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Hello
Daniele,once I used the 12au7 as mixerstage in an old tube
broadcastreceiver with very good results,much more sensitive and less IM than
the usual ECH81.I never tried it but my guess it 's very good for a hi-Z
pre-amp
73
Victor
PS.no
semiconductor is a match for a warm glowing glass tube.
Hello Victor, thanks for your reply. In fact, now I think I will
use the 12AU7 for a hi-Z pre-amp for a whip or a tuned loop (this again is
just for playing with that old glassy lady :-)). As for the BD139, I didn't
even suspect it could be used in a RF preamp, I tought of it as good for power
stages...
73
Daniele IZ5WWB
Hello Daniele,
some months ago I made a broadbanded ferriterod to cope
with heavy qrm on the 80 and 160m band.The ferite comes from a submarine and
is specified 10KHz to 4 MHZ with a symmetrical feedingcable to a ELK675 or
other receiver without a preamp.Since I don't have such a receiver I made a
balanced two stage amp with 4x BD139 grounded base.The antenna copes good with
the QRM on 80 and 160,it also performs very well on the MW broadcastband.In
the range 200-500KHz it is sometimes better than the E-probe,below 200KHz the
E-probe wins.IM is no problem but maby the noise of the amp is to high below
200KHz.
The ferriterod is about750X100mm!
I don't think the 12AU7 is a good choice for a low
inputimpedance amp...Zinput 2X BD139 is about 3 Ohm.
73
GL
Victor
PA3FNY
Hello LF,
I apologize to you all for resuming a topic that was already largely
debated here. I have seven of those quite known russian surplus ferrite
rods (20cm x 1cm) and - just for my fun - I'm planning to spend some
time in building an LF-MF antenna with them. I would like to obtain
a broadband (untuned) antenna, ideally VLF-LF and/or LF-MF,
suitable for outdoor installation, so what I would appreciate some
suggestions about is the required low-impedance amplifier, similar to those
used in air-core broadband loops, if it does make sense of course. Any
ideas? I found an unused 12AU7 in my drawer, could it be
effectively chosen as the active device (low
voltage) for such amplifier?
Thank you for any feedback
73
Daniele IZ5WWB
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