[TowerTalk] 3CX800A7 vs. GU-74B

[Herbert Schoenbohm]

Peter.....Yes I know but I was referencing a 3CX800A7 which is an Eimac 
tube and today is outrageously overpriced.  The Machlette 3CX800A7 (Now 
made in China) is at least still affordable if you can get them.  K5SVC 
says he has been waiting for them on back order for months. My point is 
that many new QRO amps have the GU-74B which is a tetrodes and they are 
fairly easy to get at an affordable price.


Herb Schoenbohm, KV4FZ


On 2/11/2017 9:34 PM, Peter Voelpel wrote:
> An EIMAC tetrode 4CX800A was never fabricated.
>
> 73
> Peter, DJ7WW
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
> Herbert Schoenbohm
> Sent: Samstag, 11. Februar 2017 21:35
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] 3CX800A7 vs. GU-74B
>
> 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
>>
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