Post by Bret LudwigScratchbuildiong is the most educational and yet the least frustrating.
I'm totally with Bret here - if you have the courage, build from
scratch.
This is true, in very limited circumstances. Ludwig is a technician who
has long since forgotten how much time he spent learning the necessary
electronics. Andy is, if I recollect correctly, a psychologist, as I
am; he is such a longtime hobbyist that he too has forgotten when he
learned his electronics and how hard it was. I am now in the same boat:
it is no problem for me to design just the amp I want at any time, and
to knock it up with chasses and parts I have, and to develop and tune
it just so. But I think that, if I hadn't started with kits, I would
have given up in frustration, trying to learn the basics of electronics
and the ins and outs of components and construction all at the same
time. A stepwise approach makes more sense; that is why so many people
learned on secondhand gear and I now recommend starting with a kit or a
cheap, developable Chinese amp.
My own recipe for a starter kit is at the bottom. Meanwhile a few notes
Post by Bret LudwigIt's what you'll end up doing so you may want to cut out the
intermediate step of rebuilding a Dynaco or Leak amp (interesting and
instructive as it is - I think we've all been there). Before doing
anything, buy "Valve Amplifiers" by Morgan Jones off Amazon (e.g.) and
read it and use it as a reference all the way through your build.
You need two copies of each of Jones and the RDH so you can keep one in
your lavatory or beside your bath or on your bedside table, wherever
you do most of your reading.
http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS%20Bookshelf.html
The kernel of Andy's advice is mad, magnificently mad but mad none the
less. Since I'm an economist as well as a psychologist, you'll forgive
me if I point out the obvious, which is that Mad Andy Evans wants you
to build with tubes that are no longer made, in limited supply, already
very expensive, likely to become more expensive. Nobody can accuse me
of speaking up for anything less than excellence, or of being cheap but
advising newbies to build with obsolete DHTs is one fast way to drive
them screaming from the hobby.
Still, big chasses and octals is good advice. Here is an amp for all of
a third of a watt...
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Post by Bret LudwigYou want to make some basic decisions. These are the ones I recommend.
a) Chassis. Think BIG, then BIGGER. If you are seriously going to carry
on building you may like to start with two chassis - one power supply
and one circuit. Your absolute minimum chassis size is a 2U 19"
rackmount case - get a solid one, if possible with one side predrilled
full of holes. You can use two of these - power supply and circuit.
Your power supply should be approximately 300vDC for the HT at a
minimum of 200mA. Feel free to chuck in as many chokes as you like -
you can use 100mA ones and split the power supply into two channels.
Use one set of chokes for the output and another for the input - four
chokes of 100ma is nice. Forget electrolytics - buy ordinary motor run
polypropylene caps, sizes 40uf are fine. Start with solid state
rectification, but leave room for valve rectifiers which you may want
to add later. For umbilical use 8 pin Neutrik Speakons or Amphenol
sockets.
b) Single ended or push pull. I'd suggest push pull - possibilities of
many many circuits to copy and higher output. Can be as good as
anything if you use DHTs (directly heated triodes). I'd go a step
further and suggest balanced all the way through, with three stages. In
Morgan's book you'll find some constant current sinks to build which
you put under each of the first two stages (page 134). If you have a
single ended input, normal phono plugs, ground the second grid of the
first valve (again in the book).
c) Get some decent output transformers. start with Hammond or something
simple, but leave plenty of space to try something else. I'd suggest 5k
to 6.6K in push-pull.
d) Valves to use. If you want to aim for very good, use all octal
sockets - don't even consider the smaller nine pins. If you want to aim
for the best use all UX4. Plan your chassis layout for twelve UX4
sockets - big holes here. Get the right hole saw to put in your drill
to drill out these holes, and imagine that each valve is 50mm across,
so allow at least that plus another 15mm - 20mm space in between
valves. This is why I'm talking big chassis. You are now ready for your
super amp. The valves you can use go from 76 which is normal 6.3v
heaters to fancy stuff like 26, a DHT, which will blow your mind away,
but need special DC supplies. Get some cheap bench power supplies and
use 26s - you'll just love them. They take 1amp on the heaters so any
bench power supply that does 0-30 at 1amp is OK. Run the heaters in
series in push pull. Or stay simple - one huge toroid at 6v will power
a whole lot of valves. For output you can start with 807 in triode,
using UX5 socket - same cutout as UX4 so you can swap later for a 2a3
or 300b.
e) use a three stage circuit - plenty about. Go to www.audioasylum.com
and hang out there and ask for suitable circuits, or work one out from
Morgan's book. Look for circuits for a 6SN7 and substitute two 76 for
one 6SN7.
This may all be too advanced, but if you take the plunge and build big,
you will eventually make your super amp using the same chassis and
componants you started off with - nothing wasted, nothing to throw away.
Okay, here we go with a sensible, flexible setup for a starter kit for
a beginning DIYer.
First, decide on SE or PP. I suggest PP for the flexibility and
slightly lower cost. (SE can be cheap too--
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--but unless you have a lot of money it can be very limiting.)
