Andre Jute
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 Posted:
Wed Dec 29, 2004 3:38 am Post subject:
Bedside and booster nostalgia, thanks Carlo! was Preamplifi |
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Carlo <masunta@wind.it> wrote:
| Quote: | http://www.audiocostruzioni.com/a_d/elettroniche/pre-marco-211-2/pre-marco-211-2i.htm
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I use a 211 as a bedside lamp. That filament glows nicely!
But I must admit I am tremendously impressed with its use as a preamp. I don't
like the 211 as much as the 845, mainly because to get any real power you have
to run it in A2 whereas the 845 delivers all its power in A1. The 845 is also
intrinsically both more linear and warmer. (1) However, for small powers the
211 is one of that handful of superbly linear amplification devices known to
man that tubies rightfully revere (300B, 845, 6SN7).
Furthermore, there is something that is often overlooked by amateur tubies,
which is simply that a driver tube requires power or the output section of
the amplifier will be bandwidth limited. The calculation is the same as for
slew rate. Even a modest 300B requires 6mA or so on the plate of the driver,
is happier with 8mA, and works better still with 10mA. Bigger tubes like the
845 require up to 20mA on the driver plate to work properly. This is the reason
I fall about laughing every time I see an 845 driven by a 12AX7; I know it
will sound like shit before I even switch it on. I can provide math if anyone
is interested.
This is the reason for booster amps. I use a version of my KISS amp with a
switch on the output as a booster amp for my 70W SE SV572-3 and -10 amps. A
booster amp is nothing but a poweramp used as a preamp. The switch turns the
output transformer into a choke by disconnecting the secondaries and cuts in
a cap to couple the 300B to the SV 572-xx grid. It sounds superb.
If you kids want to try the bedside lamp 211 or 845 or SV572 at home, remember,
you connect only the filaments, not the kilovolt to the plate.
Andre Jute
(1) I may just be prejudiced by experience. As a boy between school and college
I travelled the length and breadth of the US on one of those Greyhound 99 dollar
go-anywhere six-month tickets, for the full six months. (I'm often told those
tickets were 199 dollars but my memory is firm: 99 bucks.) Before I was even
out of Boston, I won a fine Sony radio in a dice game. It was my companion
for six months when there was no one interesting on the bus. I listened to
a lot of local stations. Smoky jazz. When I got into tubes many years later,
the 845 reproduced that sound best. I learned, on RAT I believe, that many
of those stations used 845 as their transmitting tubes. I also came into all
the tubes and transformers for setting up a forces radio station in Berlin
during the airlift, left in Cork by the Army Air Transport Command when suddenly
the crisis was over, sitting in a warehouse for decades until I rescued what
I wanted.
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