Here's the inside of a Harmony H303 amp I did up for a friend.
It's a very simple three tube series string widowmaker that could deliver a jolt, but someone had their head screwed on straight at Harmony and it came equipped with a factory installed isolation transformer that is a necessary item if you have one of these creations and it doesn't have one.
The filaments are connected in series which gives you 35v, 50v and 12v for the 35W4, 50C5 and 12AU6 tubes. The total output's about 3.5w into a smallish speaker. Take the 97 volts and add enough resistance to make up 120 more or less, and use the chassis as a return path.
Very effective, cheap and dangerous if you are connected to the chassis by a guitar cord or a microphone. It was a variation on the All American Five radio which, if it sat on a shelf and came in a plastic case was perfectly fine. But not a guitar amp.
An isolation transformer does just that-it isolates the chassis from line voltage.
It was a gut job because the output transformer was wasted as were the paper capacitors and the electrolytics.
What was interesting was the collection of mud dauber nests that were inside it.