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AKSA 29th December 2003 05:10 AM

Unity Gain Tube Buffer
I wish to make a unity gain tube buffer, preferably cascaded two stage, which will handle up to 100Vpp input/output.

To keep input impedance high, it would be better to use feedback injection from the output of the second stage to the cathode of the first stage .

If OLG of the two stages were around 100 (40dB), then a feedback ratio of this size would give vanishingly low Zout and distortion. However, devising an input stage which will handle 100Vpp is no slouch.

My thinking would be that a differential input using medium mu tubes followed by a larger tube running say 15mA for output might be best.

Does anyone have any experience/thoughts on this topology?



fdegrove 29th December 2003 05:34 AM


It's been ages but I once used something similar as you describe using a 6DJ8 cascaded into a 6BX7 to drive a 211....

Details of which must be somewhere at the end of my little tape but it worked nicely.


Sch3mat1c 29th December 2003 06:32 AM

Err... wth you need 100V for... in any case... what you need two stages for, this screams CATHODE FOLLOWER.


Will 29th December 2003 06:45 AM

I'd be interested if someone can post a schematic on a 6922 tube buffer with constant current source. ;)

AKSA 29th December 2003 08:56 AM

Thanks Guys,

Frank, thank you for your thoughts. But a 6DJ8/6BX7 would not handle feedback down to unity gain, I don't believe so anyway.

Tim, no, I don't want a Cathode Follower, because the feedback is local, and I need something with interstage feedback, unless....

Could an anode follower do it? Anyone know how it might be configured with say a 300V B+ to input and output 100V with feedback around the stage?

Of course, this might mean a pentode like the 12GN7, but it might just be possible if it could be configured to avoid input overload. That's the tough one; input overload.



Sch3mat1c 29th December 2003 09:03 AM

wtf everyone got against local FB anyways

dhaen 29th December 2003 09:18 AM

Try English Tim...:scratch:

fscarpa58 29th December 2003 09:59 AM

why do not try
a 6DJ8 White Follower ?

low distortion, low output impedance
high Vpp


Sch3mat1c 29th December 2003 09:54 PM


Originally posted by dhaen
Try English Tim...:scratch:
No need. Point being a CF is the ideal circuit element to use here, I don't see any reason to add more distortion or complication by using two stages. Nor the 100V requirement.

Here's some :scratch: :scratch: :scratch: :scratch: :scratch: back at 'ya.


SY 29th December 2003 10:01 PM

Tim, Tim, Tim. It's much cooler if you use a complicated circuit with a fancy name, preferably one originally developed for a 1942 vintage table radio. Why do something simple when there's a hard way to do it? Besides which, a fancier circuit will have a few extra poles in it, which can lead to many happy hours trying to stabilize it. A CF is just no fun.

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