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Mecanical PSU hum

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icebear

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Joined 2003
#1
What is the main reason for the mecanical hum from tubeamplifiers? From my mullard 5-20, a rarther irritating humming is heard even a couple of meters from the amp. The psu is pretty much like the standard circuit. I have a bigger cap after the chocke. Else it's the same.

bjorn
 
#2
It's usually the mains transformer. They buzz from a variety of reasons. Sometimes, it's loose laminations. Check to see if the cans or brackets are screwed down tightly. It's more likely that the core is saturating. Mains transformers are often designed to operate very close to saturation (especially toroids). Unfortunately, a bit of DC in the mains is sufficient to push the core into saturation, and that makes it buzz. A clue to this can be that the transformer buzzes during the day (when there's a heavy load on the mains) but dies away after eleven at night.

The problem isn't confined to valve amplifiers, anything with a transformer in it could have the problem.

Of course, I'm assuming here that there isn't a fault on your amplifier causing it to draw more current than it should do. Check the temperature of the mains transformer.
 
#3
I have raised the catode resistors from 470 to 545 to decrease the current. The current trough the EL34 should now be 50mA each, instead of 70mA with the 470ohm resistors. The transformer is 150mA type, so with the 470 ohm catoderesistor, it's defenetly in saturation. But it should be ok now?

bjørn
 
#4
Might be OK. Unfortunately, saturation isn't like overfilling a beer glass (on or off) it's more like putting too many chillies in a curry; the overlap is quite broad. Does the transformer still buzz? If it does, frankly, I'd reset the EL34 currents so that they are operating ideally, and start looking for a new mains transformer. Some people seem to be particularly allergic to transformer buzz, so it might be worth your while to have a transformer specially made, and specifically require the transformer manufacturer to operate the core at a flux density significantly below the knee on the BH curve. Sadly, this is more expensive...

If you do have a transformer specially made, you might as well garner some other improvements and have a separate transformer (with foil electrostatic screen) for the audio valve heaters. The HT transformer should have the 5V heater for the GZ34 on it, and to allow for the fact that GZ34 are becoming difficult to obtain, make the 5V winding 3A capable so that it can power GZ37 etc. A foil electrostatic screen isn't so important for the HT transformer. An alternative to the electrostatic screen is a split chamber bobbin.
 
#6
When you lower the current you also lower the mutual conductance, but mu stays the same, so you raise ra. But the output transformer and load stayed the same, so you have applied a heavier load to the valves. In theory, this raises distortion and lowers available output power. In practice, you'd need to measure it to see whether the difference is significant.
 
#7
In an amplifier I had that had a similar problem, I desoldered the mains transformer, took it apart and jammed the bobbin hard against the lams with an aluminium zigzag wedge. Cured the buzz: in that case, the old wedge was too loose.
 
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