From: none@ntlworld.com   
      
   On 13/10/2010 12:37, flipper wrote:   
   > On Wed, 13 Oct 2010 11:45:37 +0100, 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?   
   >   
   > It shouldn't have.. unless it wasn't connected to ground or the tube   
   > has an incredible amount of grid leakage.   
      
   You may possibly have been right on the first point or there was an   
   oscillation I never knew about because there were no ferrites on wire to   
   g1. Not sure which was right.   
      
   I reconnected the 1K0 resistor, but this time I put two beads near g1   
   pin. So, that's a slightly different setup, because before the 1K0 was   
   wired in without the beads.   
      
   Anyway I also now wired in a switch to short out that 1K0 resistor or   
   not as required, and either way the plate current is (as you say it   
   should be)precisely the same.   
      
   So, perhaps the tubes are still good.   
      
   I think to check for gassiness I need to get a reading of grid current.   
      
   I have a 200uA meter. Should I place this in series in the g1 circuit   
   and get a reading?  Or read voltage across the 1k0? If so what voltage   
   would I be looking for? How should I get an indication of grid current?   
      
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