Thank you WombCannon, Drew and Vince for your suggestions. Presently, I am doing nothing about the problems with my Vox AC30S1, because the amp is not now exhibiting the problem behavior.
I bought the amp in April at the Guitar Center in Boston near Fenway Park. Because the amp is still under warranty, I went to the Vox website to find out how to make a warranty claim. The website said to take the amp to one of Vox's Service Centers and explained how to find one nearby.
There were three Service Centers listed for the Boston area: Musical Instrument Service Center in Boston; West Suburban Electronics in Newton; and High Tech Electronic Service in Methuen, which I consider to be too far from Boston to be considered "Boston area."
All three Service Centers had negative reviews about their amp service.
Also, the one in Boston is in a location which makes it impossible to park if I drive there. I cannot take the amp on public transportation. It is too heavy for me to carry more than a block. The specifications state that it weighs 54 pounds. However, it feels much heavier. I once had a Marshall stack, and it was easier for me to carry a 1960A cab and a 1960B cab up and down a flight of stairs than it is for me to carry the Vox to the street to put in my car.
Friday, I took the amp to the Service Center in Newton. The proprietor seemed skeptical of my claim. He interrogated me about the number of times the problems occurred. He became especially skeptical when he plugged a guitar cord into the input jack several times, and the jack did not come out. He told me that if the problems I described did not occur when he examined the amp, then Vox would assume that there is no problem, and I would have to pay him $ 45 for his time.
I decided not to leave my amp with him. I would not have minded paying $ 45 for him to look at the amp. However, if it did not exhibit the problems for him, he would not do any trouble shooting, and I would be left with an amp that I would have to consider unreliable.
Vince, I am not handy with a soldering iron. I am what a friend once described as a "mechanical moron." I remember years ago I used to help with the staging for my teenage daughter's ballet company. After working with me, one of the other dads commented that I was fun to work with but he would not leave me alone with a screwdriver.
Saturday, I turned on the amp, and it worked fine. So now I will "wait and see."
The guy in Newton mentioned that the intermittent loss of sound may be due to a defective tube. If/when the problem occurs again, I will try changing the tubes. I have a set of NOS tubes that I bought more than 15 years ago for a Fender '65 Deluxe Reverb Reissue. The power tubes for the two amps are different, but both use 12AX7s.
Meanwhile, I have no pending gigs at the TD Garden in Boston.
I really like the Vox AC30S1. For decades, I have played Fenders, Peaveys and Marshalls. Then a couple of years ago, I bought a used Vox AC15C1 and instantly liked the "Vox sound." However, I did not like having two channels and tremolo, so I sold it. Then Vox introduced the AC30S1 - one channel and five knobs - and I had to have one. I have been very pleased with the sound and the simplicity.