Funke W19 : GR150DA light emission

[papsdoc]

Hello,

Until recently, the light emitted by the GR150DA of my Funke W19 was stable with a nice rose tinted glow whatever tube was tested in whateverer position of the cursor. But since a couple of days, when testing a power pentode such as a 6550 or an EL84, the light emitted by the GR150DA decreases strongly (only a very small lightning is visible in the small window) when the rotactor is in position 12. When the rotactor is switched to position 13, after a few seconds ( 10 to 15 seconds), the full light reappears and is emitted normally. In position 13, I note also that the current decreases when the light reappears : for instance, when testing a strong EL84, I measure 50mA in position 12 (light almost invisible), then 28mA in position 13 (light almost invisible) dropping to 24mA allways in position 13 when the light reappears. Note also that I checked the Va during the test : the voltage does not change when the light reappears. I measure 185V in position 12 and 197V in position 13, with a fluctuation of +-2V, whatever the light of the GR150DA.
Any explanation / advice would be welcome !!!
Vincent

[Jac19]

what you see is normal. This is a parallel stabilazation circuit. So when you draw more current with the tube under test, less current flows through the stabilzer tube, and light gets dimmer. Even so it is quite usefull to see, because when it stops burning, you are below the "burn" voltage, and then you are drawing more current as intended. Also, that should never happen with a good tube.

-Jac

[papsdoc]

OK, thank you Jac for the explanation.

Vincent