Altho Ive repaired several of the 811 and 811H's I didnt delve too deep.
With an 811H that wont be going back to the customer until Nearfest I dug a
bit deeper on my own nickle and out of curiosity as Ive heard how bad this
amp was designed and constructed.
This amp had chewed up a set of Taylor 811's in 2009 and reportedly only
run at 400-500W SSB on 80M.
The customer had replaced one of the grid resistors and the B- diode then.
This time one tube was cracked 360 degrees at the base and a 50 Ohm grid
resistor was open at another tube plus the B- diode was shorted again. Now
interestingly enough he had been running it on 3 tubes for over a year after
noticing one dark tube which is the one that had the open resistor. At
50-60W drive and 400W that should be fine for long life.
OK, so I replaced the resistor and diode and then noticed that the tube with
no filament had solder melt on the pins. Sucked it out and reflowed and now
the filament is fine.
The 4 grid resistors are mounted on a pair of terminal strips at the outer
edge of the sub chassis with up to 7" leads going back to the grid pins.
Apparently the designer realized the resistors were fuses and placed them
where they could easily be changed and expected the .01 bypass caps to do
all the RF work...not exactly my idea of good engineering but this was the
same person who spent decades condemning Heaths SB-220 grid circuit and
another persons recomendations for resistors and higher value caps.
Then I noticed that one of the parasitic suppressor resistors was
discolored....yep it was open and that was for the tube with the broken
glass.
Next I noticed that there is no plate choke bypass cap! I wonder what genius
decided that. On 160M the choke reactance is only 2X the RL and about 4.5X
on 80M. I added a .0047 @ 3KV disc to be sure and with the Measurements 59
GDO in diode mode it sniffed much less RF in the PS area.
Another issue was that both 1000pf @ 7.5kV plate blocking caps had cracked
cases on both sides. The safety choke was OK so they hadnt failed but
apparently got rather hot. They were replaced by 1000pf @ 6KV NOS Centralabs
that I picked up about a thousand at a local auction in the 90's....good old
USA made.
With one tube in the sockets and 25W drive, power on 20M was surprisingly
around 200W with all 3 and with equal anode color, Id expected fireworks.
With all 3 and 60W drive it was 400W and with 4 Svetlana 572B's I keep
around for testing and 100W the output was 700W, all measurements key down
carrier.
The customer wanted 572B's and Im waiting for them to arrive. I also noticed
Ameritron now has an AL-811HD model with 572B's, that should tell you
something about the 811 reliability.
Im also going to put the grid resistors right at the sockets where they
belong.
AND I sugest that all AL-811 owners take a close look at their amps and the
trouble spots mentioned above.
Carl
KM1H
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