ON
BIASING
Choosing among biasing
strategies is quite difficult. I've tryed several solutions. Eeveryone
have those pros and con's . At the end my opinion is that the chioce is
mainly a matter of taste and may depende on the tubes you're using and
in which stage of the amplifier you decide to put fixed/self biasing.
To make a long story
short:
-
with fixed biasing the
sound is more quick, and "aggressive", bass are thight and the power is
higher. Besides there are advantages in the costancy of the working point
( expecially the "mu" of the tube is more costant with power).
-
with self biasing the
sound is more "relaxed", power is lowered, distortion raises.
In Francesca I've tryed
to set the 2a3 with fixed bias, the power and the "quickness" sensation
increased but
the sound was less
clear.
I've convinced myself
that using fixed biasing in low level stages may be done only very carefully,
othervise the following stages amplify the "errors". Best way is to use
it only in power stages, where fixed biasing may help a lot in getting
all the power from the otuput tube and mantainig the correct load on it.
I think that the
reduction of trasparency may be connected with the fact that with
fixed biasing you're injecting a voltage on the grid. If the negative
generator you're using isn't "clean", the result may be a powerful but
"confused" sound.
Noise may come fron
the diodes youre using to get the negative voltages or from intermodulation
inside the power
supply due to changes in the load. A solution to this I haven't investigated
is using tube rectifiers in negative supply.
Besides If you're
a fan of fixed biasing, "Audio coils" and other suppliers make
"input coupling" trasformers. With them you can use fixed biasing very
efficiently also with the 5842/EC97. I'ven't tryed them. The suppliers
claims that overall distortion and amplification is improved.
The choice is up
to you. And if you're still undecided you can use the following simple
solution that allows to switch between one topology or the other.
The soluction is
depicted for the 2a3 but can be used with the SV572-10 or the 5842 provided
that the appropriate cathode resistor/cap is choosen for self biasing.
While for fixed biasing a 10ohm resistor can be used for any tube.
Simple isn't it???
. Remeber to switch off HT before changing.
:
FRANCESCA
99 WITH NO FIXED BIASING
I consider this
as my little tribute to "Sakuma" philosophy ;).
As you can see all
the stages (also 572-10) are self biased. The power is a bit reduced
but the sound is definitely "round" and relaxed.
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