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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