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The driver-to-screen-tap connection proved to be just marginally stable, even with a good deal of straightening out of the LF poles. Global feedback worked pretty well, about 4-5dB, and sounded pretty good. But Richard Sears suggested a cathode-feedback arrangement for the output tube, and that is what I went with. It's a very, very good-sounding amp if I do say so myself. I've reposted the schematic and would welcome any comments and suggestions.
Follow Ups:
Claus Byrith's article, downloadable from the Lundahl website has good information on using the OPT secondary for cathode feedback. There is some DC offset created by the cathode current flowing through the resistance of the secondary, but not enough to be harmful to a speaker. I tried this with a 6EM7 amp using Hammond 125ESE's and was pleasantly suprised at how much the bass improved. It seems that one could get a damping factor with ultralinear connection of a pentode similar to that of a triode-connected pentode without resorting to global feedback.Kyle
Thanks for the reference. Nice article and very relevant. I'll study it.
Have you tried not using the cathode-feedback arrangement with the ground of speaker connected to the kt88. I did not like what it did to the bass. I read others too, did not like this configuration.
I've experimented a little with cathode feedback like this and found that the success seems to depend a lot on the transformer. I don't know exactly what property of the OT helps/hurts, only that some just don't like it.
I'm not sure what you mean--could you explain just a little further?
When your ground reference for the kt88 cathode and bypass cap is connected to the transformer secondary, i find the bass becomes boomy
Well, I'll have to do some listening to confirm this. It's certainly better than without the feedback connection (I think), and I'm not sure there's much difference between the cathode feedback and global feedback in terms of bass control. Notice that the output terminals are reversed in phase. If you connect them in phase the feedback is positive (at least this happened with my output transformers) and you get a slight increase in gain and the bass gets very loose and ill-defined. If you tried it without getting the phase right, then the sound would deteriorate, I think. If you did have the phase right and still found the bass less controlled, I'll certainly do more comparative listening to see if I can hear it.I may end up with a combination of cathode and global feedback, such as in the article above.
I tried that too, only it was the little 6V6 I threw together recently. I remember the first time I saw that design, looking at the feedback from the secondary back to the cathode and thinking what a great idea it was. In practice, I couldn't make it work. The bass was muddy, and even the upper registers seemed to lose some detail. I tried a 6BQ5 too, thinking the additional gain might help, but no go. When I switched back to conventional NFB, it was like night and day. The amp was reborn! Maybe it will work in your case; you have a different transforemr, different driver, different output, and different PS. I'll be interested to know if it still seems good after you've listened for a while.
I'm interested that the amp cited above uses both cathode feedback and NFB. Did you try both?
So as not to mislead anyone, I should add that none of my amps are UL. I don't think the sound I mentioned is created in a UL amp with NFB removed. At least, I don't remember ever hearing it. The UL connection probably serves to reduce IM to the point where the effect isn't so obvious, even with other feedback removed.
Well, now you're stretching my memory. However, I think I tried cathode FB alone, not with NFB. I was hoping it might serve as a non-global substitute. In the end, I decided the amp sounded pretty good with lots of NFB, so that's what I used.I'll add that I think NFB is generally unavoidable with pentodes. All my pentode amps (PP and SE) exhibit an odd sound when NFB is removed, a sort of paper-like rubbing sound or breathing underneath complex tones. I think it's due to IM, but that's just a guess based on some tests I performed many years ago (produced the same sound). Unfortunately, I no longer have an IM analyzer. :(
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