d9d7102e   
   From: adrian@poppyrecords.invalid.invalid   
      
   janneman  wrote:   
      
   > On Apr 16, 5:14 am, TheOctavist  wrote:   
   > > Tube Rolling:-   
   > >   
   > > Valves have certain specs, which all manufacturers of, say, an ECC83 adhere   
   > > to in terms of gain, bias sensitivity, mutual conductance etc. All   
   > > manufacturers have production spreads, so that a valve may have low gain,   
   > > another of the same type higher gain, but still within the acceptable   
   > > spread.   
   > >   
   > > Is there any objectove evidence that a say, Mullard, ECC83 is any different   
   > > in spreads and performance than any other manufacturer's.   
   > >   
   > > Is there any objective evidence that a say, Mullard, valve will sound any   
   > > different to another manufacturer's valve.   
   > >   
   > > Is there any objective evidence that valve circuits are so sensitive to   
   > > changing valves unless it's clear that it's due to the valves being at   
   > > opposite ends of their production spreads.   
   > >   
   > > Or is it yet another audiophile myth?   
   > >   
   > > and Hey Mr. Ian, you rock Sir   
   >   
   > If by 'objective evidence' you include measurements, you can generally   
   > say that performance differences can be measured with different tubes.   
   > As tube equipment is often designed with little or no feedback they   
   > are much more sensitive to individual tube parameters than ss   
   > equipment where the feedback makes sure that individual active device   
   > performance differences do not lead to measured or otherwise objective   
   > differences.   
      
   Triodes already include quite heavy internal feedback because the anode   
   potential affects the potential gradient between the cathode and grid.   
   The screening grid of the tetrode and pentode removes that feedback to   
   allow much higher voltage gain*.   
      
   Because of this, triode circuits  give reasonably stable gain without   
   external feedback, whereas pentode circuits are more dependent on   
   individual valve characteristics.  A prudent audio designer always   
   arranges feedback around a circuit containing a pentode (things are not   
   as straightforward at R.F.).   
      
      
      
   *The original purpose of the screen grid was to reduce the anode-grid   
   capacitance (and the Miller effect) which restricted the H.F.   
   amplifiaction of triodes.   
      
      
      
   --   
   ~ Adrian Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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    * Origin: you cannot sedate... all the things you hate (1:229/2)