KT88 Tubes

Having tested nearly every quad-matched KT88 tube currently available on the market, none have approached the level of detail retrieval, holographic imaging, and tonal purity delivered by the Ray Tubes Select KT88 from Apos. It stands in a class of its own. Highest recommendation. https://apos.audio/products/ray-tubes-kt88-select-vacuum-tube?_pos=4&_sid=6e37ebf16&_ss=r&variant=45395661160684

Since you are new to to tubes, I'll make some suggestions:

1) If you hear a pop or hum or change in volume. or anything strange, the amp should not be used until you have determined what the problem is.

2) When purchasing a used tube amp, you want to nail down from the seller *everything* about that unit--the age of the tubes, the last time it was serviced or repaired, and any issues the amp may have had in the past.  With vintage units more than 30 years old, it's essential that the amp has been fully serviced and restored, all capacitors replaced and any other out-of-spec components updated.  Insist on documentation.

3) Tube amps need continuous care and feeding.  That means knowing how to check and adjust the bias of the output tubes and doing so once a week.  

That said, viridian makes a good suggestion.  Swap the KT88s between channels to make sure it's the tube and not the amp.  If it's the tube, since you don't know the age of the tubes that came with it, a new matched quad would be a good idea.  I would replace all the other tubes as well.

If the problem stays in the same channel, you need to take the amp to someone for service, preferably someone who is experienced with vintage amplifiers.

I noticed on the Apos tube site they claim that 5% of these tubes when tested meet their criteria. This means that whoever is manufacturing these tubes is so bad at it that 95% of them are rejected by Apos (sorry about the redundancy). Rest assured that I'll never buy anything from these guys. I've used lots of different tubes in my amps over the years and Gold Lions seem to be the most reliable and best sounding over all. 

Apos - yeah no, not for me either. I use NOS 6550 by GE, RCA or Winged C in my KT88/6550 amps. Have had nothing but good luck with them. Best new issue are Good Lion KT88s but I'll spend more for the NOS mentioned. Just my experience & opinion 

I am new to tubes but not troubleshooting electronics.  Occasionally in my career I have used the swap method on some similar adjacent instrument only to kill that one also as the component itself was the issue.  Is that a possibility with a bad tube in this application?