This audio amp is a rebuild of my
original 3-tube audio amplifier. This amp conforms more closely
to how amplifiers were built in the late '20s. The exception to
this in this example is the 2 Arcturus blue #227 tubes. These
tubes were actually made in the '30s, however common globe 227's were
available in 1929. I do have some common globe tubes that work
fine in this amp. I am partial to the blue ones tho, as this amp
is built open breadboard style and I can show them off. I do need
to locate some kind of knob for this amplifier.
The circuit is a typical transformer coupled amplifier. The
primary of the first transformer is open ended, so any audio source can
be connected without regard to ground reference. This is helpful
when working into the output of a detector, as the headphone
connections of a detector are usually connected in series to the
B+. Although the 2-tube receiver will drive a horn, the amp gives
it a little more "uumph", especially on weaker signals. The
monitor will not drive the horn, so the amp is needed there. The
amp allows me to do T/R switching much faster than without it, as I
would have to run 2 separate pair of headphones, one hooked to the
monitor, the other on the receiver. Now all I have to do is
change the knife switch to switch the audio inputs to the amp.
I have a separate power supply to run the amp. A 2.5V AC winding
runs the filaments. A 135V winding is rectified by a modern tube
half wave rectifier followed by a choke input filter provides about
130VDC. A series of dropping resistors provide 45 volts for the
plates of the audio amp as well as the plates of the receiver.
The amplifier makes for a good utility amplifier providing enough power
to operate the horn speaker.