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Zian 23rd May 2012 03:08 PM

Bootstrapped long tail
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Hi, it's my first post here. It is a pleliminary design (values) but i'm looking for advice or contraindication. I'm thinking to replace Tail by CCS, the output stage will be 2 6550 for a 70W ultralinear and the k follower will be present anyway, but i try to have max gain and low output impedance with the 6SN7 and feedback including just this stage and output.
Thank you in advance

Osvaldo de Banfield 23rd May 2012 03:48 PM

The idea is very good, but best results will get using 6SL7 (more mu triode than 6SN7), and better is to use a constant current source at the cathode joint. But your circuit will work. Keep in mind to maintain the anode capacitances as low as you can, because bootstrapping increases the impedance of such node, and then capacitances shunts high frequencies more quickly than usual resistor load only.

Finally, I donīt like NFB returning to grid and cathode simultaneously, I would do only at the grid of second half of the LTP. (Itīs only my opinion)

Good luck.

jerluwoo 23rd May 2012 08:49 PM

Just ground the cap from the the grid of the second half ltp triode. Remove the 47k resistors in series with the diodes.

DF96 23rd May 2012 10:00 PM

If that cap was grounded then the feedback becomes pure common-mode into a differential-mode stage - it won't work. As it is it will work, although I would not do it that way; I would use the second LTP grid for feedback as Osvaldo said.

dgta 24th May 2012 05:56 AM

I don't think that circuit will drive 6550s to full output from a line level source, if any significant amount of feedback is used. I would use 6SL7 in the first stage as Osvaldo said.

Actually, I wouldn't use this topology at all, I don't see the point of the CF.

Zian 24th May 2012 10:29 PM

Oh well. To be honest I thought a 12AT7, the 6SL7 didn't come to mind. Regarding the CF I wanted some more choices, i began to think with a 12ax7 as ltp...but probably i'll use CF anyway, better more than less.
By the way, before this i'm going to build a preamp with bass and treble control(my friend asked), levels or topology are to be defined yet
Thank you again

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