Hammond 370CX Power Transformer

How I use the 370CX...

9 January 2019

First thing: A correction. There are two 6.3V CT filament windings on this transformer, one rated at 600mA, one at 2.5A (not a 5V @ 0.6A and a 6.3 @ 2.5A as stated on the description page). Rectified (and heavily filtered), the 0.6A filament winding can drive the 300mA heater current for one 12A-something-7 dual triode (or similar) with DC while leaving the higher current heater winding for output tubes (and later preamp stages) running AC. This eases the load on the transformer for the output tube heaters as rectifying needs an AC current of about 1.8 to 2 times the DC heater current. I add a small amount of resistance (value depends on what and how many valves are driven) in series with the heater windings to get as near to 6.3V at the valves as I can make it. When wired for 240V operation (primary windings in series), the excitation current is pretty low at about 50mA and when running at 40W load, the primary current rises to about 210mA. I improve this by balancing the inductive excitation component with a 0.68uF X2-rated capacitor across the primary. That gives a no load excitation just below 20mA, a small reduction of loaded current and helps filter line noise. I also put a 430V MOV directly across the primary to catch switching surges and clean more rubbish from the mains. In series with the primary is a CL-130 inrush current limiter (50 ohm NTC thermistor, similar to a lot of Fender amps) and 500mA slow-blow fuses in both active and neutral lines (just in case the building wiring is backwards...). I also put a DC blocker (big anti-parallel capacitors with anti-parallel silicon diodes in parallel) in series with the primary. This blocks the occasional residual DC on the mains that can cause loud transformer buzzing (and high primary currents) as the core saturates. I haven't seen that happen with this transformer, but it has with others, so I do this as a preventative measure. While X2 capacitors and MOVs should fail open-circuit, the fuses protect against them failing short as well as protecting against shorts in the primary. The thermistor limits inrush current which could be considerable because of the fairly large capacitor across the primary. Overall impression: The transformer seems efficient and looks well made. The enclosed design suits mounting above the chassis and it looks fine when used that way. The dual 6.3V heater windings were a big factor for me choosing this transformer instead of some other option of similar power capability. Having made similar measurements on this and some other transformers leads me to think this is a well-designed unit.

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