Hum on Tube Amp - Can't find source

If not, it's on your mains.  Get engineer in.

Without anything plugged in it is unlikely to be a ground loop.

If nothing is plugged in (ICs) then it would be difficult to get a loop, so difficult to get a ground loop.

A DC bias on the AC line, usually comes across as transformer hum.
I ‘m not sure how it would come out of the speakers though.

Dunno… multiple tap points on the amp?
Changing glow is not encouraging, but it is useful.

Some tube expert might chime in. Like Atmasphere if you ask him nicely…

holmz    I didn't say it might be a ground loop, although it may be.

I did not say you did.

I said it. 😎

If OP reports correctly the hum is exactly 60Hz, the coincidence is too much.

If we had an Ishikawa chart we would see that a ground loop is  on the list of causal mechanisms. But we have possibility of one without the RCAs connected.

So that leaves incoming power, or the amps are jacked up, or some high current magnetic field near the amps... or ???

That's why I suggested to test the rig in another location

That helps identify the power and magnetic field.
If it sucks in location #2… then it points stronger to the amp being jacked up.

The sound in that video sounds like “motorboating”.

The sound in that video sounds like “motorboating”.

holmz you beat me to it

@jea48 Are you trolling with a lure too 😂

 

imhififan @ervikingo

Have you tried connect a different pair of speakers on your Manley Snappers?

Another good point. The manual states: Recommended Speaker Load: Optimized for 5 ohms

Do you know what impedance your speakers are rated ervikingo?

Once the noise starts you can’t bias them as the voltage cycles.

To me, this is the most concerning factor in this whole scenario.

Here’s a few other tidbits from the manual:

-For the power tubes in our amplifiers, after a few years if you notice a small revolt going on where several of the output tubes are misbehaving or getting hard to bias, you might consider doing a full re-tube.

-Three tube types that are electrically and plug-socket compatible with the 12AT7A dual triode include the 6021, 6679, and the ECC81 (I find it a little interesting that they made no mention of a 12AX7 here).

-Never operate the amplifier without a speaker load, or suddenly disconnect the speaker load while the amplifier is producing a signal. This risks punch-through of the enamel insulation covering the transformer’s internal magnet wire. Permanent damage to the output transformer may result 

-Additionally, it is best to cut power to the Snapper when not in use rather then leaving the power on indefinitely. This will enhance tube and system life.

With the motorboating clue, it would not be unthinkable that those small tubes are involved.
Do these have coupling capacitors?

Something like a broken solder trace could easily be involved to create a situation where the amps is motorboating - maybe even from trying to account for DC.

@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 yeah… shopping for a gravel/adventure one now.

Crickey, they can be around the price of 3-4 of the mono blocks.