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Old 8th November 2004, 01:08 AM   #1
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Default Boostrapping?

I'm modifying my 4 x KT88 Push Pull prototype at the moment.

I'm changing the driver stage to 12AU7 cathode follower to direct couple drive the output tube grids.

It's been suggested to me that I also use the cathode follower to bootstrap the previous common cathode stage.

Do I do this the same way you would a solid state circuit - that is split the anode load in half and capacitively couple the cathode of the follower to this junction? - Is the a better/alternative way?

Any links to example circuits/opinions/theories etc. gratefully received.

Cheers,
Ian
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Old 8th November 2004, 02:50 PM   #2
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Are you looking to increase the swing/gain of the previous stage so that you have enough gain/swing to fully drive the ouput stage with the follower??

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Old 8th November 2004, 02:57 PM   #3
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Quote:
Originally posted by bear
Are you looking to increase the swing/gain of the previous stage so that you have enough gain/swing to fully drive the ouput stage with the follower??
That's the old Mac trick you're thinking of?
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Old 8th November 2004, 09:17 PM   #4
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It made sense for McIntosh to bootstrap because they needed the swing, but it compromises stability, so I'd only do it for a good reason. Do you have a good reason?
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Old 9th November 2004, 02:40 AM   #5
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Default Response to above

No - I don't need the signal swing. It was suggested to me that I would achieve better linearity. I also hoped to learn something - and I have, don't do it unless its necessary.
Thanks,
Ian
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Old 9th November 2004, 03:28 PM   #6
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Quote:
Originally posted by EC8010
It made sense for McIntosh to bootstrap because they needed the swing, but it compromises stability, so I'd only do it for a good reason. Do you have a good reason?

Yeee Ha!

Positive feedback compromising stability? Hee hee!

But it was a great trick, try to follow the bouncin' ball for that schematic...

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Old 9th November 2004, 06:46 PM   #7
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Default Re: Response to above

Quote:
Originally posted by gingertube
No - I don't need the signal swing. It was suggested to me that I would achieve better linearity.
Just the opposite- you trade off distortion for swing and gain. The opposite of NFB.
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Old 9th November 2004, 11:29 PM   #8
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Just so's you don't get the wrong idea, bootstrapping doesn't always reduce linearity. A mu-follower is an example of bootstrapping where the cathode follower at the top bootstraps its load resistor, making that same load resistor look larger to the lower valve, reducing distortion of the lower valve, and because most of the stage's distortion is produced by the lower valve, bootstrapping reduces total distortion..
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Old 12th November 2004, 03:46 PM   #9
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Wow...


Look at that! A cat that knows audio!!

Smart cat.

Nice kitty...


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