[TowerTalk] 3CX800A7 vs. GU-74B

[Herbert Schoenbohm]

Thanks to all that responded to my inquiry about using the GU-74B's (a 
tetrode) in triode socket configuration as a replacement for the 
increasingly expensive and affordable 3CX800A7's.  All agree it could 
not be done or should not be done especially in the case of Alpha 87A 
which has a very extensive firmware protocol.  Now here comes the good 
news, at least for some of you....Many recently newly manufactured QRO 
amps are using the GU-74B's (tetrode) and they are still going at a 
reasonable price.  In fact there are 10 of them on eBay today for a 
reduced price of $195.00 each from one vendor.


Herb Schoenbohm, KV4FZ


On 2/5/2017 6:09 PM, Herbert Schoenbohm wrote:
> 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-74B's 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
>