[TowerTalk] 3CX800A7 vs. GU-74B

[Keith Dutson]

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 at contesting.com] On Behalf Of
Herbert Schoenbohm
Sent: Sunday, February 5, 2017 4:09 PM
To: towertalk at 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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