The UBT-1 is a single-ended output transformer intended
to be used in high-fidelity vacuum-tube audio amplifiers.
It has a low primary impedance and a high primary
current capability, making it suitable for use with lowimpedance triodes or pentodes. Its relatively high power
and current capability is complemented by outstanding
frequency response characteristics.
The UBT-1 is manufactured in the United States to high
quality commercial standards. The core is grain-oriented
M6 (Microsil®) silicon steel. It is protected from moisture
by a full vacuum-impregnation with varnish. Every UBT-1
transformer is tested for: inductance, turns ratio, and
high frequency response.
triode-connected pentodes connected to the UBT-1 were
Total Plate Output
* – Plate Voltage is the voltage between plate and cathode or filament.
The total DC plate supply voltage should also include about
25-30 volts for voltage drop in the UBT-1 primary, and drop
across the cathode bias resistor, if any.
Primary impedance: 1600 Ω
† – Both sections paralleled.
Secondary impedances: 4, 8, and 16 Ω
The UBT-1 can also be used with low-impedance pentodes
or beam power tubes. A 13 watt amplifier has been
successfully built using a 6KG6/EL509 running 270 volts
on the plate, at 150 ma, in “enhanced-triode” mode.
Maximum DC primary current: 160 mA
Maximum output power: 13 to 18 Watts (depending on
minimum expected low frequency response).
Primary resistance: 165 Ω nominal, 25° C
Primary inductance: 8 Henries nominal (at 160 mA)
Frequency and phase response: see graphs on next page.
Size: 3.375" x 4.25" x 2.85" (95 x 108 x 86 mm)
Weight: Approx. 5 lbs (2.25 kg)
The UBT-1 transformer is a low-impedance transformer
that is designed to be used with high-current, low-voltage
tubes. It is definitely not suitable for use with tubes such
as 10, 211, 845, etc. A computer program was written to
simulate a single-ended triode output stage. Using this
program, operating points for the following triodes or
For best sound, it is recommended that the output
transformer be driven by an impedance somewhat below
the impedance that gives the maximum power output.
Also, be aware that the optimum output impedance for a
triode power amplifier is highly dependent on the plate
supply voltage, the optimum impedance rising as the
Due to the high current flowing through the primary, it
is normal for the UBT-1 transformer to become warm
during operation. It is strongly recommended that a fuse
be used in the B+ connection to the UBT-1 transformer.
The relatively high voltage drop across the primary could
prevent the AC line fuse from blowing if the output
tube shorted or lost bias. A B+ fuse will help protect the
UBT-1 data sheet rev 1.0
UBT-1 Single-Ended Audio Output Transformer
transformer, as well as increase safety. A suggested value
would be approximately twice the worst-case expected
As with any output transformer, the UBT-1 should always
have a load connected when driven. If there is no load
and the output tube is over-driven, the high inductance
of the primary will cause extremely high voltages to
be produced, damaging either the output tubes or the
The maximum power available from the UBT-1 is
primarily limited by core saturation at low frequencies.
At 15 watts output, the core begins to saturate below
approximately 50 Hz. However, users of the UBT-1
Serial No. 0069
dB relative to 1KHz
Even though the UBT-1 transformer can be the basis for
excellent sounding amplifiers, it is meant to be used with
relatively efficient speakers. If used with inefficient speakers
(less than 85 to 90 db/watt), used in a very large room, or
used to play at deafening levels, a 10 to 15 watt amplifier
will have insufficient power. That said, a single-ended
triode amplifier, particularly without feedback, can sound
much louder than its power ratings suggest.
PHASE SHIFT (degrees)
Audio Precision System 2
Phase shift in degrees
report no lack of bass repsonse in actual usage for
amplifiers of about 13 watts output. If the UBT-1 is
being used in the high-frequency amplifier of a bi- or
tri-amped system, then more power can be obtained,
although the maximum current specification should not
be significantly exceeded.
Note: This response is for a typical transformer - the actual response may vary, especially at the high frequencies.
4.25 " (max)
108 mm (max)
Although the primary B+ lead
is the same color as the 16
ohm output lead, it is grouped
with the blue plate lead.
12 " (305 mm)
0.17 " (4.4 mm) dia.
One Electron Transformers are exclusively distributed by:
Antique Electronic Supply
P.O. Box 1000
Yerington, Nevada 89447 USA
6221 South Maple Avenue
Tempe, Arizona 85283 USA
FAX: 1-480-820-4643 or 1-800-706-6789