typical problem and typical solution - 99% of ALL the problems Bob and I get are bad solder connections.
Could be the way you bought the amp (an old one) or could be the way you put a new one together.
Something to think about - based on my 50 years of experience in electronics.
You are better off learning (thru practice, of course) good soldering techniques before doing a project.
There are literally dozens of soldering videos on YouTube - some of them are really good!
AND you are better off using high heat for 2-3 seconds then low heat for 5-10 seconds.
After buying a new 25 or 30 watt solder iron every year for about ten years, I bought a 40 watt Weller temperature controlled iron,
been using it for about ten years now. I run it at nearly full temp (I'd say 11 on a scale of 10, you know, like the movie joke)
but it's 4.5 on a scale of 5. A good solder joint at high heat never takes more than 2-3 seconds. Practice!
This is kind of funny but it has stuck with me for 50 years, because my high school electronics teacher was a little crazy.
He didn't use a typical 30 or 40 watt solder iron. He used a 140 watt solder gun with a nichrome wire installed instead of
the usual huge copper element (those things were made to solder chassis parts, not PCBs) but he could solder anything in about 1/2 second with that heat.
Any longer and of course the nichrome wire tip would break, just like a fuse does.
The key - you have to heat the PCB AND the resistor/cap/whatever at the same exact time, AND use flux or good solder (here's my pitch for Kester 44).
Put a small amount of solder on a freshly cleaned tip EVERY SINGLE time you solder a connection, and then you'll see.
If the tip isn't clean, or there is not a tiny amount of solder on the tip, you don't get heat transfer to the board or the parts leads.
Then it takes 5-10 seconds, meanwhile the board and the part get burnt. Do it hot and fast, not cool and slow !!!!!!!