6550 tube in an ARC Ref 5 SE

Hi folks, I’m trying to learn some about amplifier design and function here.

My story: I have an ARC Ref 5 SE. I stopped getting sound from it in August. Wasn’t sure what was wrong, so sent it to ARC for repair. Took a while, apparently due to their factory moving or something. Got it back last weekend, reported as “general repair” on the invoice; inside looked like they had cleaned it. Reinstalled the tubes and ... still no sound. Looking at the unit while on, I noticed no glow from the 6550 tube. It had about 1500 hours on it, so could have reasonably been at the end of its life. I therefore bought a Tung sol 6550 from the tube store and installed it. Now I see glow again, and the sound is back.

Ok, so I’m not gonna grouse about ARC not apparently testing the tube, rather, my interest is piqued, and I would like to learn here. The 6550 and its accompanying 6H30 are listed as part of the “voltage regulator” circuit as part of the power supply. My question is this: what exactly does this circuit do in a preamp, and why does the sound stop when the 6550 fails? Please explain in English, as I have no background in electronics. Just trying to learn here, and thanks in advance for teaching the newbie... the internet was pretty unhelpful on this question