[SBE] Continental 816

[Cowboy]

On Thursday 28 August 2014 12:02:35 am Barry Thomas wrote:

> I'm fairly certain it's driver-related. 
> At first I thought it was just bad 4cx250b tubes...They were old.  I Replaced them but still have the same results. Both tubes are operating and seem balanced.  I've got a good match from the exciter into the driver stage and the driver plate tunes into the final, although PA grid current is low (of course). 
> 
> Driver and PA Plate and screen voltages are fine. Cathode currents of the two drivers are balanced but low. In fact All driver current readings are low by 50-70% EXCEPT driver screen current reading which is PEGGED.  Backing off the power to 40% final output un-pegs the meter but it is still at the max scale. I know I should remember exactly what this means but it has just been too long.  Or maybe the meds have killed too many brain cells. 

> Any suggestions on the FIRST things I should check in the driver stage?  

 Inadequate drive from the exciter. ( or a fault in the driver grid circuit )

 The excessive screen current suggests that the screen is acting as the plate.
 ( and will burn out rather quickly due to excess dissipation )
 Either excessive screen voltage, inadequate plate voltage, or insufficient drive.
 You should be able to get about 10% driver output with no screen voltage at all,
 and that's the way I'd run it until you get the driver grid up to normal, or better.
 By your description, the driver stage doesn't have sufficient drive from the
 previous stage. I'd look hard at the exciter, and everything between the
 exciter and driver grids.
 Everything else is behaving as expected from your description.

-- 
Cowboy

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