The 8874 cooler is 1.625" diameter and the 3CX400U7/8961 is 2.0625" which is
the same cooler used on the 8930 tetrode. Those are measurements I just
made, I use the 8930 in a AM-6155 432 amp.
At the much higher HF efficiency (only ~ 40% at 900 MHz) it should run quite
cool, even in high duty cycle modes as compared to the 8874.
Carl
KM1H
----- Original Message -----
From: "Paul Decker" <kg7hf@comcast.net>
To: <amps@contesting.com>
Sent: Tuesday, July 03, 2012 8:05 AM
Subject: [Amps] 3CX400U7 in Alpha Linear?
>
> I've thought about doing a similar thing in my 8874 amp with the U7, it
> seems like it should work just fine, and the price of the tubes is really
> great. As a test I had thought about just creating some sort of socket
> adapter out of bread board so I wouldn't have to modify the amps socket.
> One thing about the 3cx800U7 though is the anode cooler is slightly larger
> in diameter than the 8874 so you have to keep that in mind in some cases.
>
> Paul,
> KG7HF
>
>
>
>
>
> Date: Thu, 28 Jun 2012 21:19:34 -0400
> From: "Carl" <km1h@jeremy.mv.com>
> Subject: Re: [Amps] 3CX400U7 in Alpha Linear?
> To: "Martin Sole" <hs0zed@csloxinfo.com>, <jerry@tr2.com>, "'amps'"
> <amps@contesting.com>
> Message-ID: <A2AD225EB3FC46D0A7DBD94B113D98CF@computer1>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> reply-type=original
>
> I use the U7 in a combined pair of surplused Eimac cavities for 903 MHz
> and
> have had no problem finding a pair of NIB spares for $20 each.
>
> Ive thought often about using them should my Alpha 76CA finally wear out
> the
> 8874's.
>
> One way would be to make a fingerstock grid contact mounted to the chassis
> as the first step after removing the 3 sockets. Properly done thats all
> that
> will be holding the tubes in place....a pressure fit.
> If you can find some shorted screen bypass rings for 4CX150/250 or
> 8072/8121/8122 style tubes or variants of either family you can quickly
> make
> a GG ring by soldering small copper strips between the finger stock and
> ground section in several places.
>
> The stock chimneys and exhaust stacks can be reused.
>
> Then attach some copper tubing or HB clips to the heater and cathode
> terminals and push on connector for the other heater pin; those will be
> the
> only things below chassis.
>
> Bias voltage "may" need to be changed as it runs 12V at 903 but at HF the
> stock value may be OK. At HF its still otherwise rated as an 8874.
>
> Since this will only be going to 10M rocket science isnt needed.
>
> A bit more work than 3CX800's but considering prices and future
> availability
> its an alternative to consider.
>
> Carl
> KM1H
>
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