1750D Hammond PP Übertrager Fender Tone-Master 026478

Hammond 1750D – 100 W PP output transformer for Fender Tone Master 026478

The Hammond 1750D is a 100 W push-pull output transformer providing approximately 2.35 kΩ plate-to-plate and secondary taps for 4, 8 and 16 Ω. Hammond documents it for the valve-powered Fender Tone Master and lists reference 26478.

The output stage uses four 5881 or 6L6WGC valves in total. A separate galvanically isolated feedback winding can be incorporated into local feedback where required by the original circuit.

Suitable applications

• Repair of a Fender Tone Master using reference 026478 or 26478
• Documented Twin Red Knob applications with a corresponding circuit
• 100 W parallel push-pull stages using four 5881 or 6L6WGC valves in total
• New PP amplifiers requiring approximately 2.35 kΩ plate-to-plate after full load-line calculation
• 4, 8 and 16 Ω loudspeaker loads
• Circuits using the separate feedback winding with the correct phase

Technical data

Product type

Push-pull guitar-amplifier output transformer

Model

Hammond 1750D

Documented equipment application

Fender Tone Master; the Hammond series list also names Twin Red Knob

Reference number

Hammond: 26478; BTB format: 026478

Primary impedance

2.35 kΩ plate-to-plate with centre tap

Secondary impedances

4, 8 and 16 Ω

Transformer power rating

100 W

Documented output-valve count

Four 5881 or 6L6WGC valves in total

Output-stage topology

Parallel push-pull with two valves in each half

Primary inductance

Approx. 10.2 H at 1 kHz, 1 V and open secondary

Primary DC resistance

Approx. 67.91 Ω between brown and blue

Secondary DC resistance at 4 Ω

Approx. 0.300 Ω between black and green/yellow

Secondary DC resistance at 8 Ω

Approx. 0.690 Ω between black and green

Secondary DC resistance at 16 Ω

Approx. 0.860 Ω between black and yellow

Feedback-winding resistance

Approx. 0.025 Ω between orange and white

Primary leakage inductance

Approx. 5.66 mH at 1 kHz with the secondary short-circuited

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

2,000 V RMS

Temperature range

-40 to +105 °C

Winding type

Paper winding

Lead length

At least approximately 305 mm or 12 inches; red centre-tap lead at least approximately 635 mm or 25 inches

External dimensions A/B/C

Approx. 102.9 × 94.4 × 88.9 mm

Mounting dimensions D/E/F

Approx. 88.9 × 63.5 × 76.7 mm according to the manufacturer drawing

Mounting slots

Approx. 4.75 × 7.62 mm

Hammond shipping weight

Approx. 2.95 kg

Manufacturing note

Made in Canada according to the current component drawing

Primary, secondary and feedback connections

Brown

Marked primary winding start to the first plate group

Red

Primary centre tap to B+

Blue

Second primary connection to the opposite plate group

Black

Marked secondary winding start or loudspeaker common

Green/yellow

4 Ω loudspeaker tap

Green

8 Ω loudspeaker tap

Yellow

16 Ω loudspeaker tap

White and orange

Separate feedback winding; white is marked as the winding start in the manufacturer drawing

Feedback winding

The feedback winding is not an additional loudspeaker output. Use it only where the particular amplifier topology includes a corresponding local feedback circuit.

Its phase is critical. Reversing the winding can turn negative feedback into positive feedback and cause severe oscillation, excessive level or component damage. Follow the applicable original circuit diagram exactly.

Selection for a new amplifier

The 2.35 kΩ plate-to-plate load must suit the total output-valve count, plate and screen voltage, quiescent current, operating class and intended output. Four 6L6-family valves are not automatically compatible with every 2.35 kΩ circuit.

Each output valve in a parallel push-pull stage requires controlled quiescent current. Individual control-grid stoppers and normally individual screen-grid stoppers support stable and even current sharing.

Safety notes

High plate and signal voltages may be present on the primary. Install the 2.95 kg transformer in a closed, touch-safe enclosure on a sufficiently rigid chassis.

Questions about selection and use

Which output transformer suits a Fender Tone Master using reference 026478?

Hammond 1750D is documented for the valve-powered Fender Tone Master and provides 2.35 kΩ plate-to-plate, a 100 W rating and loudspeaker taps for 4, 8 and 16 Ω.

Which 100 W PP output transformer provides 2.35 kΩ plate-to-plate and 4, 8 and 16 Ω taps for four 5881 or 6L6WGC valves?

Hammond 1750D provides this combination and is documented for a parallel push-pull output stage using four output valves in total.

Can the separate Hammond 1750D feedback winding be used for local feedback?

Yes, the separate feedback winding can be used in the intended amplifier topology when its ratio, reference point and especially its winding phase follow the circuit diagram exactly.

Is the Hammond 1750D suitable for a newly designed parallel push-pull amplifier using four output valves?

Yes, the Hammond 1750D can be used in a new four-valve design when the 2.35 kΩ load, 100 W class, plate voltage, quiescent currents and feedback circuit all suit the complete amplifier.