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cathode feedback transformer and damping factor

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Hi Guys,

I'm about to build KT88 PP amp based on LL1620PP CFB output transformer. Schematics is simple d3a as driver > interstage LL1660S (as phase splitter) one pair of KT88 (triode straped and fixed bias). > LL1620PP CFB (11,5 kR/8R)

From rough calculation seems that I'm able to achive about 3,2 DF which is rather bad score ;-(

I wonder how much I can gain DF using CFB ?
How to calculate exact figure ?

For a valve amplifier 3.2 DF is actually not a bad score at all. I've seen DF's for pentode output amps rated as low as 0.1! Yours is 32 times better. It is the nature of the beast that valve amps generally have quite low DF. People often overrate DF. But it is true that higher DF in triode and UL is one of the reasons people prefer it.

But you really shouldn't overrate the importance of high DF once it is above a certain minimal level. (Straight pentode output generally IS too low in Df.) If you have gobs of gain in an amp and then cancel most of it in feedback at the OP transformer you will have very high DF. But if you're using nGFB then you may as well have a sand amp because that high DF will also translate into the sound of sand amps.


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The LL1602 has rather high cathode feedback ratio of 25%. Need to be careful because the driver might not be able to provide the full swing anymore. Also triode connection might not be the best choice. UL with cbf will reduce Zout to similar levels and you would get the advantage of higher Pout.
For both triode and UL connections I guess you are looking for more that 200 V peak-to-peak drive and the improvement in DF would not be massive. The problem is that you still have the DC resistance of the transformer in series. If you really want high DF loop feedback is necessary. I do not think high damping factor is important or better.....
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