From: info@turneraudio.com.au   
      
   On Wednesday, 27 February 2013 23:11:52 UTC+11, Alex Pogossov wrote:   
   > Thanks Patrick, It is a useful advice to specify AC voltages where   
   applicable. Will try next time. We all are free to publish in any style we   
   like. My general direction was (I thought) to promote relatively simple   
   solutions where adding a transistor    
   dramatically improves tubed circuit without adding complexity.. With this   
   boosted cathode follower, you almost always can solder the bjt into an   
   existing pure cathode follower. Literally, only one extra solder lug -- to   
   support the base of the bjt --    
   will have to be added. And the output impedance and distortion reduces   
   significantly. And a few quick comments: Using a bjt for boosting is better.   
   It is crucial to use bjt as a current amplifier, not voltage amplifier.    
      
   ***Don't let me spoil your posting style. And don't let me ever stop you   
   asking questions. If I had a quid for every time someone said to me " Patrick,   
   for heaven's sake man, please be clearer" then I'd be rich. A building site   
   manager once said to me,    
   24 yo, and with a tendency to bullshit, "Son, say what you mean, and mean what   
   you say..." The words are still echoing around in my head, some 43 years   
   later. Now, sometimes ppl might think I'm tactless, no empathy either. Nope,   
   It just means I've left    
   out the waffle&bullshit.   
      
   From what little I could see, the bjt in your booster does act as a current   
   amp because you have a higher source drive from 33k // 12AX7 Ra to drive the   
   bjt base with grounded emitter, so Rb input is very low. So the 12AX7 anode   
   voltage change is low,    
   probably negligible, thus allowing the 12AX7 to work as a follower.    
      
   JFET as a boosting element will give worse results both in distortion and   
   output impedance.    
      
   ***Well, sure there would be a big difference between bjt and j-fet. J-fet Rin   
   at gate is high like a triode grid. J-fet Gm is much lower than a bjt.    
      
   But for me to really assess what you are saying means I have to build your   
   circuit and measure it carefully. I'm time limited, and have a very full list   
   of other interests, and, I've never been keen to do other people's homework   
   when I have such huge    
   amounts of my own.    
      
   Besides it will be prone to oscillation, as you correctly mentioned, due to a   
   pole in the hi-Z gate node. Interestingly that a tube with a bjt for boosting   
   gives the best results and the widest range of applications -- can work up to   
   high signal voltages    
   with low distortion and low output impedance. Other combinations are inferior,   
   with only perhaps boosted emitter follower being better on distortion, but   
   things may go wrong if the signal source has large impedance and the "main"   
   following transistor has    
   Early effect (beta modulation).    
      
   **It all seems like you are using a tiny tube current change to change a base   
   current in bjt to get a bigger collector current change so bjt acts like an   
   active current source load for triode, and its sort of PCFB which is then   
   regulated by the NFB of    
   the follower connection. But with just a 12AX7 with say idle Ia = 1mA, then   
   class A current change can only ever be +/- 1mA peak, and anything more gives   
   high THD. What you have is a tube white follower, super-charged with a bjt   
   which has high ac current    
   gain and hence strong action in the 0.7mArms region. One has to wonder why   
   more ppl don't use white followers, or your idea of boosting. More than one   
   idea of boosting can and do exist.    
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)