This particular radio appears to have an isolation transformer which makes it somewhat safer, but some older tube radios are actually connected directly to the mains line with no isolation transformer (this was done to save costs).
Those are especially dangerous to prod around in without the proper safety precautions when they are turned on, or even just plugged into the wall.
- All an isolation transformer (which is an external component you have to supply; it's never part of the radio) does is make the mains AC voltage inside the radio "float" in contrast with the voltage coming from the wall outlet (which is of course referenced to the actual ground outside your house). This makes things a little safer because if the chassis becomes live for any reason then there won't be a potential difference between the chassis and the ground that you're standing on, meaning you won't suddenly find yourself forming part of the circuit if you touch the chassis. In these old radios it's super easy for the chassis to become live so this is a good precaution.
- Isolation transformers do nothing for safety concerning the dangerously high DC voltages which the tubes run on. In the amp itself, the AC voltage comes in, hits a power transformer inside the amp (to get nice high voltages for the tubes), then is rectified into DC, then goes into the circuit to drive the tube plates and do all the other stuff it has to do. You can still get an almighty and quite easily fatal zap if your body provides a path between high DC voltage and the chassis (which is also the DC ground). Hence the rule, only work with one hand in a powered-on amp, because at least that way if you touch something then 350V of DC can't travel up one hand, through your heart, and back out the other arm.
- The final thing you might be referring to ("some older tube radios...") around cost savings is the archaic practice of skipping the power transformer entirely and connecting the wall AC voltage directly to the rectifier. This requires various compromises in the circuit (the DC voltage produced is quite low) and you _have_ to connect these with an isolation transformer or you're really asking for trouble - the chassis is all but guaranteed to be live. The original makers in the '40s and '50s, at least keen to be seen to try to avoid killing their customers, "solved" the problem by using plastic control knobs. Here's a good article: https://antiqueradio.org/safety.htm
Yes, you can still receive a fatal shock via one arm to the other even with an isolation transformer.
It's not true to say that it does nothing for safety, though. It provides isolation vs ground and prevents the circuit from being completed through your feet and the floor:
https://electronics.stackexchange.com/questions/17496/how-is...
It also limits the total current to whatever the transformer can supply before voltage starts to sag.
I didn't, I said it did nothing for safety concerning DC :) In the prior bullet I explained an iso transformer's contribution to safety regarding AC.
(Feet/floor not involved in the DC side. DC ground is the chassis, regardless of whether that's floating with respect to AC ground or not.)
EDIT: I'll double down on the pedantry 'cos this stuff does matter. There's no isolation transformer in this radio - it's a piece of benchtop equipment the author (wisely) chose to use. As I was getting at in my comment, I can't think of any radio or amplifier that incorporates an isolation transformer: at the time they were designed nobody cared, and nowadays the law requires you to include a separate chassis safety ground (for class 1 devices of course - class 2 being double insulated which is all but impossible on anything you could plug an external audio input into).
Ah I think I see what you're getting at. Are you saying if your feet are at the same potential as the chassis then there's an alternative DC path to ground through your body, which the iso transformer breaks?
That's a good point, needed to think about that one a bit!
EDIT: Hmm, in that circumstance you'd be safe (from grounding DC through your feet) if the chassis was live! If the chassis was connected to AC neutral then your body could offer a path for high voltage DC to complete the circuit back to the chassis, but not if it was connected to AC line (hot). Quite an interesting thought.