General > General Technical Chat

Appliance power switch in neutral line - legal in aus?

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TimFox:
On vacuum tube radios, the power switch was usually mounted on the back of the volume control, so that one turned the unit on by twisting the single knob clockwise off the detent (in the switch), then turning it back down after the tubes warm up. 

Medved:

--- Quote from: TimFox on March 10, 2021, 12:14:16 am ---On vacuum tube radios, the power switch was usually mounted on the back of the volume control, so that one turned the unit on by twisting the single knob clockwise off the detent (in the switch), then turning it back down after the tubes warm up.

--- End quote ---

There it was a must to have it on the "cold" side, as it was really barely 1cm from the signal with no shielding within the pot whatsoever. Plus these had to be really dirt cheap (talk about "planned obsolescence" being a new thing, when these radios connected the scale illumination bulb in a way once it burned, it took the rectifier tube with it in a short time; circuit originated from ca 1936).
But even on the combos it played a role, because those usually required way better S/N then the radios, so even if it was 5cm away, it still caused hum. Plus often the wires were in various bundles, so not that far apart.

TimFox:
The 35 V 150 mA heater rectifiers used in US 110 V series-string radios had a tap for the panel bulb to allow operation with a burned-out bulb.  Radio repairmen were urged to replace the bulb to impress the client with an obvious improvement.

vk6zgo:

--- Quote from: Medved on March 10, 2021, 12:31:08 am ---
--- Quote from: TimFox on March 10, 2021, 12:14:16 am ---On vacuum tube radios, the power switch was usually mounted on the back of the volume control, so that one turned the unit on by twisting the single knob clockwise off the detent (in the switch), then turning it back down after the tubes warm up.

--- End quote ---

There it was a must to have it on the "cold" side, as it was really barely 1cm from the signal with no shielding within the pot whatsoever. Plus these had to be really dirt cheap (talk about "planned obsolescence" being a new thing, when these radios connected the scale illumination bulb in a way once it burned, it took the rectifier tube with it in a short time; circuit originated from ca 1936).
But even on the combos it played a role, because those usually required way better S/N then the radios, so even if it was 5cm away, it still caused hum. Plus often the wires were in various bundles, so not that far apart.

--- End quote ---

I can only speak from the Australian viewpoint, where the standard in those years was to use a transformer power supply.

Oz tube radios switched the active ("hot") side of the Mains, removing power from the transformer primary.

The switch was normally in a separate part of the pot body, with just the mechanical part that operated the switch protruding between them.

In the usual 4/5 "valve" superhet, the audio out of the detector is at a relatively high level, thanks to the substantial IF gain, so any hum induced was a long way down, in comparison.

In any case, the "cold" side of the Mains in a transformer power supply did not return to chassis earth (ground), the common connection between those being at the Mains entry to the house, allowing hum or other noise pickup on the house wiring, if that was really a concern.

The only time I have seen the dial illumination lamps connected directly to the rectifier are with those rectifiers which were directly heated with 5v.
Probably to use up old stock, some older radios using these had 5v dial lamps directly connected to the rectifier filaments-------and the HT line!!
A "trap for young players", indeed!

When a dial lamp burnt out, it normally went open circuit, so I fail to see how that could affect the life of the rectifier tube in such a design.
If the insulation failed, maybe that would do it.

james_s:

--- Quote from: vk6zgo on March 10, 2021, 01:54:52 am ---When a dial lamp burnt out, it normally went open circuit, so I fail to see how that could affect the life of the rectifier tube in such a design.
If the insulation failed, maybe that would do it.

--- End quote ---

IIRC the bulb was wired in parallel with a tapped segment of the tube heater, so if the bulb burns out the voltage across that segment rises due to the higher resistance. This causes that part of the heater to run hotter and burn out.

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