[ARC5] Re: 1626 tubes

[Tino Zottola]

Hello Bob,

Actually, someone did come up with the idea of a
transformer-less audio amplifier more than 50 years
ago.

In 1952, Julius Futterman came up with the OTL (output
transform-less)amplifier consisting of many (6 to
12)triodes configured into two parallel groups driving
a speaker directly. The plates of group #1 were fed by
B+(about +150 volts), the cathodes of group #2 went to
B-(about -150 volts) and the speaker was connected
directly between the junction of the group #1 cathodes
& group #2 plates and ground. The circuit drive
speakers from 4, 8 and 16 ohms impedance depending on
many tubes are present.

I have seen several variations of the circuit using
cheap power supply regulator dual power triodes like
6080, 6082 and 6AS7G.

I have even saw a version with no power transformer
where the filaments are wired in series and the line
voltage is doubled to create B+/B- as was done with
cheap AC/DC television sets of the 1950's and 1960's.

Audio-fools complain about the circuit generating a
lot of heat (each tube has a 2 amp filament), tube
failure destroying speakers (normally functioning
tubes balance and 0 amps flows in the speaker with no
signal)and an aoverall so-so tonal performance of the
circuit.

Regards Tino, VE2GCE

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