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UBT-1 Single-Ended Audio Output Transformer Description 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. Specifications triode-connected pentodes connected to the UBT-1 were determined: Plate Total Plate Output Quantity Tube Voltage* Current Power 3 2A3, 6B4G 295 V 140 mA 11.5 W 2 300B 350 V 155 mA 14 W 2 6CA7/EL34 350 V 115 mA 8W 3 6CA7/EL34 400 V 150 mA 14.5 W 4 6W6GT 275 V 130 mA 10 W 4 6BQ5/EL84 305 V 120 mA 8W 4 6CW5/EL86 275 V 130 mA 10.5 W 2 6550/KT88 385 V 140 mA 10 W 1† 6AS7G/6080 210 V 115 mA 8W * – 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) Application Information 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 voltage rises. 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 www.one-electron.com 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 operating current. 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 transformer. 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 +1 +50 Serial No. 0069 +40 AMPLITUDE (dB) +0 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. +30 +20 -1 -2 -3 -4 8 tap 1 +0 4 tap -10 800 -5 -6 -20 -30 Black Brown -7 -8 +10 PHASE SHIFT (degrees) 50 100 200 500 -40 16 tap Audio Precision System 2 20Hz Phase shift in degrees +2 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. 1KHz 2K 5K 10K 20K 50K 200KHz -50 Note: This response is for a typical transformer - the actual response may vary, especially at the high frequencies. 4.25 " (max) 108 mm (max) Blue Plate Brown 16 Ω Green Yellow Black B+ 8Ω 3.375 " 86 mm 4Ω 3.0 " 76 mm 3.75 " 95 mm 2.25 " 57 mm COM 2.75 " 70 mm Brown Although the primary B+ lead is the same color as the 16 ohm output lead, it is grouped with the blue plate lead. Lead Lengths: 12 " (305 mm) Primary Leads Secondary Leads 2.85 " 72 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 www.one-electron.com 6221 South Maple Avenue Tempe, Arizona 85283 USA Tel: 1-480-820-5411 FAX: 1-480-820-4643 or 1-800-706-6789 www.tubesandmore.com