1750A Hammond SE Übertrager Fender Reverb 125A20B Ersatz

Hammond 1750A – reverb driver transformer for 125A20B and 022921

The Hammond 1750A is a small single-ended output transformer for valve-driven spring-reverb circuits. Its current type-specific data sheet states a 25 kΩ primary impedance, an 8 Ω secondary impedance and a 3.5 W transformer rating.

Hammond lists the 1750A as the reverb driver for compatible Twin Reverb and Twin Reverb 100 circuits and identifies Fender references 125A20B and 022921. It is not the main output transformer for the power stage and should not be selected as a speaker transformer merely because its secondary is rated at 8 Ω.

Suitable applications

• Repair of compatible Twin Reverb and Twin Reverb 100 reverb-driver circuits
• Replacement applications associated by Hammond with references 125A20B and 022921
• Fender-style single-ended valve stages driving an 8 Ω spring-reverb input
• Restoration projects whose electrical ratings and mounting dimensions match the original unit

Technical data

Product type

Single-ended reverb driver transformer for valve amplifiers

Manufacturer application

Reverb driver for compatible Twin Reverb and Twin Reverb 100 circuits

Fender references

125A20B and 022921

Primary impedance

25 kΩ in the current type-specific data sheet and current series table; the Hammond product page differs by stating 22.8 kΩ

Secondary impedance

8 Ω

Transformer power rating

3.5 W

Operation

Single ended

Winding documentation

Bobbin wound according to the current part page and series table; paper wound according to the Twin Reverb application page

Frequency response

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

Distortion figure

Less than 1.5% at 70 Hz under the test conditions used in the Hammond data sheet

Primary inductance

31.0 H typical between blue and red at 1 kHz, 1 V with the secondary open

Secondary inductance

39.09 mH typical between black and green at 1 kHz, 1 V with the primary open

Leakage inductance

1.54 H typical on the primary at 1 kHz, 1 V with the secondary shorted

Primary DC resistance

1,065 Ω between blue and red, typical

Secondary DC resistance

0.966 Ω between black and green, typical

Dielectric strength

1,500 V RMS

Temperature range

-40 to +105 °C

Construction

Open-frame chassis mounting

Flying leads

Primary and secondary leads at least 6 in or approximately 152 mm long

Dimensions

Approx. 50.8 × 36.6 × 44.3 mm according to current drawing dimensions A × B × C

Mounting

Approx. 43.7 mm mounting centres; mounting-hole diameter approx. 4.75 mm

Shipping weight

Approx. 0.18 kg according to Hammond

Connections

Primary winding

Blue–red; blue carries the polarity dot in the current connection diagram

Secondary winding

Black–green for 8 Ω; black has the same indicated polarity reference as blue

Comparison with the Hammond 1750AX

Hammond 1750A

Reverb driver transformer with an approximately 25 kΩ primary, 8 Ω secondary and Fender references 125A20B or 022921

Hammond 1750AX

7.5 kΩ-to-8-Ω version for the separately documented stand-alone 6G15 reverb circuit and Fender reference 125A12A

Selection and installation notes

The 1750A is intended for an application-specific reverb driver stage. The primary impedance, valve quiescent current, supply voltage and spring-reverb input impedance must all suit the exact schematic.

Current Hammond documents state both 22.8 and 25 kΩ. This discrepancy is less critical in a like-for-like repair than in a new design because Hammond explicitly identifies the 1750A through the Fender references. For a new circuit, use the current 25 kΩ drawing as the starting point and verify the operating conditions.

The 31 H and 95.49 H inductance figures were measured at substantially different voltages and frequencies. They must not be treated as a production discrepancy or directly interchangeable specifications.

Hammond does not state a general maximum primary DC current in the current 1750A data sheet. The part should therefore not be repurposed as a universal 3.5 W single-ended speaker transformer without a complete calculation.

Hazardous plate voltages may be present on the primary wiring. Installation and testing require suitable expertise, a disconnected mains supply, safely discharged filter capacitors, touch protection and adequate insulation clearances.

Questions about selection and use

Which reverb driver transformer suits Fender references 125A20B or 022921?

Hammond 1750A is the documented reverb-driver reference for 125A20B and 022921. Its current data sheet specifies approximately 25 kΩ primary, 8 Ω secondary and 3.5 W; the amplifier revision, mounting and wiring must match the original.

Which transformer couples a valve-driven Fender reverb stage to an 8 ohm spring-reverb input?

Hammond 1750A is intended for this application-specific single-ended connection. Connect the primary between blue and red and the 8 Ω spring-reverb input between black and green; the complete amplifier schematic remains decisive.

Can the Hammond 1750A be used in compatible Twin Reverb and Twin Reverb 100 circuits?

Yes, Hammond explicitly lists the 1750A as the reverb driver for these amplifier families. The application reference concerns the reverb-driver circuit and not the main output transformer of the power stage.

Is the Hammond 1750A suitable as a replacement when the original transformer is marked 125A20B or 022921?

Yes, Hammond documents both numbers for the 1750A. Before installation, compare the exact amplifier revision, mounting pattern, lead colours and existing reverb-driver circuit.