My 2nd post on this, I deleted the first, poorly written late at night.
I'm chasing my tail lately on Tetrode project.
It stopped amplifying despite correct voltages at tube pins and a reasonable zsac but in retrospect (below) I recognize the bias voltage was nowhere close to datasheet values when bias set by zsac only. When bias V set by voltage alone, no plate current at all. Yesterday I found that the "D-2 safety diode from B minus to ground (bypassed with 50v ceramic caps a'la W8JI website) went short. This was the 2nd diode to fail short in several months (1st was snubber across T-R relay, a 4000 series but both diodes from Amazon, Bojack branding). Will try again later today with new D2 diode. Larger size diode? Before I lost function a few days ago, I was making good progress with decent PO (low drive into DL).
Is there anyway the shorting of D-2 could have damaged tubes? Plate transformer rated 1800v @ 400mA FWB while datasheet reports 500mA max plate current for 2 tubes. I'm hoping tubes are OK. When I think about D-2 going short the only effect would be plate meter accuracy showing too low because of D2's added conduction. Is my thinking sound on this? Is proper Bias a combo of voltage and current? I always looked at it as: 'datasheet is approximate but zsac is best'...............I think I'm learning that Bias is more of a combination of volts & mA within the datasheet range, that is best.
I'm chasing my tail lately on Tetrode project.
It stopped amplifying despite correct voltages at tube pins and a reasonable zsac but in retrospect (below) I recognize the bias voltage was nowhere close to datasheet values when bias set by zsac only. When bias V set by voltage alone, no plate current at all. Yesterday I found that the "D-2 safety diode from B minus to ground (bypassed with 50v ceramic caps a'la W8JI website) went short. This was the 2nd diode to fail short in several months (1st was snubber across T-R relay, a 4000 series but both diodes from Amazon, Bojack branding). Will try again later today with new D2 diode. Larger size diode? Before I lost function a few days ago, I was making good progress with decent PO (low drive into DL).
Is there anyway the shorting of D-2 could have damaged tubes? Plate transformer rated 1800v @ 400mA FWB while datasheet reports 500mA max plate current for 2 tubes. I'm hoping tubes are OK. When I think about D-2 going short the only effect would be plate meter accuracy showing too low because of D2's added conduction. Is my thinking sound on this? Is proper Bias a combo of voltage and current? I always looked at it as: 'datasheet is approximate but zsac is best'...............I think I'm learning that Bias is more of a combination of volts & mA within the datasheet range, that is best.
