Hum issue with my Ampeg SVT-CL

Hi guys, I am having some issue with my newly acquired Ampeg SVT-CL (Vietnam make), and I am hoping someone may be able to help me out. Long story short, I bought the amp used from a local store 2 days ago, tested it out last night in my apartment and noticed that there’s a mellow humming sound coming out of the speakers when the amp’s taken out of the stand-by. I didn’t notice it at the store because I was surrounded by kids shredding their metal licks. This hum is not affected by master nor gain knobs, it’s faint but steady, almost reminds me of rumbling noise of a fridge. By the way, I did test several different outlets in my apartment, all with the same results. This most likely is not going to be a big deal in live situation as once the volume knobs are over 2, the hum is basically drown out, but none or less, it really bugs me, and potentially may be an issue if used in recording. I was wondering if this is normal of a SVT?

I have had numerous amps most recently an Aguilar DB750, also a few tube ones as well, such as Orange OR120, and Traynor YBA-1a and none of them produced hum like this. From what I can gather from searches of same topic that this could be a preamp tube issue picking up 60 cycle hum? The store has 90 days return/repair policy, but they do not cover tubes if they were the cause. I am deciding on if I should gamble and spend the money on replacing all the preamp tubes, but what if it’s the cap or filter,…etc. Not sure if it’s all worth it, maybe I should just return it as this potentially could be a lemon. What do you guys think? Also, I have never played an SVT in a quite place before (ie, my apartment), so maybe the hum is normal. i am torn.

Thx in advance for all the comments and help.

 

Here's a video of the issue that I am talking about. I apologize for the volume quality, it was recorded with my phone. You will likely need to use a set of headphones with the volume crank, but you can definitely hear the rumbling hum once the amp's taken out of the stand-by.

 

Okay, I just changed the driver tube, 12ax7 with an older Sovtek that I happen to have on hands and the hum was much louder. It was unbearable, I felt like I was going to throw up. Perhaps the driver tubes picking up 60 cycle hum?

 

The background noise tapered down at the end, and at the last second or two, I turned the stand-by back on, you can hear a singnificant difference with the hum on and off.

 

Okay, I reset all the driver tubes, and the hum was still there. Worst of all, the bias lights went completely haywire.

Bias 1, both red and green were on.
Bias 2, none of them were on.

Scared sh** out of my pants, I quickly grabbed a flathead screw driver and did some tweaking, and guess what, I was able to balance both sides to green and the hum is now gone. So, it turned out to a bias issue after all.

 

Turned out it was a bias issue, even though initially both lights were green it wasn't balanced somehow and were casuing the hum. Thanks for the links by the way, additional resources are always helpful for future reference.

Just thinking of all the potential issues that can happen to these SVTs sends chills down my spin.