227 Audio Amplifier

Audio amplifier

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.