From: pallison49@gmail.com   
      
   Big Bad Bob wrote:   
   > Phil Allison wrote:   
   >>   
   > > Most recent models from PV, Fender and VOX run their EL84s waaayyyy too   
   > > hot and burn them out in short order.   
   >   
   >   
   > keeps costs down, but requires frequent tube replacement.   
   >   
      
   ** Plain bad design IMO - I modify ones that come my way to solve the problem.   
      
      Cost SFA to do in the factory.   
      
      
   > and so they sell tubes.   
      
      
   ** Not likely, there is way too much competition in the tube market for tubes   
   branded Fender or Peavey to dominate.   
      
      
      
   > A good power output tube should last for YEARS, without obvious signs of   
   > failure.   
   >   
      
   ** Well, thousands of hours of normal operation is quite possible.   
      
      
   > the GE book tells me that EL84/6BQ5 has a plate dissipation of 12W. It's   
   > a pretty fair bet that class AB operation would give you 12-15W RMS with   
   > properly designed "everything".  Squeezing more than that out of it is   
   > going to kill the tubes in a short period of time.   
   >   
   > the ref design in the GE book has 300V power supply and 8k p-p   
   > transformer load (AB1) with a 130 ohm cathode bias resistor, for 17W   
   > output [probably heavily distorted 17W].  I'm sure you could work out an   
   > AB2 design with fixed bias to crank it up even higher, but that's not   
   > necessarily good for the tubes...   
   >   
      
   ** FYI,  I have a prototype 2x EL84 output stage here that delivers over 50   
   watts with low THD and no overheating. The trick is to use a split supply of   
   325V and 650V for screens and plates respectively.   
      
   There have been commercial amplifiers that did much the same.   
      
      
      
   .... Phil   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)