Here’s an
amplifier circuit that based on the BUZ11 can
be replaced by an IRFZ34N and an ECC83 can be used instead of the ECC88. In
that case the anode voltage should be reduced slightly to 155 V. The ECC83 (or
its US equivalent the 12AX7) requires 2 x 6.3 V for the filament supply and
there is no screen between the two triodes, normally connected to pin 9. This
pin is now connected to the common of the two filaments. The filaments are
connected to ground via R5. If you’re keeping an eye on the quality, you should
at least use MKT types for coupling capacitors C1, C4 and C7. Better still are
MKP capacitors. For C8 you should have a look at Panasonic’s range of audio
grade electrolyts. P1 is used to set the amount of negative feedback. The
larger the negative feedback is, the flatter the frequency response will be,
but the smaller the overall gain becomes. This is the figure of the circuit;
With
P2 you can set the quiescent current through T2. We have chosen a fairly high current
of 1.3 A, making the output stage work in Class A mode. This does generate a
relatively large amount of heat, so you should use a large heat sink for T2
with a thermal coefficient of 1 K/W or better. For L1 we connected two
secondary windings in series from a 2x18V/225 VA toroidal transformer. The
resulting inductance of 150 mH was quite a bit more than the recommended 50 mH.
However, with an output power of 1 W the amplifier had difficulty reproducing
signals below 160 Hz. The distortion rose to as much as 9% for a signal of 20
Hz at 100 mW. To properly reproduce low-frequency signals the amplifier needs a
much larger coil with an iron core and an air gap. This prevents the core from
saturating when a large DC current flows through the coil.