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ebay 6J1 valve pre-amp - circuit diagram/gain
ebay 6J1 valve pre-amp - circuit diagram/gain
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Old 8th December 2017, 11:05 AM   #11
DF96 is offline DF96  England
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To turn this into a cathode follower you need to move the 4.7k anode resistor to the cathode circuit. Put it between R14 and -28V, so the valve still gets its bias from R14. Take the output from the cathode.
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Old 8th December 2017, 11:34 AM   #12
Vix is offline Vix  Yugoslavia
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I have bought exactly this circuit and it sounds disapointing. High output impedance and a lot of distortion if driven harder. So, before simply throwing it to a trashbin, I wanted to try to convert it to a buffer. I am not ready to waste a lot of time on it, so a couple of quick modifications will do, but no more. I already replaced those yellow input/output caps with better ones. Also, I reconnected a 470k resistor as it was not referencing the grid to ground, but to -24v supply (it is powered with +-24v supply via voltage doubler from 12v ac).
So, I guess tha I have to bypass that 4.7 k anode resistor (connect anode directly to +24v supply) but then I am wondering about the value of cathode resistor (200 ohms) on that schematic? What should be its new value?
Of course, output will be taken from the cathode...
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Old 8th December 2017, 12:38 PM   #13
DF96 is offline DF96  England
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No point in using "better" caps in a poor circuit. The existing circuit is correct in referencing the grid to -24/28V, but the cathode follower version should not.

As I said, the 4.7k anode resistor should be put in the cathode circuit. The 200 ohm resistor stays, as that provides bias.
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Old 8th December 2017, 12:53 PM   #14
Vix is offline Vix  Yugoslavia
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Referring to the schematic in post #3, then it turns out that 4.7 k resistor gets connected in series with tha 200 ohm one? (Or, can 200 ohm resistor be removed, 4.7 or 4.9 k shouldn't make a difference)...
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Old 8th December 2017, 01:54 PM   #15
DF96 is offline DF96  England
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Yes, 4.7k in series with 200. The grid resistor goes to the junction.
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Old 8th December 2017, 02:05 PM   #16
Vix is offline Vix  Yugoslavia
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Thanks!
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Old 8th December 2017, 06:50 PM   #17
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Vix View Post
Also, I reconnected a 470k resistor as it was not referencing the grid to ground, but to -24v supply .
If you do that the current in the tube becomes very low, not good.It is the 100k at the output that need the change.
The power supply treats the transformer very badly, single phase rectifier for the heaters
Here are the originals with 100k mod and two alternative supplys.
Also a very simple buffer mod
Mona
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