Pre-amplifier
Click here to see my previous preamp..
This
is my current pre-amplifier.It all started out as a " joke". On the
rubbish-shelf was a push-pull mono-amp (being used in the past for a subwoofer)
on the un-employment list. Also a pair Lundahl LL1660 interstages were spare.
It occurred to me that this was a waste... why not build anything else in that
chassis? Due to its mono-icy, almost nothing else than a pre-amp could be build
on that cahssis, using much of the same components.. One problem: I planned
to use the UX227 (which can handle a max of 250V on the plate) in a choke load
configuration with a power-transformer having a 540V center tap secondary....
So I decided to use some VR-tubes to come across that difference. The power
supply now features an AZ1 rectifier with mesh-plates, 12mF oil (GEC), a 10He/200mA
(Pamerco) choke, again 12mF oil (GEC) and then an OA3/OD3 combo/per channel.
This results in about 180V on the 227. Why 180V, the 227 can handle 250V: why
not use a OC3/OD3 combo? Well, I've been there and can tell you that 180V simply
sounds better! Less "stress", if I may put it this way.. The LL1660
is used as a choke-load together with an UltraTone 0,1mF capacitor as
output. The interlinks being used are a mere 40cm long (as you can see in the
pictures) and the two conductors are held apart to avoid any cable capacitance
as the output impedance is fairly high. The connectors used are Lehmo types.
These are very nice build Swiss connectors which plug both plus and minus in
and out at the same time (avoiding the big HUM at (de)connecting) and, more
important, they don't have the big clump of iron at the negative side which
phono plugs tend to have..
This was not as a joke as presumed..... This Pre-amp sounded
far more better than my current pre-amp.... And I mean way better! And
then to know that my (now previous) pre-amp has a separate power supply and
a mu-stage 30! This is really the way to go....
This pre-amp has been auditioned by several friends, as well
as visiting several friends, with the same result: Bingo! I'm already into tweaking:
DHT's? Faster power-supply? etc etc. Here
you can have a look at the schematic... (by the way: the 5R4GY is
being referred to as rectifier...)
Here an idea of
tweaking on this one...
PS: the wooden feet are worth giving a try: made of "underlayment" (used for floor building, made of pine..) Let me know how it sounds.
03-09-2002
Very nice! That is, the UX227 did a marvellous job, but, it is indirect heated......
So I found a real killer tube. It is a tube that was used in Telegraph centrals.
It is a Siemens Aa, a so called "Post tube". This means a really long
life time with very good reliability. The word is that they where guarantueed
for 10.000 hours, and that measurements at that point must still met certain
criteria. I have Aa's of both Valvo and Siemens. The Valvo was used first, and
the quality of that one decided me to lend two Siemens Aa's. They have the same
open structure as the Ba's, although it is not visible due to the metalspray
coating on the glassbulb. This tube has a very open sound! The Valvo has push,
authority and very sweet highs, but the Siemens adds a whole heap of openess!
This is a real winner! There is a real disadvantage though: With its 30K Rp
in a choke load configuration you must be very carefull with the interlinks....
But taken that into account: The Siemens Aa is my choice !
20-09-2002
Outch! Being impressed with the swapping of a Tamura for my
Lundahl LL1660 (soundswise), it occurred to me 
that
choosing the Siemens Aa on my preamp is like making the "last"or "definitive"
preamp as no other company except for Lundahl can provide me with a choke or
line-out/interstage 1:1 with reasonable induction.... Now that can not be! So,
still being number one, the Aa has to make room for another tube, solely for
the tweaking purposes..... Luckily I found four 4019A STC's. These are the successors
of the WE101 being manufactured in England at that time.... Also "Post
tubes"!! Very good building quality, and more important, also very good
sounding! Not as good as the Aa, but with some tweaking I think I can manage
them to sound even better... I ordered for some Iron at Tribute, which is underway
as I am typing... Those are a C-core powertransformer and two amorphous supply
chokes of 25He/20mA each. These are to replace the junkyard which is currently
on my preamp (see picture above). I added a picture of the 4019A to show the
looks of this tube, one thing at least that is better than with the Aa.....The
schematic of the preamp with the 4019A is here.
_______________