Some output tubes (211 and 845) need a very high drive voltage with low distortion. Some tube freaks have used the 300B tube as a driver but it is very expensive. Others have tried to use a triode connected EL34 but it is not linear enough.
A pentode driven on the screen grid is very linear. I have tried this concept in a new driver stage, as shown on the accompanying schematic. The first two tubes V1 and V2 form an asymmetrical mu-follower. V3 is a screen grid driven pentode. Adjust the potentiometer R9 to 350V DC at the plate of V3. You can use many different tubes for V3.
I made distortion measurements for some different tube types with the results given in table 2. EL36 and 5881 are the two best tubes. The linearity of these two tubes in this "enhanced" mode is even better than for a 300B tube used as a driver.
The sensitivity of this driver stage is so high that it can be used direct with a CD player. Use an 100k ohm pot as volume control. Both the 400V and 700V supplies should be well filtered, maximum ripple 2mV.
You can of course use higher voltage than 700V for better linearity. The EL36 tube can withstand more than 1000V. The 6DN6 tube can also be used for very high operating voltages. Never use more than 1000V B+ supply voltage and be sure to set the anode voltage to 50% of B+ voltage. Use a separate filament supply for V2. Be careful, high voltages are dangerous!
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