Push-Pull
6L6 Class A
This my first Power Amp Project I ever did. When you look into the scheme you will see that totally it has only 2 stage, the first stage is differental phase splitter using 12AT7 and the second stage is power stage using 6L6. It produces about 17 watt in pure A class operation. You woun't find NFB here because I don't like it.
Input Stage
The
input stage is differential input phase splitter using 12AT7 and supplied
by LM317 in the form of current regulator.
This kind of phase splitter has very good balance, stability and high power
supply rejection ratio. Beside that both of the output took from
same output, the plates.
There are many phase splitter design commonly
use by tube audio designer such as split loaded, side branch, floating
paraphase, and also differential type. Split loaded phase splitter is most
commonly use because it is very simple, only need one tube. But according
to my experience observing the character of split loaded phase splitter
I've found that it has some disadvantages :
- Unbalance, as the input frequency goes
up the output from both of the plate amplitude become
asymetrically.
- Refering to the both input of the
power stage that drove by the phase splitter, both output of the
phase splitter perform different
character.
Plate output performs high impedance
and narrow bandwidth, cathode output performs low impedance
and wide bandwidth
I apply 6L6 power pentode
in the output stage operate in pure class A and ultralinier mode.
I select 6L6 power pentode under two considerations
:
a) 6L6 power pentode provide better linearity
compare to another popular power tube such as EL34,
EL84, KT 88 etc.
b) 6L6 power tube is easy to find on the
market and the price also cheap compare to those valves.
Finally output transformer will connect the
output stage to loudspeaker with the 8 ohm impedance, and
the plate to plate impedance is 5 kohm.
