Hum on Tube Amp - Can't find source
I have a hum (60hz) I can hear on my speakers and it happens with my tube monoblocks (either of them). With or without interconnects, it even happens on either amp (have tried one at a time) with every circuit on the house tripped/disconnected, every other component disconnected from the wall (including the Internet/CaTV line) and no interconnects.
One amp has it as soon as it warms up whereas the other one is intermittent.
Hum X doesn't solve it, iFi Ground defender either, AVA HumDinger on powerline doesn't solve it either.
I have replaced the tubes and both amps were just tested at the factory. Replaced the circuit breaker, tightened every wire on the breaker box, checked and cleaned all connections to ground rod. Added a hum eliminator to the internet line.
Hum cycles a bit with the tube glow matching the cycles.
I'm waiting on the power company to come check the power coming to the house.
Thoughts?
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@holmz The 12AX7 has more gain than a 12AT7. This can affect the amount of feedback that exists in the amplifier and can also affect something called 'phase margin'. If the amp was designed to use a lot of feedback and was a bit to close to the limits of its phase margin, by adding the slight extra gain of the 12AX7 it could have pushed the amp over the edge. So an oscillation is entirely possible, simply on account of the 12AX7. This is why it can be so important to use the tube for which the amp is designed. |
@ervikingo 12AX7s trade high impedance for more gain so could be sensitive to a variety of variables. So going to 12AT7s may well solve the issue. I've also found that the brand of tube can sometimes cause hum. For example a phono section I was working on exhibited a hum a few minutes after warmup. The customer had replaced the stock 12AX7 tubes with Brimars, which are supposed to be a nice tube, but for some reason, even though the filament supply was regulated DC, the tubes would hum. Replacing them solved the problem. |
@ervikingo I had assumed that since your amps were 'tested at the factory' that they had a clean bill of health. But the first rule of troubleshooting is 'don't make assumptions'. Occam's Razor is always trying to get me to look at the simpler explanation, and @redhouse6 's comment about the filter caps is a very good one! A lot simpler than most the other explanations here, including my own. So how old are your amps?
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@ervikingo There are a number of AC line problems that can cause diodes to radiate noise which can get into the audio circuitry. Chief among these is the 5th harmonic of the line frequency (in the US, which uses 60Hz, this would be 300Hz). Harmonics of the line frequency occur when power transformers upstream from your house are loaded too heavily. If they are loaded past 50% of their ratings is where you get into trouble. If this is happening sometimes the power company will be willing to replace the transformer if they can find which one is doing it- the 5th harmonic can cause their equipment to run hotter and be less reliable. So it might be worth it to have someone from the power utility to come by and do some tests. Another way the amps could be in trouble is if the safety ground wire and the neutral wire were swapped somewhere. I've seen this problem happen in the US and it can cause a lot of buzz and hum! |
@ervikingo Yes. |
Get yourself something called a Digital Voltmeter. Set it to the AC scale and plug the probes into your AC power. Keep an eye on the voltage; when the hum shows up look at the voltage and see if its changed. Or you could also use one of these: The nice thing about this item is it can also measure the current drawn by an amplifier. It would be interesting to see what the current and AC voltage are doing when the amp hums. |
