• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hammond Power Transformer Buzz...

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I see you live in SoCal, at my place the mains has some DC which causes mechanical hum. Also, Hammod transformers are spec'd at 115V Vin but at my place the mains is about 122V. I use a power resistor in series with the primary to drop the extra 7V. It also adds DCR to the primary which brings the Hum level down quite a bit. Though I would only recommend that if your amps are on the lower powered side.

There are some "DC trap" and "DC blocker" circuits made up of a couple capacitors and diodes you can build. You can also run a search on mechanical hum and it brings up useful info. This is a common problem for people using toroids. EI core transformers are less prone to this problem, but they are not immune.
The 272JX is spec'ed at 300-0-300.... but is putting out 337-0-337... go figure.

The 272JX that is currently connected to a TubelabSE on my bench runs anywhere from 320-0-320 to 343-0-343 volts depending on the line voltage, the load, and the phase of the moon. It does not hum though. I may not have the best hearing on the planet, but I have put my ear right on the transformer. no buzz. Maybe I just got lucky and got a good one.
The 300 series have "universal primary windings". This means that they can be operated from several different power line voltages. This allows some flexibility in the output voltage by choosing a different primary tap.

The 300 series is also specified for 50 Hz operation. This means that a 300 series transformer will have more iron in it than a 200 series transformer of equal ratings. This will allow it to run cooler than a 200 series transformer, or it will allow you to run it a bit over the specs if you are running it on 60 Hz.
I had one that buzzed, I think it ended up being loose laminations. Try tightening up your end bells and see if it goes away. You can do a quick test by squeezing the edges of the bells right by the stack to see if the buzz goes away. Also, rubber insulators are a nice thing to have between the x-frmr and chassis. Don't tighten too tight, you want some absorbtion.

I also noted that when fed with 124v line voltage, the power transformers buzz louder than with 110v or 117v, and the heating could be a saturation issue due to the higher voltages.

The fact that some buzz and some don't would give a little more validity to the probability of loose laminations or end bells. Hope this helps ;)
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