Hum on Tube Amp - Can't find source

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.

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

@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.

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).

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

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