KT88 Tubes

Before buying more tubes, exchange the tubes in the right channel for the left. If the bad tube remains bad, it is a tube issue, however if the tube that you switch into the bad position red plates, it is an issue with the amp. Good luck!

IMHO, replace all output tubes if it is a tube issue.

Good advice from viridian. Always replace the entire set of power tubes unless you had an early failure in the set. If this amp is in original form internally you will need to replace caps at the minimum.

You can use 6550's in the Citation II. They are generally cheaper than KT88's. That's what I use in my pair of Dynaco Mk III 's.

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.