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In Reply to: RE: ESLs and OTLs posted by tyu on January 30, 2014 at 14:36:50
How about those Kits........Any sold yet?....an the $$ if you can say?
Yes. They are $4200 right now.
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If the amplifier has feedback and it is not handled correctly, this can cause stability problems. Futterman knew that ESLs were a likely load and it seems that he knew to be careful about how he set up his feedback to prevent oscillation.
Only OTLs i have owned are Yours ..... an have tryed the Feedback at the input of the frist input tubes grid...but always go back to no feedback....is your setup the way you are saying is the right way to use feedback...an not on the cathoid of the input tube?..... like i see on alot of tube amps...........Dose this help with stability problems on any Tube amp OTL or pushpull?
Vary cool about the Kits.....thanks for your time an for Saleing Kits again.......
The problem is not whether the feedback is on the cathode or grid.
It has to do with what frequency the feedback is no longer negative, but shifted in phase (due to time delays in the amplifier circuit) enough that it can act as positive feedback, causing the amplifier to oscillate.
This usually involves setting up some timing constants (capacitor/resistor networks) in the feedback loop, to sidestep the problem above. OTLs tend to have a lot of bandwidth and if this quality is ignored, oscillation will not be far behind. In this regard they have more in common with solid state amps than transformer-coupled tubes amps, as an output transformer limits the bandwidth.
Hi Mr. Karsten !
Except those effect derived from insertion of GNFB also THD spectrum can be altered easy , in way of generation of higher order harmonics , from 5 th to 21 , which can alter in negative way quality of overall percieved sound .
Some positive effects from GNFB exist too , mostly by lowering of Amp output impedance , but again if GNFB is over done output power tubes can easy suffer overload trying to maintain constant output voltage on sometime very low load(speaker) dynamic value , finally resulting in Amp instability .
However I think difference between SS Amps and OTL Amps is huge ! , since most of commercial SS Amps have very low open loop bandwidth , but in the same time they have intentionally very high open loop voltage gain , so many SS Amps without insertion of significant value of GNFB will be helpless to achieve wide closed loop bandwidth .
Best Regards !
__
Enlightened Evolution-Astral Projection
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