I also agree with the not grounding of the filament. It has been my
experience though, that 1N5408 diodes can be destroyed by a 3500vdc discharge
through
a 25 ohm glitch resistor. Perhaps the discharge takes too long for the
diodes to survive. Or, it is also likely that the primary of the hv
transformer
was not turned off quickly enough. It does take some time to blow the 30A
primary fuses.
I am curious about the reason for 20 diodes as the same current will flow
through all of them.
I have finally settled on using 6A diodes with three or four connected to
various spots on the b- rail in case there are some locations needing a
little
extra protection. You know the approach: belt, suspenders and velcro. Under
fault conditions, the b- voltage will be limited to the forward voltage drop
of a single diode conducting about 140A.
Last weekend, several of the 1296MHz eme group met at my qth to test our
collection of TH347 tubes in my cavity amplifier. We did test all of the tubes
for leakage before firing up and several were not up to par. We were tempted
into testing a couple that showed moderate leakage and sure enough we were
greeted by an internal plate to screen grid flashover. The 6A b- diodes all
survived this and interestingly, so did the 30A primary fuses. The amp has an
overcurrent trip on the screen grid that latches a small relay which in turn
opens the control circuit to a 40A SSR in the 240vac primary. This all acted
quickly enough that the amp was shut down before any major damage was done. The
only failure was a 1kv rated feed thru capacitor connecting the G2 voltage to
G2. It was found leaky after a flashover.
The screen grid power supply uses a string of 50w zener diodes as a
regulator (don't recommend this, it can only loosely be called a regulator)
which
have been destroyed in similar circumstances in the past. A couple of months
ago
I had added a 500 ohm 10W resistor between the screen grid and the power
supply to limit current surges into the zener stack in case of a flashover.
There are two 1N5408 diodes in series across this resistor to limit the
voltage
drop to about 1.4vdc for normal screen current. These diodes are back biased
during a flashover condition. Apparently this resistor saved the zener stack
during our tests last weekend as there were no failures there.
I am hoping that a G2 feedthru capacitor rated for full plate voltage will
take care of the remaining failure mode in this amplifier. If it does, then I
will be one happy camper!
73,
Gerald K5GW
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