The Harmony 189 schematic

 I've had a small Harmony model 189  amp here for repair and I quickly learned there's no schematic available. There were the fragments of one pasted inside the cabinet but the most important parts of it were long gone.

After some looking around I found that the amp was similar in tube lineup to the later Harmony 303-that is, 12SQ7-50L6-35Z5 and the RCA manual tells me this should produce a rockin' 3.8w of raging power.

So I drew up a schematic for the use of interested folks and here it is.

Unlike other series string filament amps both the H189 and the H303 and H303A utilized an isolation transformer which was a significant safety feature. That's not the case with other so called "widowmaker" amps like the 303B which have a live chassis and no isolation. Those amps are unsafe and can deliver a nasty jolt if provoked.

This idea was an outgrowth of what the radio folks call the All American Five-a five tube superhet radio circuit developed by RCA in the prewar era and popularized after the war with a cascade of tabe radios that were cheap and cheerful. The idea was simple. String together filaments to amount to about 117 volts and you can plug right into the wall sans transformer of any kind, thus 12SA7-12SQ7-12SK7-35Z5-50L6.

Of course with guitar amps you don't need the radio stuff so you can omit the 12SA7 and 12SK7 but you need a big resistor to make up the difference and you still have juice on the chassis. If it was a plastic radio that sat on the shelf who cares? But you're connected directly with a guitar cable or worse a microphone.

The only cure is an isolation transformer, and these are readily available. The Triad N68X can be bought from Newark Electronics for as little as fifteen dollars.