Re: Cretinous idea - The combined volume pot and mains SW.
 

Its easy enough to do. I did it like that for input switching of RCA sockets.
All the bits came from the junk box. Amplifier was 4 X 807 per channel and built in twin power supplies with large choke filters. I couldnt lift it :laugh2: Hence it now being stripped out.

Joe

Re: Cretinous idea - The combined volume pot and mains SW.
 

I started using the word "pessimise" some 25 years ago when taking items from work to the EMC test house. The blokes there would scan the frequencies looking for peaks of noise, and then rotate the antenna, the work piece etc, to, in their words, "optimise" the height of the peak on the analyser. I wanted the thing to pass, so my name for their "optimise" was "pessimise".

Re: Cretinous idea - The combined volume pot and mains SW.
 

It was never that big a problem .

The electron beam, for that short while, didn't really do much harm.

What did cause the classic burnt "spot in the middle of the screen" were the negative ions, which were also, unfortunately, emitted, but not deflected by the magnetic deflection, so continued on their merry way to hit the screen "dead centre", all the while the CRT was in use.

They travel slower than electrons, but are far more massive, & are much more prone to damage the phosphor.

Earlier CRTs had "ion traps" where the electron gun was initially pointed off centre, then the beam was deflected back into the right path by a fixed magnet.
the ions were not so deflected, & hit the side of the tube.

This was a complex system, & was later made obsolete by "Aluminised" screens, which were sufficiently "ion burn" resistant to do without.

Re: Cretinous idea - The combined volume pot and mains SW.
 

Maybe not a big problem, but it was definitely possible to have a spot burn in from the electron bundle if it was still focussed at switch off.

Re: Cretinous idea - The combined volume pot and mains SW.
 

I remember an old television at my grandparents house which was originally bought in New York in the 1960s. It's the only TV I've ever seen that had a 'turn-off-spot' which lasted for up to a minute or so as I recall (it was a long time ago). I never noted any burn-out effect in the area where the spot persisted. But it's the only TV I've ever seen to exhibit that effect, so I'm assuming that most televisions of later date either had a purposely designed circuit to avoid the spot (i.e. a 'spot killer' as mentioned earlier in the thread), or that by design the EHT dies down fast enough so the spot doesn't appear.

Getting back on topic, one advantage of the on/off switch + volume control is that the unit basically has one single everyday control, so it's a single-control user interface in more modern parlance. Logically it makes sense, you turn the volume down, and when it's so low you can't hear it you might as well turn it off completely.

Certain members of the general public would be scared stiff if they had more than one control to operate - I've heard stories which I don't think are myths of people coming in to the dealer to have them select a different channel on their radio, or a different volume level. So having the power switch and volume control in one forces them to understand how the volume control actually works.

Not having the unit potentially burst out at maximum volume when switched on would be another advantage, on the other hand with older valved equipment, the sound would come up gradually giving the user (assuming he or she was still close to the unit) time to manually turn down the volume if indeed necessary.

Re: Cretinous idea - The combined volume pot and mains SW.
 

There was the combined volume control/on-off switch which solved part of the problems. Can't remember the manufacturer, but you turned it to alter the volume, and push/pulled it to operate the switch.

It saved the wearing out of the track, and meant that volume could be as last-used.

Of course, it still needed the mains wiring to be brought close to the signal, and it introduced the new problem of knobs being out of line in one position - it it was positioned within a group of similarly-knobbed controls, that is. Though, surprisingly, with about 1/8" of movement, one knob sitting proud (or back) did not look obvious.

Re: Cretinous idea - The combined volume pot and mains SW.
 

Actually it did and is a very significant problem, that is on CRT's that have non-aluminized screens.

Not only did the aluminization protect from ion burns but it made the phosphor much more resistant to high beam current burns at switch off with an intense central spot.

I have evaluated a large number of non-aluminised pre WW2 CRT's and I can assure you the phosphor is extremely easily damaged by the turn off spot. I assume it is better with aluminization due to the thermal conductivity of the thin layer of aluminium.

For example I was able to detect screen phosphor desensitization in my 904 after only 3 or four episodes of turn off spot in an NOS 5FP4, which is why I modified the circuit to suppress the beam current at turn off.

Re: Cretinous idea - The combined volume pot and mains SW.
 

