McIntosh preamp with Hum

I still think, probably, one of the tubes is bad, and even though you have done some tube trouble shooting, I would continue...   You have 4 for line level and 4 for phono.  Try to isolate it by turning it on with different inputs selected, ie. phono vs CD.  Then, try right and left channel balance.  you should be able to isolate it down to at least a possible 2 out of the 8 tubes..  Also, try tube swapping between phono an line stage to see if the behavior changes...  Note:  while turning it on or off, do not do it quickly.  This is not good for the tubes.  Leave some time after turning it off and turning it on again.  Note: the bad tube may also be microphonic, so, you can try tapping the tubes lightly with a chop stick when the preamp is on to see if you hear noise through the speakers while lightly tapping it. 

Here is what AI says about the "warm up" sequence of the c2200.  Note: the DC offset stabilization section...  Maybe, this explains why you are getting hum during warm up ?   DC offset sounds like a hum and maybe one tube is not stablizing fast enough as the voltage increases ? 

----------------------------

When you flip the switch on a McIntosh C2200, it doesn’t just "turn on"—it performs a choreographed sequence designed to protect the delicate vacuum tubes and ensure the longevity of the circuitry.

Since tubes rely on heat to function, hitting them with full voltage while they are cold is a recipe for a short lifespan (and some nasty popping sounds through your speakers).

Here is exactly what’s happening during those roughly 30 seconds of silence:

The Warm-Up Sequence

1. The Muting Phase

Immediately upon power-up, the C2200 engages a silent electromagnetic muting circuit. This keeps the output jacks disconnected from the rest of the preamp. This is why you won’t hear any "thump" or static in your speakers during the startup process.

2. Tube Filament Heating

The preamp begins sending low-voltage current to the tube filaments (the heaters).

• The C2200 uses four 12AX7A tubes (two for the phono stage, two for high level).

• During this time, the electrons are beginning to "boil" off the cathode, a process called thermionic emission.

   

3. The Visual Countdown

You’ll notice the front panel display (and often the tube illumination) blinking or showing a "MUTE" status. McIntosh designed this visual cue to let you know the microprocessor is monitoring the internal stabilization.

4. DC Offset Stabilization

The internal circuitry waits for the voltages to reach a steady state. If the output were unmuted too early, any DC offset (leftover direct current) could potentially damage your power amplifier or speakers.

5. The "Click"

Once the internal timer and sensors confirm the tubes are at an optimal operating temperature, you will hear a distinct mechanical click. This is the relay closing, finally opening the signal path to your amplifier.

Pro-Tips for the C2200

• Patience is a Virtue: While the mute lets go after 30 seconds, most tube enthusiasts agree the C2200 doesn’t hit its "sweet spot" sonically until it has been running for about 15 to 20 minutes.

• Tube Longevity: The C2200 is known for being very gentle on its tubes. Because of this controlled startup, you can expect the stock tubes to last several thousand hours.

• The "Look": If you have the lights dimmed, you’ll see the tubes glow a soft amber. Note that in many McIntosh models, there are also LEDs under the tubes to enhance this glow—don’t worry, that’s not the tubes "overheating!"

@hjdca

Thank you so much for the time and effort you put into your reply!  I will read it over several times and will begin the process of tube isolation as you outlined.

If I find the culprit then I will be sure to post a followup here.

As promised, returning here for an update.  I was able to isolate the "Hum" that occurred when powering up the preamp, and that disappeared after the tube warmup was completed.  Over an extended period of trail and error, I found the tubes to be fine, with no microphonic feedback, and that all the connections into the Preamp were good except for ONE.   That was the subwoofer out (using one of the Main outputs on the preamp).   That led to removing those connections to the subwoofer which eliminated the hum.  Then finding that the subwoofer wasn't even powering on!  Checked the fuse, it was intact and not defective.  So, I am now in touch with SVS, and will probably need to replace the amp module on the subwoofer.  

I had not played the stereo after finding the hum and so didn't even notice the lack of Subwoofer sound.  It is a SVS PB 13 Ultra that I bought new many years ago.  Now looking forward to seeing what SVS will do regarding replacement of the subwoofer amp.