1882cb01   
   From: nowt@ntlworld.com   
      
   On 09/11/2010 23:33, Patrick Turner wrote:   
   > On Nov 10, 6:48 am, Richard  wrote:   
   >> 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   
   >   
   >>   
   >>   
   >> 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   
   >   
   > OK, then use the ferrite beads. I have not found any need during my   
   > many tests of OPT tubes over the last 15 years. I just kept leads   
   > short. The only time I've noticed RF oscillations have been in an OPT   
   > tubes in an amp circuit that had begun to overheat and glow red.   
   > Usually this had been caused by incorrect bias setting or having too   
   > low an anode load so that the AC signal Pda exceeded the rating. It   
   > can happen in amps with an OPT which has shorted turns. OP Tubes can   
   > then oscillate at some RF which saturates the tube and hasens their   
   > melt down unless the amp is turned off or a fuse blows.   
   >   
   > Have you measured the oscillation frequency? Probably it is at some   
   > some RF. The beads act to raise the inductance of the lead wires to a   
   > higher value. Beads change the effective circuit to one where RF   
   > oscillation becomes impossible due to the arrangement of L and C   
   > elements.   
   >   
   > Either leads are kept very short, or you use ferrite beads or you   
   > bypass anode to cathode, anode to grid using sufficient C values which   
   > will allow signal testing at LF.  I have always found ferrite beds to   
   > be of limited value, and one does not know exactly what value of   
   > inductance results with their use. I've built a number of SS amps   
   > using both bjts and mosfets, and the latter can often give oscillation   
   > troubles in a circuit even when leads have been kept short.   
   >   
   > Patrick Turner.   
      
   Well, this is the layout:   
      
   http://homepage.ntlworld.com/richards_internet/TEMPORARY/kt88test.html   
      
   But anyway, I've stopped oscillations.   
      
   I'm now working out an arrangement to activate the getter.   
      
   You will like it. (Maybe if it works: :c)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)