it may well reduce the output impedance but that is not the same thing at all.Īpologises for 'teaching my grandmother to suck eggs." but not everyone understands this enough when they examine a circuit. It wont drive 600 ohms without a transformer. so the use of CF configuartion or feedback (local or global) doesn't give the stage anymore ability to drive current into a load. The 6SN7 cathode follower should, if properly designed, handle a decent-sized signal (5-6Vac) into 5k or higher. Fedup with it thinking to move to cathode follower for preamp application. This is clearly seen by looking at the V/I curves (plate curves) of the valve at the chosen operating point and then plotting where the input signal moves it from that operating point. 1 Ive been trying to fix mufollower with 6sn7 always some sort of hum and little hf noise. One thing people need to understand is that regardless of configuration or feedback a valve cannot deliver current into or volts accross a load that goes outside of that permtted by the valves intrinsic V/I regime. Run the beastie with high curent and it will sound good. So it goes: CCS loaded parallel 6SN7 gain stage > direct coupled to parallel 6AS7G cathode follower output stage. like run the input at low Av to reduce the Millar capacitance and load it with a choke loaded CF second stage. Of course it probably sounds OK in an all valve system where it isn't asked to drive long cables or low input impedance but then why use this design as there are recognised much simpler ways to get good sound out of a 6SN7. I have included three schematics based around a 6SN7 the tube I would like to use. If you use impedance coupling then the CF doesn't load the driver except with many megohms and there is virtually no rotation of the AC load line in the. in short its a horrible application for a 6SN7. Frankly I would never direct couple to a cathode follower, unless there were some bias issue I was trying to work around, because it messes with the bootstrapped input impedance of the CF. It really doesn't use the 6SN7s stregths (exceptional linearity when lightly loaded but run hot) and walks into its weaknesses (high Rp, relatively high input capacitance, poor current drive capability) the feedback doesn't change the V/I regime the valve is operating under and just exposes it to more current demands that it cannot meet so adding to the TID that the feedback introduces. I've been trying not to comment on the design as I think it is horrible. Circuit design is configured with 12AU76SN7 x2 for amplification and 6SN7 x2 (cathode follower) for the driver stage to powerfully drive the large-scaled. It's in some of the 40s and 50s textbooks too - so it has a certain pedigree that runs from before the 80s revival diy scene. ![]() This requires that the grid is at the dc-potential determined by voltage divider 5k6 and 2k7, not at ground potential. The bias voltage is generated with the 5k6 resistor at the cathode. JRB defines the meaning of plate (anode) follower and why it has been used. There is definitely an error in the drawing concerning the dc-blocking capacitor at the grid of 6SN7 cathode follower.
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