I purchased my 87A from ETO as well, in 1995, and the original tubes are
still going strong. At one point I had a problem that appeared to implicate
one of the tubes, so I replaced it with another tube I got from Alpha (which
had changed hands by then.) The 87A worked OK for a while, then I started
having a slew of problems (all kinds of fault codes) that I eventually
isolated to the new tube. It was arcing. The arcing was getting worse, so it
was clearly a gassy tube. The tube carried a guarantee from Eimac, so I was
able to return it to Alpha for a free replacement.
That tube is still sitting on the shelf because while troubleshooting I
determined that there was nothing wrong with the original tube I had
replaced. The problem had to do with intermittent contact between one of the
pins and the socket, possibly aggravated when I removed the tubes while
performing an earlier repair.
Troubleshooting the gassy tube was very difficult because the arcs generated
a bunch of random fault codes, none of which pointed to the tubes. Most
implicated the power supplies. I ended up lifting many components from the
HV and LV boards to check them, but to no avail. In the beginning, the
faults occurred when transmitting, but eventually happened immediately after
warmup. Then they started happening as soon as the power switch was flipped.
That made me suspicious that it had to do with HV applied to the tubes. I
finally decided to see what would happen if I started the amp without the
tubes. As I recall, the amp made it through warmup, but faulted because
there were no tubes installed. I tried removing one tube and the amp made it
through warmup OK. At that point I knew it was the tube I had removed. I
confirmed it by reinstalling the original tube.
It's awfully hard to abuse the tubes in an 87A because of the protection
circuitry, though an extreme amount of continuous duty might shorten the
life. I think it may be a matter of luck whether a given tube eventually
fails or lasts virtually forever.
73, Dick WC1M
-----Original Message-----
From: Keith Dutson [mailto:kdutson@sbcglobal.net]
Sent: Sunday, February 12, 2017 10:44 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 3CX800A7 vs. GU-74B
I purchased my first 87A from ETO (Ehrhorn). The tubes are original and the
unit still puts out a steady 1.5KW, even on RTTY, which requires a second
fan. I often wonder why people are having to replace the original Eimac
matched tubes. Could it be that those units have been abused by some
method?
73, Keith NM5G
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Herbert Schoenbohm
Sent: Sunday, February 5, 2017 4:09 PM
To: towertalk@contesting.com
Subject: [TowerTalk] 3CX800A7 vs. GU-74B
I have been trying to buy just one single triode 3CX800A7 to fix a problem
with a sick Alpha 87A. Several match pair are available but used and will
only be sold in pairs. The new Eimac version of this tube are still
available but cost as much as the amplifier did years ago! The less
expensive Machlette (Chinese) version of this tube, although advertised on
eBay, are reportedly not in stock and a 60-90 day waiting period. However,
Svetlana (Russian Federation) made many thousands of the GU-74B which is
there version of the Eimac tetrode 4CX800A. The GU-74B's are plentiful on
eBay for a third the price of a 3CX800A7. I have successfully before used
spare tetrodes (4-400's) as substitutes for 3-500Z's in several amplifiers
by merely making sure the screen grid was connected to the grounded grid at
the socket. My question is if the
GU-74 could be successfully operated as a triode. I have not found a
print of the socket for the GU-74B and if there is a difference requiring a
socket change this idea would be a non-starter. Many EU hams are using the
GU-74B in home built amps and now more and more commercial amps have chosen
them as their PA tubes. Please let me know if the idea of using a GU-74B is
even feasible. A single 3CXP800A7 (medical version used in MRI's) if good,
would also solve my problem if I could only find a source.
Thanks,
Herb Schoenbohm, KV4FZ
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