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-   -   Diff drive (http://www.diyaudio.com/forums/tubes-valves/134981-diff-drive.html)

bigwill 16th December 2008 04:38 PM

Diff drive
 
Came up with a simple idea today, wondered if it has ever been tried. The basic idea is having a differential driver (not a phase splitter) that can take feedback from the plate to linearise and lower the impedance of a pentode or tetrode (or further linearise a triode)

See image for the circuit I thought of

bigwill 16th December 2008 04:48 PM

Oops, assume the bottom triodes are upside down :D Forgot to make the cathodes look different

On another side note I'm amazed how inarticulate I feel today after being around the people I work with :xeye:

HollowState 16th December 2008 06:20 PM

Bigwill,
I assume that you wanted negative feedback to accomplish your goals. Following it through from tube to tube, it seems to me that you've got positive feedback as drawn. Perhaps cross coupling the feedback points would more efficacious.

Victor

bigwill 16th December 2008 06:33 PM

Quote:

Originally posted by HollowState
Bigwill,
I assume that you wanted negative feedback to accomplish your goals. Following it through from tube to tube, it seems to me that you've got positive feedback as drawn. Perhaps cross coupling the feedback points would more efficacious.

Victor


Hmm, are you sure?

Input signal goes to first grid, positive going signal lowers anode voltage on first valve, causing the output valve to switch off more thus also having a positive direction on ITS anode, which is then coupled to the other side of the differential, turning on that side more and turning off the driving side to cancel it out

Is this right?

HollowState 16th December 2008 06:44 PM

Following the phase:

plus in to 1st tube

neg out at plate

neg in at 2nd grid *

pos out at plate

pos in at output tube

neg out at plate

neg in at feedback grid

neg in at cathode *

same phase equals positive FB

bigwill 16th December 2008 07:01 PM

Damn, it seems you're correct :D

Back to the drawing board!

HollowState 16th December 2008 07:06 PM

The design is interesting, just cross couple the feedback and you'll be cool.

lineup 16th December 2008 07:06 PM

for a differential pair
we want the same phase on both inputs
to get negative corrective feedback

this will strive to keep the diff pair both input voltages in balance

so, i think the circuit is correct .. ??

Bandersnatch 16th December 2008 07:19 PM

Quote:

Originally posted by lineup
for a differential pair
we want the same phase on both inputs
to get negative corrective feedback

this will strive to keep the diff pair both input voltages in balance

so, i think the circuit is correct .. ??

That is how I read it.

I put a little arrow at each input and output. Up for increasing and Down for decreasing is simple enough...:)
cheers,
Douglas

bigwill 16th December 2008 07:35 PM

I think I've changed my mind, I'm pretty sure it's negative feedback


Starting at the diff pair


signal goes UP

left hand anode goes DOWN

causing OUTPUT tube anode to go UP

driving right hand of diff pair UP

causing ITs anode to go DOWN

thus making the left hand anode opposite go UP, against its downward... ness :D


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