Tech question regarding anomalous and intermittent 60 cycle hum

I am going to doubt that you have a transformer humming. Ime, a power tranny that hums does so due to loose laminates, and one can feel the hum in the transformer.
The hum which your amp exhibits makes me suspect that there is a bad connection or a weak filter cap. Tech time, imho.

 

I'd try plugging it in somewhere else in your house or taking it somewhere else completely and seeing if it has any effect.

AFAICT there can be things turning on and off in your house that can change how much noise you're amplifying.

You didn't mention if you've got it on a good surge protector/filter/whatever/etc.. that can affect this kind of stuff.

I have a THR I often carry around to practice away from my house. I've noticed there are some locations where the power is WAY cleaner than my house.

 

^^^

Try to find something in the house/building that’s turning on and off. I have this problem when the dishwasher runs. The hum comes and goes as it switches wash cycles.

 

My Cornford Hurricane developed a very nasty hum. The culprit was a cable whose retaining clip had failed, leaving the cable flapping around too close to a transformer. Simple fix involving a blob of silicon sealant. Interference can come from some strange places; my Tivoli radio, for example, picks up radar pulses on FM from my local airport, 5 miles away.
Years ago I gigged regularly in a bar close to a railway line, and my amps would pick up track-circuit signalling pulses as a series of rapid bleeps.

 

Kudos on curing the problem. It would be of interest to know which Millard was going south. The hum was due to the imbalance of current draw when the problem exhibited itself....so I would think only one of those Millard’s is defective. Bad control grid??
I take it that the bias reading procedure reads both tubes at once...and 66ma indicates a current draw of 33ma per tube. Fwiw, this is one situation where measuring current draw via the transformer shunt method would reveal which tube has the problem.
Knowing which tube had the problem would lead one to making sure that that tube was making good contact in all of the pin sockets. Taking measurements on the plate, screen grid and the control grid during the hum situation might have been revealing. A tube tester might provide some insight...maybe not due to the intermittent nature of the proble.

 

A day late and dollar short since the problem was unrelated.

But I recommend every electric player - especially those that gig or move amps from room to room - own a $7 outlet tester and check every power outlet before plugging anything in. Especially in older houses, 3-prong outlets have NO ground connection - 3 holes are meaningless; you have to check.

And a huge number of outlets in both old and new houses have some outlets where the polarity is fouled (polarity is a "term of convenience" in this context) - the "hot" and "neutral" supply wires are connected backwards on the outlet. There is a reason for one plug side to be wider than the other, and that's to ensure neutral and hot are supplied to the equipment correctly. "Neutral" is NOT an extra ground wire, but having the outlet reversed can cause some dangerous issues, especially in some vintage amps.

An outlet test has 3 simple colored lights that tell you if the outlet is grounded and wired correctly. I check the power source every single place I play; if I can't access a correctly wired source I use an isolation transformer or don't play.

Glad it was easy. Good lesson - check tubes first when hum is present. But while it's unusual for a newer amp to have a filter cap fail, it DOES happen - that would have been my next item based on the description of the hum.

Because it was intermittent and varied it was very unlikely to be due to some piece of nearby equipment, especially anything run continuously.

 

This^^^. I have seen “pops” caused by connection issues. Get the elecrolytics replaced in the power supply and if it has push on connections in there, fully disconnect and reconnect ALL of them. First thing I do servicing pcb amps (and older printing systems I work on at my personal accounts.) You have no idea how breaking a poor connection and reconnecting has corrected intermittent issues.

 

“I have seen pops caused by connection issues”...
I overhauled a ‘67 Bandmaster last week that had a pop issue. There was a loose connection on the standby switch....the wire was sitting in a hole in the solder at that connection post....maybe for all of these 51 years.