dbfa38a0   
   From: nowt@ntlworld.com   
      
   On 17/10/2010 04:34, Patrick Turner wrote:   
   > On Oct 13, 9:45 pm, Richard  wrote:   
   >> On 12/10/2010 21:37, Richard wrote:   
   >>   
   >> I said   
   >>   
   >>> I'v wired the KT88 triode connection. Anode and screen are at same   
   >>> potential because they are connected.   
   >>> There is a 270R cathode bias resistor, which serves to effectively place   
   >>> a bias on g1, because g1 connects to "earthy" end of the bias resistor.   
   >>   
   >>> Okay, with this setup (which may be deficient, but I would not know why)   
   >>> anode current goes way up to 240mA, the voltage output from PSU drops to   
   >>> about 250V. Under these conditions bias resistor develops 64.8V.   
   >>   
   >> Okay, I made a change which altered the readings significantly.   
   >>   
   >> I have a triode connection, ferrite beads on anode, screen and grid   
   >> wires, but I had only a 1K0 grid stopper next to g1 pin on the tube   
   >> socket. A 270R cathode bias resistor.   
   >>   
   >> Ia was 240mA   
   >> Voltage across bias resistor was 64.8V   
   >> PSU voltage 250V (dropped 60V due to overdrawing PSU design current)   
   >>   
   >> I took out the grid stopper, and simply placed ferrite beads on the grid   
   >> wire.   
   >>   
   >> Now the figures are:   
   >>   
   >> Ia 90mA   
   >> Voltage across bias resistor is 24.3V   
   >> PSU Voltage is 310V   
   >>   
   >> Removing that 1K0 grid stopper altered the whole thing. Things look much   
   >> better.   
   >>   
   >> Why did removing the 1K0 resistor in g1 circuit change things so   
   >> significantly?   
   >   
   > I think you may be a long way from knowing enough to answer your   
   > questions by yourself and part of the problem may seem to be that you   
   > have no oscilloscope to monitor whether or not there are RF   
   > oscillations or what frequency they may be, or whether your ferrite   
   > beads have any real effect at all. Bypassing ALL electrodes to a   
   > common ground point using say 0.47uF 630V rated plastic caps should   
   > eliminate any possibility of RF oscillations.   
   >   
   > Your obsevations so far lead me to think you have an almost completely   
   > fucked tube.   
   >   
   > What happens when you test new tubes, or other tubes of the same age?   
   >   
   > There are many questions you should be asking, IMHO.   
   >   
   > Patrick Turner.r   
   >   
   > i.0uF plaPlaicin   
   >   
      
   There was strong oscillation without the ferrite beads. Even oscillation   
   with the 1KO grid stopper. That made the tube conduct heavy current.   
      
   When beads were put on every pin, stopped oscillations dead. Now the   
   tube can be tested.   
      
   There is in fact 4uA ionic current.   
      
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