Hi all,
I was commissioning my YD1336 23cm PA over the weekend. Testing
with 2kV supply, G3SEK triode board for protection/bias/etc.
When first powered up, before activating PTT, I noticed that it
drew ~1A when the EHT came up. A bit of head-scratching later and
a few more (quick) tries it seemed to fix itself and biassed up
nicely when PTT activated. Tuned it up, very pleased with 400W
output (from low EHT), went to bed happy.
Next day, having tidied wiring and installed properly, it was back
to 1A on RX mode and this time it stayed there. Some hours of testing
later I changed the valve (tube) and immediately all was fine
(except this one only gives ~300W).
What I'm wondering is where this 1A was flowing? The cathode has a 10K
series resistor when no PTT - I even tried disconnecting it from the
bias circuitry to make sure. I didn't get the 1A unless the heater
was running. If it was flowing through the grid then there must
be a pile of molten metal there (5mA max rating), yet it gave 400W
out after some seconds of the 1A treatment. Any ideas? I'm baffled.
73, Neil G4BRK
|