Yo amps
Interesting debate on the 4CX1600B tube. This is a real workhorse tube
and it's too bad it has been dropped by the amp makers due supply
problems.
I never heard from anyone that had the tube "bang" and found it still
good or bad unless I missed it from the postings.
The history: QRO HF-3KDX 4CX1600B
WW phone the amp had been on for 4 or 5 hours but mostly S/P, low
duty cycle. While idle the amp "banged". Sharp report, like a firecracker.
The fault light came on. I believe I saw a flash in the room but not certain
anymore.
The report was sharper then my normal 4-1000 "bangs". The 4-1000's
seem to want to "bang" after sitting idle all summer, and usually happen
only once. Those pops sound more muffled and occasionally open the HV
glitch resistor.
A low voltage transformer feeds two power supplies. One is a 160 VDC
that runs the TR switching relays. The other is -160VDC bias for the tube.
On transmit the bias supply is routed through a DPDT relay to a stack of
Zeners for -60 VDC.
Both supplies are protected by fuses and both of these opened violently
if the black vaporized coated glass is an indicator. Both diode stacks
showed about .2 ohms short on the transformer side of the stack. The
output side of the +160 supply for the relays was shorted to about 10
ohms and the output side of the bias stack tested okay.
Only the fuses, two diode stacks, and tube was replaced which returned
the amp to service. Nothing else was damaged, that I could find anyway.
So the question remains is the tube good or will it just blow those parts
again?
Following the AMP reflector's discussion and in the name of science I will
re-install the tube and jump back but only after the contest season. This
amp has to be taken almost all apart to get to the LV/BIAS board including
removing the front panel. All easy to do but putting it back together is a
royal pain in the cathode.
I will report back, thanks for the interest.
73 Rich KL7RA
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