Troubleshooting an RCA SHF-4

The unit itself powers on. The turntable spins, and the tuner lights up, but it produces no sound. It has a tube amp. I removed the tubes, and inspected them, and one of them, a 5U4GB has a silvery dark spot on the side of it. I'm not much of a tube guy, so I don't know if that means it is bad or not. At first I assumed it did, so I started looking on Ebay for a replacement, and one of the ones advertised there stated that it tests good, but it has the same kind of silvery dark spot on the side just like the one I thought was bad. I have no idea how to test them. Is there anyone in the Indianapolis area that knows someone who can? Is there something else I should be checking on the amp itself?
 
The silvery dark spot is called the "getter", and is there specifically to absorb stray gas molecules left over from the manufacturing process, as well as to absorb any stray gas that may be generated internally as the tube ages with use. You want it to be silvery. If it's any other color (typically white), the tube is gassy and thus bad.

There are lots of possible reasons for your lack-of-sound problem. I strongly suggest you start with this thread:

http://audiokarma.org/forums/index.php?threads/how-does-a-tube-amp-work.443915/

...to get your head wrapped around some of the basics. And be careful, tube amps are chock full of high voltage that can kill you quick. If you don't know what safety precautions to take, I strongly suggest watching some of the How-To primers on the Uncle Doug youtube channel.
 
Pics?

Read this about electron tubes:

https://en.wikipedia.org/wiki/Vacuum_tube

No sound issues can be several things but usually in my experience with tube stuff, no sound = no B+ (high voltage to the plates of the tubes) but could very well be something else. Got a DVM?
 
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