Next buy two Hammond 17x10x4 inch ali boxes and six or so 17x10 covers
and about a dozen each of 6x4 and 4x4 ali plates (they're covers for
smaller boxes you won't buy). Buy some slotted 3/4 in angle ali. Brace
the two flimsy Hammond boxes with the ali on all edges. Bolt ali angle
strips across the width four inches apart. The less than an inch open
at end will be to lead cables in while you do development work. Don't
be tempted to buy steel cases; they'll just skin your knuckles and
blunt your tools and discourage you before you even start work. These
two boxes are in any event just covers for electronics. It is easier to
build on the big flat coverplate stiffened by ply, and then just to
bolt on the box as safety cover. But for the time being you will build
on the little plates section by section and bolt them onto the
framework. One box is your power supply, the other the amplifier.
Decide to use only octal tubes. The 6SN7 is greatest signal tube and
small power tube too that God ever made, and the 6SL7 the sweetest. The
EL34 is the greatest power tube God ever made, and that includes the
300B. The 6SN7 is in production in Russia and China, and so is the
EL34; the 5SL7 is in plentiful cheap supply and equivalents are also
available. These are just samples: there are many more octal tubes
worth your attention for one reason or another, like the KT88/6550 for
more power, for instance. All the really desirable tube rectifiers are
octal.
A stereo PP amp needs a minimum of four octal sockets on the chassis (I
will publish such a design in the next few weeks) but all you'll drive
with it will be a pair of headphones or some small multimedia speakers.
A realistic stereo PP amp needs at least input/gain, phase splitter and
power tube sockets, which adds up to a minimum of six sockets for
single-ended inputs. For balanced inputs, or at least long-tail pairs,
which open the way to balanced input, and differential gain stages, you
need two sections or one whole envelope per stage, so now you need
eight sockets for the signal section. You might want a buffer or an
extra gain stage for some reason, so add two more sockets for a grand
total of ten. Unused sockets can just sit there. You fit these sockets
two to a 4x4 ali plate (you clamp them all and drill them all five
plates at once) or even six to a 6x4 ali plate -- it depends on what
circuitry you want to attach to the socket pins and standoffs or
component strips you also bolt to the same plate. Each stage or group
of stages is built complete on the plate so that it has only wires
leaving for signal in and out, power and earth (to a star ground near
the mains input). The complete plate is bolted onto the angle ali over
the open end of your cover box. Input and output and control circuitry
also goes onto these or separate plates; you don't drill the main box
for the RCA, five-way connectors, mains input, switches and volume pot
until you have a humfree arrangement.
The power supply needs four sockets in case you ever want to build a
rectifier bridge. You won't. The Graetz bridge with one fullwave
rectifier and two diodes that I show in many of my circuits--
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--works indistinguishably from a full tube rectifier bridge, but the
other sockets will be handy if you ever want to build a tube regulator,
or for separate power takeoffs or for plugging in relays. You can buy
plugs to fit octal sockets and they're well rated and cheap, an easy
solution to a perennial problem. Some come with a big cover for
building silicon electronics inside and I have used them for building
constant current sinks.
Next you need very, very flexible transformers. In the States UBT sells
a universal transformer roughly equivalent to the old Tango 808 but of
course vastly cheaper. In Europe it isn't really worth going for
anything less than cut C-core Lundahl transformers, which are a couple
of magnitudes up the scale from the UBT; Lundahls may seem a little
pricey at first but they are the best-sounding transformers in the
world, the top models are the most flexible, and in that light they are
a stunning bargain. Buy the Lundahls with multiple windings on the
secondaries which can be connected to reflect a vast range of primary
impedances and connected to virtually any speaker impedance. Or, if you
know you will stick with the stunning EL34 and its relatives, in the
States you can buy Plitron transformers designed by Menno van der Veen;
the same design is sold by �mplimo in Europe. A pair of Lundahls or
Plitron/Amplimo vanderVeens will last your lifetime. Menno has
published some really good amp designs to use with these transformers;
Plitron sells his book with these designs--
http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS%20T%26T.html
--and also a PCB to speed your work if you're in too much of a hurry to
hardwire.
A Lundahl LL1651 power transformer is beefy, well-made, silent, and
with four 3.1A filament supplies and a 250-0-250V supply of 430mA is
very flexible. The Lundahl mains and output transformers all fit the
same bolt footprint and their bases are tapped, so you just drill three
4x4 plates all at once.
A choke is a really good piece of iron to have. The Lundahl LL1638 is
very flexible since it is wound with two coils which can be used in
series for 10H/200mA or in parallel for 2.5H/400mA; it can also be
split across the positive and negative rails to give you a quieter
amp--
http://members.lycos.co.uk/fiultra/KISS%20134%20by%20Andre%20Jute.htm
http://members.lycos.co.uk/fiultra/KISS%20135%20by%20John%20Byrns.htm
Two chokes gives you the opportunity to investigate the attractions of
choke input power filters, which changes the sound appreciably. The
choke, or a pair, sits on a 6x4 plate together with holes for bolting
down the associated caps.
Now you're ready to design and build an amp. I suggest it be PP Class
A1 trioded EL34. That's simple to design and build, and will give you a
decent output. If it isn't enough power for your speakers, you can
rewire for class A/B and get 24 clean watts. If you need more than 20W,
you should build better speakers--
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You will eventually get there anyway when your amp will be so much
better than your current speakers that the speakers will become the
bottleneck in your system. There's a lot of design information here:
http://members.lycos.co.uk/fiultra/The%20KISS%20Amp%20INDEX.htm
You might also want a root around in my general hi-fi design site:
http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm
Good luck.
Andre Jute
Visit Andre Jute at http://members.lycos.co.uk/fiultra/
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