Boostrapping?

[gingertube]

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

 

[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??

_-_-bear

 

[SY]

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?

 

[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?

 

[gingertube]

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

 

[bear]

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...

_-_-bear

 

[SY]

Re: Response to above

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.

 

[EC8010]

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..

 

[bear]

Wow...


Look at that! A cat that knows audio!!

Smart cat.

Nice kitty...


_-_-bear