[TowerTalk] 3CX800A7 vs. GU-74B

[Herbert Schoenbohm]

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