On 12/18/2023 9:35 AM, W7WRX wrote:
Those are Z5U caps.? They are a poor choice for coupling RF with
high current..? No wonder they failed.? Can't believe Alpha
would do that. I have repaired several 8410s and I don't even
remember those blue caps being used.
They done it; here's the BOM listing:
2 CDX-2221-C 3 C1-2 C7 CAP,CERAMIC DISK,0022uF,6KV 20% VISHAY
Here's the actual parts listing:
CDX-?©\2221-?©\C 3 CAP 2200PF 6K VOLTS 20% .375 L.S. RADIAL
I don't know the vintage of my amp, but it's early enough that it
had the VTX-118s in it. I've since bought a pair of 1500s but
haven't installed them yet.
?Really should use a cap that is rated for high RF current.? The
idea of using six caps for coupling is just bizarre.
I agree; I was flabbergasted when I first saw disc ceramics being
used as the plate blocking cap in a high power amplifier in my
ALS-800 (in fact, they, too, are blue, just like the Alpha's caps;
must be something about blue disc ceramic caps that is attractive
like a Siren to amplifier designers? 8-D ); it has a pair of
something, I didn't note what they were. I looked in the parts
list and schematic but I didn't note whether those are changed for
the ALS-800H, which uses a pair of -800A7s for well over
legal-limit output power. I doubt they are, though.
Rember: that guy -JI designed the ALS-800/800H, so it's GOTTA be
"right", right??? 8-D
?
I get that you had them but heck man.? Just order a proper HEC
high energy RF cap and be done with it.? The amp will work
better and be more reliable.
Perhaps. I've had quite a few of those CDE doorknobs that cracked,
though, so I'd probly use one of my few Russian humungously-sized
replacements, instead. However, the layout around that area is
tight, so there's not much room for a larger cap of any type without
lenthening the leads from the plate choke to the input tuning C. I
frankly was not very impressed with the layout of the 8410's output
section at all. I'm really wondering if the 160m coil's toroids are
going to do the same melting-stunt that I've read that K1TTT's
Commander HF-2500s have done during a contest.
By
the way,? Filament voltage on that 8410 should be checked!? They
run very high Fil voltage and if your line voltage hits 250 it
could pop those tubes.? I know this because one guy kept losing
4cx1000s.? We found he had high line voltage due to the entire
neighborhood being on Solar. In the middle of the day it would
hit 260!!? Then Pop the tubes.
I've gotta go do that ASAP; it seems to me that I might be losing
a bit of power every time I turn on this thing, which could be
from the cathodes boiling away and losing emission. I'm totally
off-the-grid here, and presently have to run the Alpha from a 3700
watt Xantrex RS3000M 120VAC inverter, which runs off six
paralleled 105 A/hr LiFePO4 batteries, recharged from 2 kW of
solar panels. I've measured the inverter's output at 124VAC under
800VA load but that drops to about 118 under 2 kVA load at the AC
outlet. Obviously, that is still too high for the Alpha since it
spends the majority of its time just idling. But there's no way to
turn it down unless I get out my 2.4 kVA variac. That Alpha is a
power hog, though... I can't usually run a pileup longer than a
few hours before the batteries drop below 50% State-Of-Charge,
which, on a cloudy day, might get back up into the high 80s by
sundown the next day. I've had it down to 14% (right after the
recent ARRL 160m contest) but that day was bright and sunny, so
the batts got recharged back to about 75% by sundown.
73,
Steve K0XP
