From: info@turneraudio.com.au   
      
   On Monday, 5 November 2012 14:03:31 UTC+11, Phil Allison wrote:   
   > "John L Stewart" > 'Phil Allison >> > > >> > Sounds like by either intent or   
   serendipity the 300B and 6SN7 2'nd >> > harmonics are canceling.- >> >> >> **   
   Bingo !! >> >> Seems an eccentric Russian gent was responsible for the circuit   
   design. >> > >    
   There was an article on 2H cancellation in SE amps published about 10 > years   
   ago, I think in Glass Audio. Used the curvature of the driver to > cancel   
   curvature in the OP Tride. One would think this possible at only > one level   
   on the output. ** More    
   testing with the FFT feature on a Rigol scope: 3H is dominant from about 9   
   watts through to 21 watts. From 1watt to 4 watts, 2H dominates, then 4H, then   
   3H. If load R is changed to 16 ohms, 2H dominates all the way. 2H cancellation   
   is occurring over a    
   fairly wide range, but it is not complete. I can swap the 6SN7 with almost no   
   change. Neat eh ? .... Phil   
      
   Its actually possible to exploit the set up of a driver tube to cancel 2H of   
   the SE OP tube. But while maximum 2H cancellation plus probable 4H   
   cancellation can be arranged for the nominal load to be used, the cancellation   
   may not be much when RLa is    
   above of below nominal RLa. Roughly the same Vg input to 300B is needed over a   
   range of RLa and a typical SE triode driver makes roughly the same 2H for most   
   RLa values. But 2H is most severe in 300B when RLa is low, and then least when   
   RLa is high. Some    
   3H is generated by intermodulation process and you can end up with higher 3H   
   than you'd have if the driver triode was more linear. I found the cancellation   
   of 2H with SE tetrode & pentode amps with about 15% local CFB could reduce THD   
   to less than the    
   same OP tubes used in a PP circuit. But with the multigrids, 2H is highest   
   with low RLa, then as RLa is raised it nulls out at some centre RLa value even   
   if the driver is perfectly linear, and then for RLa above the centre value 2H   
   increases and its    
   relative phase is opposite to low RLa values. Therefore the 2H of the driving   
   triode adds to that of the multigrid OP tubes. But after spending much time   
   studying this phenomena of THD cancellatiuon, I decided it was not worth   
   deliberately setting up a    
   driver tube to be slightly non linear to give maximum cancelling, and I   
   concluded its better to make the driver as linear as possible no matter what   
   driver and output tubes may be. I found that the CFB OPT connection forced THD   
   production to be very much    
   lower than for triode or UL SET, and then you'd still get some 2H cancelling   
   with low RLa so the end result was THD equal to many class A PP amps over a   
   useful range and for a similar amount of low GNFB applied.    
      
   Nearly all SE amps with 300B are ideally used where average PO < 0.1 x max   
   clipping PO so then average THD < 0.2% and mainly 2H and lower 3H. This   
   assumes you may have 10dB GNFB. But in fact if you remove the GNFB the sound   
   change is very subtle if any    
   can be percieved at all. I have had ppl say, "Well, now we have disconnected   
   the NFB it sounds the same" - even though N&D has risen maybe +3 times for a   
   given volume level. If RLa for the 300B is say 3k6, and Ra at the bias point   
   is 1k2, then DF = 3    
   without any FNFB, not to be sneezed at. Ppl will argue the GNFB either gilds   
   the lily, or makes it into pumpkin. My conclusion is that more ppl like   
   adequate GNFB well applied than dislike it, citing better bass tighness and a   
   creamier top end, ie, the    
   NFB allows the tubes to blossom, so good GNFB is natural organic electronic   
   fertilizer....    
      
   Some amps such as the EAR509, a 100W Paravicini PP concoction with a total of   
   44dB NFB including the unity gain OPT, does not sound as well as something   
   with less NFB and far lower PO but with much more pure class A1. I think its   
   pointless playing silly    
   tricks with nearly all class B and huge amounts of NFB. But hey, that's what   
   is done with OTL amps, and ppl rave that they can't hear the veil of the OPT.   
   But they don't understand the tubes work mostly in class B and with terrible   
   load matching and with    
   huge amounts of NFB.    
      
   There is truth in that GNFB can increase the THD spectra so creating higher H   
   that were not present when no GNFB is used. But the effect seems to rear its   
   ugly head when the THD without NFB is high at say 10%, and not much NFB is   
   applied, say 10dB, and    
   10dB is quite typical for triode amp GNFB, so it can be said use of this FB in   
   a triode amp invites sonic horrors. There is a point where even if open loop   
   THD = 20% before clipping, and there was still sufficient open loop gain, then   
   increasing NFB to    
   say 26Db will usually reduce THD to 1%, and as THD is about proportional to   
   Vo, then at 1/3 full Vo and at 1/10 full PO, THD should be say 0.3%. But its   
   not ideal though to start with 20%. Its better to have open loop THD < 2% if   
   possible and then the    
   figures and sound all get better without huge dollops of NFB, and the mild   
   10dB GNFB does not make things sound worse. Along with applying GNFB come the   
   rigours of making sure the amp is stable and does nt have a peaked response   
   just below and above the    
   AF band. One does not want LF peaking to upset dynamic DC core magnetization.   
   I suspect many pp hate NFB because it forces them to think of the amp as an   
   active bandpass filter with NFB, and think about Nyquist stuff.   
      
   There have been attempts to cancel 3H production of most PP amps where   
   typically wave peaks are flattened slightly. Therefore one could make a driver   
   PP amp stage which produced oppositely phased 3H, ie, peaks on waves became   
   "more pointed" I tried all    
   this about 12+ years ago and concluded it was a totally hopeless design aim.   
   Just make it linear, said my brain, without tricks.    
   I also tried positive current FB and its easy to get a negative output   
   resistance. But THD rises even though Ro becomes negative. And if there is a   
   shorted output, the +current FB goes sky high and amp violently oscillates. No   
   sir, no tricks, right?    
   Patrick Turner.     
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)