tcsned wrote:1 EL84 (power amp tube)
1 12AX7 (preamp tube)
It's good that there's just one power amp tube so there's no need for a matched set.
what tcsned said . 1 x 12AX7 preamp tube and 1 x EL84 power amp tube. Buying tubes in pairs or quads is for amps that have two or four power tubes. Preamp tubes would not come in pairs and the reason for buying the power amp tubes in pairs or quads is to have the vendor test them and select 2 or 4 with very close operating characteristics as that they sound and work the best for the amp. Not the case with your amp but if you ever get into a 30 watt or larger amp, you would run into that.
From the picture of the back of the amp, the tubes are likely hanging upside down from the amp section housed in the top of the amp. With the back being pretty open, you could just tip the amp down on it's face and look in there and pull out and put in different tubes.
As far as the actual brand of tubes, you will likely find (if you bought it new) that they are Bugera "brand" tubes. This means the tubes are rebranded (i.e., they pay the manufactuere to put the amp company's name on the tubes). There really are only a handful of manufacturers out there. In this case, a poke around on the web seems to indicate that the tubes are re-branded Sovteks. Not sure which exact Sovtek preamp tube it would be as there are long plate, wide plate and plain old plate, i..e, there are Sovtek 12AX7-WA, -WB, -WC, etc. that all have varying charcteristics such as gain and microphonics. Those preamp tubes would really be a try it and see type of thing. And there are other brands as well. Preamp tubes are genearlly are not too expensive.
I had a Peavey Classic 30 with multiple preamp and power tubes. They were all Sovtek, which btw a lot of companies ship as stock tubes now and they seemed to drive into distortion or breakup too early for me. I wanted to "tame" the amp and make it cleaner. So I read about Steve Kimock's tone and ways he achieved it. Turns out he used a slightly different tube in one of his preamps stages. You only have one stage so your selection will be based on what you are trying to achieve.
The preamp tubes that can fit in your amp are of a family with the same pin arrangements and physical electrical characteristics but with different gain capabilities. An abridged rundown is:
12AX7 - 100% gain
5751 - 90% gain
12AT7 - 70% gain
and there are others.
I swapped a preamp tube on the clean channel to a Phillips 5751 and was amazed at how the amp cleaned up, the distortion was gone and the tone had a clear, bell like tone to it. That mellowed a bit as the tube changes I made settled in but still very clean and very clear. I did lose a bit of gain as is to be expected. But the difference in clarity and, for me, the percieved headroom was well worth it. It was a relatively expensive preamp tube but there are other brands that aren't so bad.
I would try a 5751 or a 12AT7 before heading down the power tube swapping thing, but that is just me and I'm sure there are others that would approach the situation differently.
Whatever approach you take, best of luck.