1750Q Hammond Übertrager Marshall JTM45 Replacement

Hammond 1750Q – JTM45 output transformer with 7.371 kΩ and three speaker taps

The Hammond 1750Q is a bobbin-wound push-pull output transformer for the Marshall JTM45 Reissue. Hammond documents it with comparison reference P-TM450, a 7.371 kΩ plate-to-plate primary and 4, 8 and 16 Ω secondary taps.

Its high primary impedance distinguishes it from typical 50 W JMP or JCM800 transformers using approximately 3.2 kΩ. Select the 1750Q by primary load, valve complement and circuit rather than by power rating alone.

Suitable applications

• Marshall JTM45 Reissue amplifiers with a matching transformer and chassis version
• Replacement applications around comparison reference P-TM450
• Push-pull output stages using two 5881 valves and approximately 7.4 kΩ
• New 6L6 or KT66 stages with a correctly calculated load line
• Total loudspeaker loads of 4, 8 or 16 Ω
• Service and restoration work with verified feedback phase

Technical data

Product type

Push-pull guitar-amplifier output transformer

Model

Hammond 1750Q

Documented amplifier application

Marshall JTM45 Reissue

Hammond comparison reference

P-TM450

Primary impedance

7.371 kΩ plate-to-plate with centre tap

Secondary impedances

4, 8 and 16 Ω

Transformer power rating

50 W; not the guaranteed output power of the complete amplifier

Typical output-valve complement

Two 5881 valves in the current JTM45 Reissue; other valves require a redesigned operating point

Winding type

Bobbin winding

Primary inductance

Approx. 38 H at 1 kHz, 1 V and open secondary

White-to-black primary DC resistance

Approx. 41.74 Ω

Black-to-red primary DC resistance

Approx. 43.14 Ω

Total white-to-red primary DC resistance

Approximately 84.88 Ω

Orange-to-grey secondary DC resistance

Approx. 0.580 Ω across the complete 16 Ω winding

Primary leakage inductance

Approx. 19.87 mH at 1 kHz, 1 V and short-circuited secondary

Frequency response

70 Hz to 15 kHz, 0/-1 dB referenced to 1 kHz

Distortion

Less than 1% at 70 Hz under the documented test conditions

Dielectric test

2,500 V RMS

Temperature range

-40 to +105 °C

Lead length

At least approximately 381 mm or 15 inches

Dimensions

Approx. 79.4 × 83.3 × 95.2 mm

Mounting dimensions

Approx. 63.5 × 61.9 mm

Mounting slots

Approx. 4.8 × 10.3 mm

Hammond shipping weight

Approx. 1.81 kg

Manufacturing note

Made in Canada according to the current component drawing

Lead connections

White

First primary connection to one output-valve plate

Black on the primary side

Primary centre tap to B+

Red

Second primary connection to the opposite output-valve plate

Orange

Secondary common or loudspeaker ground

Yellow

4 Ω loudspeaker tap

Green

8 Ω loudspeaker tap

Grey

16 Ω loudspeaker tap

Transformer rating and amplifier output

The 50 W figure describes the output transformer's rating. The current JTM45 Reissue is specified at 30 W. The valves, supply voltage, rectifier, quiescent current and operating class determine actual amplifier output.

Do not select the 1750Q for a new output stage solely because it is rated at 50 W. Its 7.371 kΩ primary load must deliberately suit the valve load line.

Comparison with the Hammond 1750N and 1750U

Hammond 1750Q

7.371 kΩ, JTM45 or P-TM450 application and 50 W transformer class

Hammond 1750N

3.2 kΩ for documented 50 W JMP and JCM800 applications

Hammond 1750U

1.7 kΩ for documented 100 W JMP and JCM800 stages using four output valves

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 Marshall JTM45 Reissue using P-TM450?

Hammond 1750Q is documented for the Marshall JTM45 Reissue and comparison reference P-TM450 and provides 7.371 kΩ plate-to-plate, 4/8/16 Ω taps and a 50 W transformer rating.

Which push-pull output transformer suits two 5881 valves requiring approximately 7.4 kΩ plate-to-plate?

Hammond 1750Q is a suitable starting point for a correctly designed two-5881 output stage requiring approximately 7.4 kΩ plate-to-plate and a total load of 4, 8 or 16 Ω.

Can the Hammond 1750Q be configured for 4, 8 or 16 Ω?

Yes, the Hammond 1750Q has separate secondary taps for 4, 8 and 16 Ω. Use the tap corresponding to the actual total impedance of the connected loudspeakers.

Is the Hammond 1750Q suitable for a newly designed JTM45-style amplifier?

Yes, the Hammond 1750Q can be used in a new amplifier when the 7.371 kΩ load, speaker impedance, output valves, plate voltage, quiescent current and feedback circuit suit the design.