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Old 12th March 2009, 04:56 PM   #1
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Default Cathode feedback on SimpleSE

Hi,
presently building a SimpleSE with the Edcor CXSE opt's, going to use UL for the power increase, but trying to get a handle on what exactly CFB does in this scenario. I found that George used it with the smaller transformers to improve some things, but how exactly, and would it improve/worsen the results with the better opt's? Obviously not even going to include it in this build if it doesn't need it.

One more question about the output capacitor. George uses a .22mfd, what changes using a larger one, like 1.0?
thanks
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Old 12th March 2009, 05:30 PM   #2
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Negative feedback takes a portion of the signal "downstream" and feeds it back into the amplifier "upstream", but out of phase. If the output of the amplifier "zigs" when it should have "zagged", negative feedback will put a little bit of anti-zig (aka zag) back into the front end, hopefully encouraging the output to be closer to where you wanted it. Ok, so maybe that's a bad way of describing things, but it kinda makes sense to me. Keep in mind that the voltages present on the secondary winding of the output transformer might not only come from the field induced by the primary, but also from uncontrolled motion over at the woofer.

Cathode feedback on this amplifier will help improve the damping factor, tightening up the bass and giving more control over the woofers. It might help smallish output transformers reach a little deeper than they would ordinarily be able to go.

The bottom line is that you need to try it both ways and see which you prefer. It's relatively easy to re-wire the Simple SE either way. The only tricky part is ensuring you've wired the cathode feedback with the proper phase. All other things being equal, the amp should play quieter with cathode feedback connected. If it plays louder, you need to reverse the leads going to the OPT secondary. Unlike the feedback used in some other amps, the Simple SE employs a relatively small amount of cathode feedback. On my amp, if I accidentally connect the feedback with the wrong phase, it will NOT drive the amp into uncontrolled oscillation (a dead giveaway that you got the phase wrong!).
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Old 12th March 2009, 05:50 PM   #3
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Regarding the use of a 0.22uF or 1.0uF coupling capacitor, the input impedance of the output tubes would seem to be the most significant factor for choosing a value. If you use perhaps a 0.01uF capacitor you may filter out the lower frequencies of what you are trying to amplify. A 0.22uF is probably just right for the job, but using a 0.47uF, 0.68uF, or 1.0uF probably won't hurt, but may just be more expensive to you.

In the end the main role of the cap is to stop DC current and to pass AC.

I suppose you could bypass the cap, but some undesirable effects may result.

Someone please correct me if I'm mistaken.
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Old 12th March 2009, 05:51 PM   #4
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I was about to type a similar response. I had a Simple SE with Edcor CXSE's. I used CFB on my big speakers because it sharpens up the bass. I don't use it on my small speakers because it doesn't make much difference, and may kill some detail. Those speakers have no response below 70Hz anyway. I would test the amplifier initially without it simply because it removes another variable.

Quote:
it will NOT drive the amp into uncontrolled oscillation (a dead giveaway that you got the phase wrong!).
I have managed to get some screaming with EL34's in UL mode. It is not common though.

The coupling cap is like anything in life, a compromize. .22 uF is large enough to provide flat response down to 30Hz, however there is some phase shift below 50Hz. A larger cap will reduce this, but may not make any audible improvement. If you run the amp into clipping a lot, a larger cap may influence the recovery time. I have built some Simple SE's with 1uF caps because I got a big bag of 1uF Wonder Caps on Ebay a few years ago. There is no issue with using a big cap, but I have noticed some overload recovery issues when I used the Simple SE for a guitar amp and cranked it to 11. Given the choice I would go for a good quality .22 over a generic 1uF.

Quote:
I suppose you could bypass the cap, but some undesirable effects may result.
The result would be extreme distortion and a short unhappy life for the output tubes! The output tube's grid would be driven positive resulting in glow.
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Old 12th March 2009, 06:11 PM   #5
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Thanks guys,
George do you remember which way the opt's are wound? whether I need to hook the output reversed?
thanks
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Old 13th March 2009, 02:43 AM   #6
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Quote:
whether I need to hook the output reversed?
I think that they are reversed, but I can't remember for sure. I took that amp apart to use the transformers in a 300B amp. That was a year ago. My life got so hectic last year that that amp never got finished. In fact I didn't build any amp last year.
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