1751A Hammond Übertrager Gibson GA15 8k 5,3/8/16

Hammond 1751A – Gibson GA15 output transformer with 5.3, 8 and 16 Ω taps

The Hammond 1751A is a push-pull output transformer for Gibson Gold Tone or Goldseal GA15 and GA15-RV amplifiers. It provides 8 kΩ plate-to-plate, a 15 W rating and secondary connections for 5.3, 8 and 16 Ω.

Hammond lists 73-TRAN-15W-OP2 as the documented comparison reference. The unusual 5.3 Ω tap is application-specific and distinguishes the 1751A from common universal output transformers.

Suitable applications

• Gibson Gold Tone or Goldseal GA15 amplifiers
• Gibson GA15-RV amplifiers with matching reference and mechanics
• Replacement applications for 73-TRAN-15W-OP2
• Push-pull output stages using two EL84 valves and approximately 8 kΩ
• Total loudspeaker loads of 5.3, 8 or 16 Ω
• New amplifiers specifically requiring the unusual 5.3 Ω tap

Technical data

Product type

Push-pull guitar-amplifier output transformer

Model

Hammond 1751A

Documented amplifier applications

Gibson Gold Tone or Goldseal GA15 and GA15-RV

Documented comparison reference

73-TRAN-15W-OP2

Primary impedance

8 kΩ plate-to-plate with centre tap

Secondary impedances

5.3, 8 and 16 Ω

Transformer power rating

15 W

Amplifier topology

Push pull

Typical output-valve complement

Two EL84 valves in total at a suitable operating point

Winding type

Bobbin winding

Red-to-blue primary inductance

Approx. 37 H at 1 kHz, 1 V and open secondary

Red-to-white primary inductance

Approx. 10.3 H at 1 kHz, 1 V and open secondary

Red-to-blue primary DC resistance

Approx. 300.7 Ω with ±20% tolerance

Red-to-white primary DC resistance

Approx. 126.9 Ω with ±20% tolerance

White-to-blue primary DC resistance

Approx. 173.9 Ω with ±20% tolerance

Brown-to-orange secondary DC resistance

Approx. 0.445 Ω at the 5.3 Ω tap

Brown-to-yellow secondary DC resistance

Approx. 0.548 Ω at the 8 Ω tap

Brown-to-green secondary DC resistance

Approx. 0.797 Ω at the 16 Ω tap

Red-to-blue primary leakage inductance

Approx. 31.70 mH at 1 kHz with the secondary short-circuited

Red-to-white primary leakage inductance

Approx. 15.44 mH at 1 kHz with the secondary short-circuited

Frequency response

40 Hz to 15 kHz within ±1 dB referenced to 1 kHz at 27 dBu drive

Dielectric test

2,500 V RMS

Temperature class

Up to +105 °C

Lead length

At least approximately 203 mm or 8 inches

Dimensions

Approx. 66.7 × 65.1 × 55.6 mm

Mounting slots

Approx. 4.8 × 6.3 mm

Weight in the type-specific data sheet

Approx. 0.60 kg

Lead connections

Red

First primary connection to one output-valve plate

White

Primary centre tap to B+

Blue

Second primary connection to the opposite output-valve plate

Brown

Secondary common or loudspeaker ground

Orange

5.3 Ω loudspeaker tap

Yellow

8 Ω loudspeaker tap

Green

16 Ω loudspeaker tap

Selecting the secondary tap

Use only the connection matching the total impedance of the connected loudspeakers. Insulate the remaining leads separately.

The 5.3 Ω tap is not equivalent to a conventional 4 Ω output. A new design must deliberately match the actual loudspeaker impedance to the selected tap.

Selection for a new amplifier

The nominal 8 kΩ plate-to-plate load must suit the load line of both output valves. Assess plate voltage, quiescent current, screen voltage, cathode or fixed bias and intended output together.

Safety notes

Lethal plate and signal voltages are present on the primary. Install the open transformer with complete touch protection and fully discharge the amplifier before service work.

Questions about selection and use

Which output transformer suits a Gibson GA15 or GA15-RV using 73-TRAN-15W-OP2?

Hammond 1751A is documented for the Gibson GA15 and GA15-RV and comparison reference 73-TRAN-15W-OP2 and provides 8 kΩ plate-to-plate, a 15 W rating and 5.3, 8 and 16 Ω outputs.

Which 15 W output transformer provides 8 kΩ with 5.3, 8 and 16 Ω taps for two EL84 valves?

Hammond 1751A combines an 8 kΩ primary with three secondary taps and is a suitable starting point for a correctly designed two-EL84 push-pull output stage.

Can the Hammond 1751A be configured for 5.3, 8 or 16 Ω?

Yes, the Hammond 1751A has separate secondary connections for 5.3, 8 and 16 Ω. Use the connection matching the actual total loudspeaker impedance.

Is the Hammond 1751A suitable for a newly built EL84 push-pull amplifier?

Yes, the Hammond 1751A can be used in a new amplifier when the 8 kΩ load, 15 W rating, loudspeaker impedance, plate and screen voltage and quiescent currents suit the EL84 circuit.