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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?

ervikingo
ervikingo OP
166 posts
 

Yes, with the significant other "quarantining" due to Covid, I moved to the music room since last Monday!

Have put hours enjoying music!

The (financial) downside is that i hit the vendors:

- Have 2 quads of EL34's scheduled for delivery today.  With those, every single tube on the amps is new

- Waiting for the PS Airlens to be released so in the meantime I ordered (and received) a Wiim Pro

- Ordered a budget DAC to go with it (Topping D90SE MQA) which should be delivered later in the week

Currently running the Wiim to my trusty but dated spec'd Theta Gen V and alternatively to the onboard DAC on a Lexicon MC-1.

Have to say, the Wiim sounds good!  On A/B tests from my CD transport to the same DAC there is a discernible difference (improvement).

 

atmasphere's avatar
atmasphere
11,929 posts
 

With the motorboating clue, it would not be unthinkable that those small tubes are involved.

@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 OP
166 posts
 

merits repeating: "atmosphere: This is why it can be so important to use the tube for which the amp is designed."

holmz's avatar
holmz
2,328 posts
 

@atmasphere being a guy in the land of 10k lakes, motor boating should be natural. 😎 (but it’s probably ice fishing season now,)

I was pretty sure that the hum reducer gear, and power conditioning approach, was not likely to lead to a solution… and the amps not having RCAs hooked up precluded a ground loop being possible.

And some version of an Ishikawa chart would have been helpful here.

 

In any case… it is a great outcome for the OP, and should be useful for others.

atmasphere's avatar
atmasphere
11,929 posts
 

being a guy in the land of 10k lakes, motor boating should be natural. 😎 (but it’s probably ice fishing season now,)

@holmz 

My understanding is the two happiest days in the life of a boat owner is the day he buys it and the day he actually is able to sell it. I find bicycles are much easier to manage except for that N+1 thing (N= the number of bikes needed).

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