b2670c17   
   From: apogosso@tpg.com.au   
      
   "Roger Jones"  wrote in message   
   news:251c1d57-179c-4778-a2ea-add8dfcad550@i18g2000vbx.googlegroups.com...   
   > Hi, Vacuumlanders,   
   > I just came across a very dead EL84/6BQ5 (from a P-P audio amplifier   
   > with a leaky coupling cap to its g1 - take note!) Just for interest, I   
   > cut the glass off just above the base with a Dremel-type cutting disk   
   > (use glasses and gloves!) Then, I pulled off the plate assembly to   
   > look at the electrodes.   
   > The RCA tube manual calls this tube a "Beam Power Tube" but there are   
   > NO beam plates, rather just a lose spiral grid 3. (Indeed, the word   
   > tetrode is NOT used by RCA) But to me it looks like a power pentode.   
   > Perhaps the rather open g3 spiral and supports may cause electron   
   > beams, but it's not clear to me how this would happen.   
   > Any comments on this and the name?   
   > Cheers,   
   > Roger   
      
   If I understand correctly, the main feature of a beam tetrode is the same   
   pitch and alignment of the control grid and screen grid. This creates the   
   beams and helps keep screen current low.   
      
   On the other hand, to keep the screen grid electron interception small   
   enough, the screen grid can not be very dense. Therefore the control grid is   
   also not dense. Thus the transconductance is not that high (per each mA of   
   plate current) -- probably about 10% (mA/V) of the plate current (mA): about   
   5mA/V per 50mA plate current.   
      
   If you want a high efficiency high-gm tube, the control grid has to be   
   tight, but the screen still can not be too tight. So the hi-gm beam tetrode   
   is impossible. It will be a pentode, though some pentodes can have   
   suppression plates or frame instead of a suppression grid.   
      
   EL84 has 11mA/V, a tight control grid and never can be a true beam tetrode,   
   even if some manufacturer fits supression plates instead of the third grid.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)