10 gauge power cord. Too much power for tube amp?

Just to clear up any misconceptions about AC supply from a wall outlet. If you connect a properly operating tube amp, or any other amp to a 20 amp AC wall outlet, with a 20 amp rated cord, and that device only requires 3.5 amps to run, all it’s going to draw from that 20 amp supply is 3.5 amps. Notice I said ’properly operating’. If it has an internal fault that causes it to draw 20 amps, it will try to draw 20 amps and then your problems start. Under normal operating circumstances, no electronic device will draw more current than it requires regardless of how much current is available. Voltage is a different matter. It will try to use whatever voltage is presented to the wall outlet. When trouble shooting a problem like this, the very first step should be to verify that the outlet is properly wired and to see how much voltage is being supplied. The tester erik_squires linked is exactly what you need for that task. If the outlet is good, then you visually inspect the power cord for damage and use a multimeter to test it’s continuity. Easier still, just try a different cord of the same size/rating and see if the problem is still there. As others have suggested, you could still have a defective small signal tube or, more likely, a rectifier tube if that model of McIntosh uses one. I personally would replace all the small tubes one at a time before shipping the amp off to McIntosh for repair. To that end, I always have a full replacement set of tubes available for my tube amp.

Not a tech expert but I’ve had a signal tube go bad in an integrated amp as well as in a DAC and in neither case did the component shutdown. Also, I have 11 gauge power cords on my 3 tube integrated amps, zero issues.

Just for clarification, the larger the gauge number or awg, the smaller the wire is, not bigger. For example, 10 awg is a larger diameter than 12. As mentioned previously, when you check the outlet’s wiring, make sure the ground wire is firmly connected. If you have an outlet tester, use it. If you aren’t using a high quality outlet like Hubbell, now would be a great time to install a Hubbell hospital grade outlet. Most 20 amp circuits use 15 amp receptacles unless it is a dedicated circuit. Hope you don’t need the amp repaired.

Electrical devices will attempt to draw whatever current they need - and no more than that. 12 Ga. is likely just fine for anything reasonable but having 10 Ga. won’t "hurt" anything. Just a bit overkill. I mean, your own house wiring from the junction box is only 12 Ga.

Moonwatcher, unfortunately, it isn’t safe to assume the home has 12 gauge romex. If the panel has 20 amp breakers, then the wire is probably 12 gauge. If the breakers are 15 amp, the wire is probably 14 gauge. This can vary by local codes and builder. Some builders try to save money and use both 14 and 12 gauge based on the intended use.