Hammond 125E
A universal output transformer that keeps turning up. Builders found that a pair of them, wired carefully, does real work.
Overview
The 125E is a universal push-pull output transformer from Hammond, rated at 15 watts of audio each. Keep that rating in mind, because the interesting thing about the 125E is what happens when you use two of them. The primaries can be parallel-connected so that they reflect half the data sheet impedance when similar secondaries are used. At least one experimenter reasoned that if you can parallel resistors, capacitors, or inductors to reduce impedance, you can do the same with transformers. Wire the primaries blue-to-brown and brown-to-blue, reverse the secondaries so the signals still add, and a pair of 125Es will happily serve a push-pull amplifier that a single 125E couldn't touch.
The pairing trick has a wrinkle. In many lower-cost audio transformers the secondary is wound around one end of the primary, which dumps extra stray capacity onto one side of the primary. In the Hammond 125 series, that end is the brown lead, and the interwinding capacity between primary and secondary amounts to more than 200pF. In a single-ended circuit you orient the primary so the parasitic capacity sits at the power supply lead, away from the plate connection. In a push-pull amplifier that capacity lands on one of the signal leads and disturbs the high-frequency balance of the output stage. The fix is to wire the two transformers' primaries and secondaries in opposite phase to each other, so the parasitic capacities balance out. Make the other possible connection and square-wave response turns visibly unsymmetrical, with output dropping off well before distortion shows up. Because the primaries are reversed, the secondaries have to be reversed too, so the signal still adds and drives the speakers properly.
There's a DC resistance mismatch as well. Each half of the primary in the cathode circuit differs by about 6Ω in the 125E, which is why one published build paralleled a pair of 12Ω, half-watt resistors to compensate. With that in place, output tube DC balance can be measured by putting a voltmeter across the test points and adjusting the bias pot for a zero reading. Do it with no signal present and the gain turned down, since normal operation puts very large AC voltages between those points.
The performance numbers for a pair of 125Es are honest but not spectacular. In one measured push-pull build running 6L6GCs as tetrodes with two 125Es on taps 1 and 5, the amplifier produced 1 watt at 0.14% THD at 1000Hz, with a damping factor of 2.725 at the 1-watt level. At 100Hz the same pair gave 1 watt at 0.25% THD, 10 watts at 0.60%, 30 watts at 2.30%. With 6550Cs as tetrodes, the pair managed 1 watt at 0.20%, 10 watts at 0.40%, and 30 watts at 1.00%. Distortion at high levels and low frequencies climbs because of the limited iron in these transformers. The damping factor also takes a small hit compared to a larger custom-wound output transformer, because of more copper losses from the higher winding resistance in the signal path.
A single 125E is rated for 15W, and the manufacturer's own documentation notes that two of them used together are "still a very usable alternative" to a big custom transformer, with the caveat that a proper 125F would be welcome. For builders chasing maximum output from a given pair of tubes, the 125E is a compromise. For builders chasing a working amplifier on a reasonable budget, it's a well-documented one.
The secondaries follow the standard Hammond configuration of two windings allowing 4, 8, and 16Ω loading. Connecting secondary taps 1 and 5 of each transformer in series into an 8Ω load reflects roughly 1500Ω to the output tubes. That load suits 6L6GCs well, though the damping factor won't match what a dedicated custom transformer delivers.
The 125E became a building block in a specific school of DIY amplifier design. The circuits that used it took feedback from the cathodes of the output tubes rather than from the transformer secondary, which kept the 125E itself out of the NFB loop. That avoided a pair of DC decoupling caps, since the output tube cathodes sit barely off ground potential. The trade-off is that the amplifier needs plenty of gain elsewhere, because the output stage demands a large driving voltage, around 250V measured at the driver grids for a 25W output. With a 6SL7GT input/phase inverter and a 6SN7GT bootstrapped driver, the input required for full output comes down to about 1V.
None of this is unique to the 125E. What is unique is the documentation: the transformer appears in published build articles with measured performance tables, wiring diagrams, and candid notes about its limitations. Audio Express (August 2004, p.22) described how the feedback loop could be modified via a CR pair in the 6SL7 input cathode circuit to adjust high-frequency response. That paper trail is why the 125E remains a known quantity among builders who work with push-pull tube amplifiers, even though a single unit is modest by any measure.
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