Originally Posted by Analog Addict (Post 1017013)

I communicated with Dennis and he said take the three prong cord and a pair of pliers and break off the ground plug, which is the bottom round prong. That is floating the ground.

He them said to take the now two prong plug and flip it over so that it is oriented in reverse from the normal orientation when it has three prongs. This is called reversing the polarity. Now test the amp and see what happens.

Originally Posted by Ruleof72 (Post 1017053)

Ok, I removed the ground plug and oriented the plug reverse in the outlet. I had no interconnects connected, just the speakers. When I turned on the amp I got the same result, an audible hum from the speakers after about 10 seconds of the amplifier being powered up from cold. The loudness of the hum dropped (after about 30 minutes of the amp being on) to a level that was still just audible if I place my ear within a foot or so from speaker but not so loud that I can hear it from several feet away.



Any ideas?

Originally Posted by bearcity (Post 1017062)

The anecdote comes to mind where the man goes to the doctor and complains his arm hurts when he wiggles it a certain way and the doctor’s response is “don’t wiggle it that way.”

Not sure why you have your head 1’ from the speaker?

Originally Posted by Ruleof72 (Post 1017081)

I agree, having to get within 1' from the speaker to hear a slight hum isn't worth worrying about. It's the fact that for the 1st 30 minutes or so (until the amp is warmed up) the hum is audible from my listening position about 8 feet away. That doesn't seem normal but this is my 1st tube amp so I have no reference point to know if it's an issue or not.

Originally Posted by Analog Addict (Post 1017089)

Have you tried different tubes? And perhaps I missed it, but what kind of speakers are you using, and what's their efficiency?

Originally Posted by Analog Addict (Post 1017129)

You don't need to use KT88s. You can use 6L6s, 6V6s, and even 6F6s. I'd use a 5V4 with 6L6s and a 5U4 with the others

Originally Posted by Ruleof72 (Post 1017136)

My plan is to try some other power tubes but for now all I have are the KT88's. I will give a longer listen with the KT88/5U4 combo and see if that makes a difference.

Based on everything I've tried, is the hum coming from the power, the transformers, the tubes or some combination?

Originally Posted by Bombadil (Post 1016566)

I noticed that listing. It is a little tempting. Not much of a jump in power from my Inspire but would be running the output tubes more in their optimal range than in mine, where they are pushed to their max.