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
----- Original Message -----
From: "Martin Sole" <hs0zed@csloxinfo.com>
To: <jerry@tr2.com>; "'amps'" <amps@contesting.com>
Sent: Thursday, June 28, 2012 4:04 AM
Subject: Re: [Amps] 3CX400U7 in Alpha Linear?
> Jerry,
>
> I've seen the U7 tubes on occasion but whilst cheaper than the A7 8874
> variant I'm not sure they are really any more plentiful. How many sets of
> tubes do you expect to get through in say 20 years, 30 years?
>
> The question of re-tubing the 78 has been on my mind since I've owned one,
> best part of 10 years now. I don't think I'll do anything about it until
> the
> existing tubes die, which realistically might be a loooooong time. If it
> should come to pass then my pondering leads me as follows:
>
> The 78 is a decent RF deck, it's not perfect but it's pretty much all
> sorted. The bandpass arrangement is very convenient and probably the next
> best thing to a (much more expensive) auto tune amp. It's a one piece
> desktop box and it's pretty quiet both the fan and the switching
> especially
> in a radio room with CQ WW on 20 metres blasting out the speakers. If I
> put
> on the headphones the amp get magically even quieter! It's not the
> lightest
> or smallest but it makes my Alpha 77 look positively huge never mind the
> floor standing Henry and Collins.
>
> Given that I feel going to 3CX800's is really only a half-way house,
> certainly in this part of the world they are as rare as 3CX400's and a
> pair
> of 800's would run not much less than 3 similar 400's, sooo how about
> something a bit more radical. A single 8877.
>
> It needs a new plenum box. Well that's just some folded sheet aluminium
> with
> cut-outs/holes. The 8877 will fit if the box is made for it. The 8877 has
> a
> 6.3 volts filament as does the 8874 and though I haven't look at the
> sheets
> probably has a similar total current requirement. Hopefully the 8877 could
> use the same broadband input transformer that the 3 8874's use, might need
> a
> bit of rework. The big problem would seem to be the HT, the 8877 wants a
> lot
> more volts to play nicely. I think, though again this is just supposition,
> that the 78 transformer will work okay. The 3 8874's when pushed seem to
> be
> able to draw about 500mA each, that with the key down A0 style, in the SSB
> position the HT is at or around 2000 volts Converting the HT supply to a
> voltage doubler might be a feasible option. If it's possible to make 3500
> volts whilst drawing 600mA this should give about 1200 watts, similar to
> what I see key down with the 3 8874's.
>
> Evidently a lot more work than the 3CX800 idea but not I think
> unreasonable,
> if it would work. With 8877 availability likely to remain good for now it
> might just be worth it.
>
> Just an idea
>
> Martin, HS0ZED
>
>
>
>
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Jerry Kaidor
> Sent: 28 June 2012 05:30
> To: amps
> Subject: [Amps] 3CX400U7 in Alpha Linear?
>
> Hello,
>
> I've seen the 3CX400U7 out there pretty cheap. But those don't have
> pins, they have rings. Anybody considered replacing the 3CX400A7s
> (when they die) in an Alpha linear with the U7's? I'm thinking it would
> take a new RF deck - no big deal, it's just a metal half-box with the
> three
> tube sockets. I think the U7's are probably deeper underneath than the
> A7's.
>
>
> - Jerry Kaidor
>
>
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