Rmu)/(1+Rmu*gfs)
Zout = (7600 (tube 26)+470) / (1+470x 24mS for 2SK3557)
Zout = 8070 / 11281 = 715R as Zout not far the 860R measured
Ale's formula:
Zout = (Rp+Rmu)/(1+Rmu*gfs)
Zout = (7600 (tube 26)+470) / (1+470x 24mS for 2SK3557)
Zout = 8070 / 11281 = 715R as Zout not far the 860R measured
I have an old commercial pre based on 12b4a and its a wet dog. Wouldn't mind improving on it . I can use plate choke or anode current source or both and tweak it a little. I don't want to hack the design completely.
Psu is a 83 valve plus RC string. Regulated 6.3V filament.
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Hi analog_sa,
Do you have an exemple (schematic) for such a dht+mosfet follower?
Sure. Seems like LTspice lost the 4-pin triode symbol, sorry about that, no time to fix it atm. The actual mosfets i use in the ccs are IXTP-U-Y01N100D. Tube is 10Y.
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I just finished a 6C4C line stage which is very interesting - exceptionally lean, clean and detailed. Just about the complete opposite of my 26 stage which is full and lush. It's different from anything I've heard or built. Intriguing. For what it does it's quite remarkable, but it isn't full and lush at all. Very good on classical music and piano, wide precise soundstage. Dynamic and effortless - it's an output tube after all. Low gain and 800 ohm output impedance. Svetlana quality tubes. I could probably put more current through it. KBG PIO output caps.
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If you want a line preamp with gain of less than 10X, and you don't want to use negative voltage feedback, then 6N30P and 6N6P will end up with too much gain (at least 12X, even with cathode degeneration). That's why there's interest in using power output tubes for a line stage. There are several on diyAudio including a 12B4A version, a 6V6-triode version, and several others. 4P1L-triode also fits this category, but I know andjevans has tried that and other similar things.