I vividly remember those slowly receding spots in the middle. I stayed with an aunt back in 1953, and she had a TV; I saw the "interlude potter" a few times on that TV..
later(1964/5) I played with my first old TV (which I did get to work briefly), and read about angled guns and ion trap magnets. Adjusted them on sight for optimum picture.
later, I read about aluminised tubes, guns now in line again, and no bother with ion burn.
However, I have seen (and caused at least once) a burned out central spot on a colour TV tube when the scanning stopped, but the beam did not (coils unplugged?).
Les.

Re: Cretinous idea - The combined volume pot and mains SW.
 

Back to the title of this thread!!

I have worked on guitar amps that have the tremelo pot ganged with the mains switch.
This is mounted smack bang in the middle of ALL the pots.
It does cause hum. My method of curing it ( ALMOST completely ) is to wire the on/off switch in balanced microphone cable. The two conductors obviously carry the mains voltage, and the screen is connected to ground at the feed end. ( IEC socket/mains cable etc )

Joe

Re: Cretinous idea - The combined volume pot and mains SW.
 

If we are concerned about hum resulting from mains live wiring then twin-core mainscable with an in-built screen is available. This has the advantage over mic cable that it meets the regulatory requirements for mains operation. Otherwise sliding copper braid over the outside of ordinary mains cable also works. It doesn't fix the problem of noise from the switch mechanism itself leaking into the volume pot track, of course.

Cheers,

GJ

Re: Cretinous idea - The combined volume pot and mains SW.
 

In times-past, when doing stuff for clients-best-unmentioned we bought a load of seriously-screened moulded 13A-to-IEC309 leads from a company called "Rendar" with the option for the foil/overbraid tied to protective-ground at either the IEC309 end, the 13A end, or both.

[Yes it was horribly expensive - think modern 'audiophile' pricing for power-leads - but it was a "Cost-plus" contract and the UK taxpayers were paying for it]

I'm sure that if you had loads of money available you could source suitable foil/braid-shielded 2-core cable for wiring modern single-pole mains-switching-pots. Or you could just redesign the audio stages to be less-sensitive below 100Hz?

Re: Cretinous idea - The combined volume pot and mains SW.
 

You could indeed roll off the low-frequency performance, if that was unimportant to you. It might save you £1.48 plus VAT and P&P https://tm3connections.com//index.ph...l-screen-mains.

Cheers,

GJ

Re: Cretinous idea - The combined volume pot and mains SW.
 

Philips used those sometimes.

Re: Cretinous idea - The combined volume pot and mains SW.
 

When I restored a valve Armstrong amp it had screened audio cable carrying mains to the switch on the volume control. I replaced it with ordinary mains cable over which I slid some braid screening and some heatshrink insulation. I felt safer that way.

Peter

Re: Cretinous idea - The combined volume pot and mains SW.
 

OK, prewar CRTs are certainly a different story.
They were mostly originally similar to, or in fact, were, CRTs designed for Oscilloscope use.
Even with much more modern 'scopes, I well remember the adjuration to "turn down the intensity!"

Pretty much anything recognisable as a "TV" CRT from the 1950s on will not normally have a problem.

The kind of "burn in" I have seen over the years, is when a tube is deliberately underscanned to ensure a correct 3:4 ratio picture, or those tubes which have displayed a "quad screen" display for a long time.

I had a big Philips which had the latter problem, although it was really only obvious when the picture background was static -----fairly rapidly moving backgrounds masked it enough to be tolerable.

Re: Cretinous idea - The combined volume pot and mains SW.
 

Many decades ago I had a dream wherein I created the word "colomendy". I spent years searching for it but it doesn't exist except in my dream where it means "doing something for someone which they badly need but doing it in the worst possible way, on purpose" . For example, a librarian filing every book in the wrong place, a car mechanic disconnecting everything so the fix takes far, far longer or an electrical engineer deliberately mixing mains wires and volume pots...all are colomendic behaviours.

I will let you have my word for free.

Re: Cretinous idea - The combined volume pot and mains SW.
 

Most televisions from the 1960's and 1970's used combined on/off volume controls. It did not seem to cause to many problems with hum pickup even on some of the televisions that had a decent size and quality speaker fitted.
From the servicing point of view they could be a pain to change when the on off switch failed.