A
simple Class A Tube Amplifier
This is one of
the
simplest (and nicest) project i've ever made. I suggest it to all the
first-time
DiYers
In my opinion, Its
main advantages are:
-
low price and
good quality
-
components easy
to find
-
easy costruction
-
(above all)
lot of fun in changing parts or
experimenting
new tubes or changing bias point or driving tubes or... (see
modification
section in this page)..
The design has been
made by Nuova Elettonica, a famous italian distributors of DIY kits.
you can check the
price/availability in their site:
http://www.nuovaelettronica.it
The kit number is LX.1240 KIT
The issue number
is
182
Description: FINALE classe A a VALVOLA
Price: 185.000 lit. (ca. 100U$)
This little
monster
delivers about 8+8w using a pair of EL34 SE in ultralinear mode.
The output
thansformer
is rated 2300ohm and can be used to build a lot of project using famous
tubes, like 2A3 and 300B. Or with a couple of EL84 in parallel SE.
Also the supply
with its 330V dc of output can be used to power a lot of different
tubes.
Here is the main
Schema. The number written in pencil are the voltages measured in my
kit.
They're in pair beacuse i've a separate supply for each channel, so
every
amplifier is completely indipendent.
and
the power supply:
The part list is
follwing
N.B. there's an error in the components R10. the value should
be 150ohm 3w.
MODIFICATIONS
Here comes the
funny
parts..... ;), i've I've changed almost everithing, i suggest you try
it
to... you can experiment a lot using this kit end learn much more
too...
EASY/LOW COST
MOD.
-
TUBES I've tryed
several
types, but, leaving back the different suppiers (svetlana, chinese..)
one
of the best and easyest improvements is using a tetrode/triode instead
of a penthode. What you loose in bass power and
amplification
you gain in clarity and smoothness of music. Changing to tetrode is
very
simple: the tetrode version of the EL34 is the 6CA7
-
By changin the
heater
connection of the ECC82 (cut the shotcircuit of pins 4-5 and connect
them
to 6,3V, leave pin n°9 floating) you can experiment a broad range of
other tubes, including 6CG7 (the noval version of 6SN7) and ECC88/6DJ8,
ECC83 etc.. this is a nice way to taste the different "voice of tubes".
-
You can also
experiment
EL34/6Ca7 in triode connection: Simply remove R12 from the ultralinear
tap of the output trasformer and connect it (trough the resistror R12)
to pin 3 (anode) of the EL34.
-
you can also
try
to take away the feedback by cutting the loop or taking away the RC
network
R11-C8. To avoid ringing/oscillating is better to use low-amplification
factor tubes when doing this mode. I've tryed this with the 2A3 and it
was one of the best discover: no feedback gives "life" to music.
ADVANCED/COSTY
MOD.
-
Double the power
supply.
This is necessary to improve channel separation and bass response. To
tell
the truth I've do it in firrst place, before any other mod.
-
With separate
supply
trasformers you can do the last improvement:substitute output tubes
with
2A3. What you need are only tubes, sockets and a couple of resistors do
decrease heater voltage to 2,5V. (unless you use the 6,3V version of
2A3).
The scheme should
look
like this:
The power supply
should be changed as follows
The resistor RF
should
be calculate with ohm's law, allowing for some loss if you use long
wires
for the heather. I used 1ohm resistor each.
The bias poit of
the 2A3 can be choose from the sheets. I used a bias point published in
another project, this used Rk=820/2W. For CK you can use any low-z cap.
Value must be 100-470uF. At the end I choose a network of 10X5,6uF
mylar
capacitor.
I've also paralleled
the supply caps with 30uF paper-oil... just to get the taste..
LISTENING
IMPRESSION
Not much to
say...
after building it and the first series of (low cost mods)
me/myfriends
began to belive that valves had a good chance to be the right device
for
sound... after I put the 2A3 and took away the feedback we had no
doubt... tube were the winners...
From then on,
we think that building DIY amplifiers with tube was the right
direction....
that lead me to "Francesca" and the 572-10.... and my friends to
"The Triodino" and 300B........ but this is another
story....
:).
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