From: info@turneraudio.com.au   
      
   On Tuesday, 19 January 2016 05:52:28 UTC+11, tub...@gmail.com  wrote:   
   > Hello!   
   > Which tube is equivalent to 1/2 12BH7?   
   >    
    Mr Wieck typed....   
      
   "The standard equivalent in use today is the 12AU7, with perhaps a little bit   
   less gain on the latter.   
   There are those who find that they are indistinguishable as drivers in   
   low-power amps, favoring the 12BH7 over the 12AU in higher-power amps."    
      
   Just who says the 12AU7 "the standard equivalent" to 12BH7? The 12BH7 has a   
   much lower Ra and higher Gm, and although both BH7 and AU7 are able to be used   
   in the same audio circuit along with 6CG7, the BH7 has a significantly higher   
   Ea swing because the    
   Ra curve for EG1=0 is a more vertical line than in the AU7 or CG7.   
   I built and sold countless new and re-engineered amps to my customers, and   
   some used BH7 for driver tubes, especially where drive voltage to output grids   
   exceeded 50Vrms max. McIntosh used 12BH7 to drive 6550 output tubes where   
   drive voltage could exceed    
   150Vrms, because the 6550 was set up with its anode and cathode Vac being from   
   1/2 of the two OPT primary windings for anode and cathode, ie, where Ea =   
   +450V, Va-k in each 6550 could be 400Vpk, ie 280Vrms, so Va = Vk = 140Vrms,   
   with opposite phases.   
   If the 6550 gain = 10, then Vg-k = 28Vrms, so Vg-0V = 28+140 = 168Vrms!!   
   To achieve this easily, the dc carrying anode RL of BH7 in MC60 = 12k and fed   
   from a same phase point on OPT with 140Vrms, ie, the BH7 anode was   
   bootstrapped to make its anode load an effectively a much higher number of   
   ohms, about 100k, while keeping Ea    
   of BH7 about +380V. Its a case of mild positive FB, which tends to boost THD,   
   but because the BH7 has low Ra, the PFB effect is minimized because the Ra of   
   BH7 and Rla 12k form a resistance divider reducing PFB by -12dB approx.   
   But no matter how how high the value of BH7 RLa, the triode still has to swing   
   the rather high anode swing, and my load line analysis reveals BH7 is better   
   than AU7 at this job. I figured BH7 could swing just over 230Vrms max, and AU7   
   over 205Vrms.    
   McIntosh would have sussed out all this long ago. But now most ppl will miss   
   making distinctions between tube types because they have forgotten how to do   
   load line analysis and they refuse to ask WHY very much and they make a   
   statement based on    
   superficial assessment of tube properties.   
      
   I have not included the consequences of McIntosh's use of a cathode follower   
   after BH7 to directly drive output tube grids, but they do have CF and with   
   bootstrapped cathode RL, so that output tubes can be driven a bit AB2, so   
   squeeze the very most    
   anyone can from a 6550, while keeping the total anode load ohms for BH7 high   
   as humanly possible, and affordable, without using a separate choke feed to   
   BH7 anodes. The CF has high gain but follower connection reduces this to just   
   under 1.0, so its gain    
   effects on BH7 gain may be neglected here in this context.   
      
   The AU7 used to replace BH7 in McIntosh MC60 will mean gain of driver stage is   
   reduced slightly, because Gain A = µ x RL / ( RL + Ra ) and the higher Ra for   
   AU7 always means gain will be lower. If the Va = 168Vrms max, and BH7 gain =   
   16, then its grid    
   drive = 10.5Vrms, and AU7 might need 12Vrms, but all this is while the OPT   
   voltages have not sagged much with load; when they do sag, the AU7 gets less   
   bootstrapping so its gain falls more than BH7,and correctional effects of GNFB   
   is reduced. Its just    
   another reason why McIntosh favoured BH7.    
       
      
   So just how you use 12BH7 or 12AU7 should be carefully considered if you want   
   the best possible performance, ie, wide Va swing, low THD, and reliability,   
   and low Ra effect on bandwidth and FB.     
      
   But where the BH7 or AU7 are used in circuits for low Va swings including   
   preamps, their function is very similar, and both are similar to 6CG7 / 6SN7 /   
   6BL7 and maybe a few ECC euro twin triodes. All these medium µ triodes produce   
   hi-fi audio    
   amplification so easily......    
      
   Patrick Turner.   
       
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)