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Suggestion for a tube buffer
Suggestion for a tube buffer
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Old 8th January 2018, 07:49 PM   #11
Koifarm is offline Koifarm  Netherlands
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Quote:
Originally Posted by indaco View Post
A last question, somewhere I read another way to raise the voltage...at the secondary output of the first transformer another transformer, set reversed so 12V->220 , was connected to get the desired voltage. Is this a good idea? It seems you may risk to burn something...

I read it here: Tube buffer
I used it to, read below.

6N16 12x preamp srpp/cathode follower with printed circuit board design.
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Old 8th January 2018, 08:08 PM   #12
stocktrader200 is offline stocktrader200  Canada
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i am getting 155v from a voltage doubler hung off the 50v dc supply and using 32v winding from sx-780.
the 155v is regulated down to 140v for noise free B+.
I am using 10uf / 250v caps for the AC connection. The output sags about 5v (150v) with a 20ma load for 6922 preamp.

inversing the transformer works well as long as you use equal or less input voltage and keep the load at or below the transformer VA rating. ( watts of load)

Last edited by stocktrader200; 8th January 2018 at 08:28 PM.
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Old 8th January 2018, 08:09 PM   #13
GoatGuy is offline GoatGuy  United States
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Not really. Consider the attached diagram. Better mathematicians than me have demonstrated that the input impedance can be VERY high with cathode-followers. I prefer them more controlled, as indicated. With a 330 kohm bias-to-grid ballast resistor, the input impedance will be REQ (51 Kohm) + 330 khom or about 380 Kohm. High enough.

gg
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Old 8th January 2018, 09:03 PM   #14
indaco is online now indaco
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Ah ok. Never seen it before, for the signature I suppose is your invention So, it is certainly worth trying (as other here confirm that works well for the purpose)
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Old 8th January 2018, 09:29 PM   #15
indaco is online now indaco
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Quote:
Originally Posted by Koifarm View Post
Yes, I took a look. It's a good PSU topology, with double PI filter at the output...seems well regulated.
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Old 9th January 2018, 09:02 AM   #16
DF96 is offline DF96  England
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Quote:
Originally Posted by GoatGuy
Consider the attached diagram.
The diagram says medium to high mu triode. For good performance as a cathode follower you need high gm, not high mu. For use with lowish supply rails and lowish value load resistances (as seen in that circuit) you need lowish mu.

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With a 330 kohm bias-to-grid ballast resistor, the input impedance will be REQ (51 Kohm) + 330 khom or about 380 Kohm.
Where does the 51k come from?
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Old 9th January 2018, 09:50 AM   #17
GoatGuy is offline GoatGuy  United States
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Quote:
Originally Posted by DF96 View Post
The diagram says medium to high mu triode. For good performance as a cathode follower you need high gm, not high mu. For use with lowish supply rails and lowish value load resistances (as seen in that circuit) you need lowish mu.


Where does the 51k come from?
Ah… mu and gm. They're quite different. Yet also related 'sort of'. But you are right: high gm is needed. Thanks.

51 kΩ come from? 220 khom || 68 khom. Thevenin equivalent resistance of a resistive voltage divider voltage source. For others (I hardly need remind you, esteemed one!), a voltage divider for all intents and purposes acts like the divided voltage source in series with an equivalent resistor. Good ol' Dr. Thevenin came up with that. Useful abstraction.

GoatGuy
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Old 9th January 2018, 12:05 PM   #18
DF96 is offline DF96  England
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Your diagram shows a decoupling cap at the junction of the 220k and 68k. Assuming an appropriate value for this, the 51k only applies at DC and subsonic frequencies.
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Old 9th January 2018, 12:14 PM   #19
Eli Duttman is offline Eli Duttman  United States
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Quote:
The diagram says medium to high mu triode. For good performance as a cathode follower you need high gm, not high mu. For use with lowish supply rails and lowish value load resistances (as seen in that circuit) you need lowish mu.
Biipolar PSU and ECC86/6GM8, with (possibly) CCS loading of the CFs?
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Old 9th January 2018, 12:56 PM   #20
kodabmx is offline kodabmx  Canada
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I use this design myself. Active loaded CF Buffer. Adjust the 1u cap to suit your load and desired LF cutoff.
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Last edited by kodabmx; 9th January 2018 at 12:58 PM.
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