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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 11th Oct 2019, 4:48 pm   #1
Diabolical Artificer
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Default Cretinous idea - The combined volume pot and mains SW.

After trying to trace a hum/noise issue on a guitar amp, looking at all the usual potential areas like layout of AC heater wires, grounds, possible htr to cathode SC's etc etc, I found the problem - the combined pot and mains switch.

Never occurred to me before, but thinking about this, it is a monumentally stupid idea to put a source of high voltage AC right next to a pot that often connects straight to the control grid of a high gain IP stage. I mean why bother laying the heater wiring into the corners of the chassis and wiring the valve socket heater pins properly, keeping the mains tfmr away from the OPT and orientating it 90 deg, etc, etc and then plonk 240v RMS 1/8" next to the wiper and wire that goes to the grid. Madness.

Andy.
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Old 11th Oct 2019, 5:09 pm   #2
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Not only that, but every time the amp or W.H.Y is turned on the pot's wiper is moved over the track, eventually wearing it down. A completely separate OFF/ON switch would be better.

Some radios gang the OFF/ON switch with the tone control.
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Old 11th Oct 2019, 5:13 pm   #3
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Default Re: Cretinous idea - The combined volume pot and mains SW.

"every time the amp or W.H.Y is turned on the pot's wiper is moved over the track, eventually wearing it down" Good point Graham. We get loads of "pot/SW" wanted threads but very few requests for the other pots in a radio set or whatever.

Andy.
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Old 11th Oct 2019, 5:26 pm   #4
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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Some radios gang the OFF/ON switch with the tone control.
Which in many cases still means there is mains-voltage AC and slow-level-audio wiring in close proximity so the opportunity for hum is still there.

(the combining tone-control and on/off does however reduce the wear on the low-level part of the volume-control pot and hence the usual 'scratchiness').

I've seen at least one radio where the on/off was combined with the wavechange switch - meaning hum-modulation of the local-oscillator!
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Old 11th Oct 2019, 5:36 pm   #5
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Default Re: Cretinous idea - The combined volume pot and mains SW.

"where the on/off was combined with the wavechange switch - meaning hum-modulation of the local-oscillator! " this was more or less the case with this SW/pot. It formed part of a tremelo circuit, it was modulating the combined tremelo/signal, so you had a tremelo, tremelo sort of effect!

A.
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Old 11th Oct 2019, 7:06 pm   #6
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Default Re: Cretinous idea - The combined volume pot and mains SW.

I've always felt that combining sw. and pot in that way doesn't make sense ergonomically either. After all, when you want to turn off, you don't want to change the volume or tone setting as well.

I always liked the push-pull switches on the volume control of the old Beolits - could be switched on/off without altering settings, and with battery operation no mains proximity problems.

Mike
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Old 11th Oct 2019, 7:13 pm   #7
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Absolutely cretinous. stupid in two ways.

I realy do want a new word adding to the English language: "Pessimised"

Like optimised, but in the opposite direction.

David
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Old 11th Oct 2019, 7:15 pm   #8
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Default Re: Cretinous idea - The combined volume pot and mains SW.

If the switch is single-pole then you can switch the neutral and at least the electric-field part of the 50Hz noise signal is then all but eliminated. Safety arguments can be made against switching the neutral rather than the live. But at least one major vintage manufacturer's pre-amp will only meet its hum spec if you do this (I had that confirmed from the horse's mouth when I spoke to one of their service staff).

Cheers,

GJ
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Old 11th Oct 2019, 7:24 pm   #9
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Default Re: Cretinous idea - The combined volume pot and mains SW.

I suppose the thinking is that you dont get a nasty surprise and wake everyone up when you forgot how loud you had it last time it was used.
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Old 11th Oct 2019, 8:09 pm   #10
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Built in obsolescence?
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Old 11th Oct 2019, 8:15 pm   #11
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Depends on the design life. Twenty years I'd say for valve equipment.
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Old 11th Oct 2019, 9:03 pm   #12
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Yep - crazy .... almost as bad as the concentrically sleeved volume and tone control shafts on some Grundigs [E.g. the Type 97's] - which always seem to seize solid [but at least they usually use 'piano key] on/off switches on those]!
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Old 11th Oct 2019, 9:19 pm   #13
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Default Re: Cretinous idea - The combined volume pot and mains SW.

It is also poor practice to runs mains wiring up to a control panel area with signal control switches on it, both from the perspective of hum pickup & and electrical safety. I think they even recommend against this practice in Horowitz & Hill in the construction section.

It's better to have the mains power switch on the rear of the instrument and this is where switched & fused IEC sockets come in handy.

Some manufacturers like Tektronix & Philips got around this issue in their instruments by having the power button connected to a long plastic arm and the mains switch itself nearer the rear of the instrument. Also another trend was that the power button has a passive mechanical optical indicator that changes when pushed.

For all of my mains powered projects now I just used the switched & fused IEC socket on the rear panel.

I don't think it's legal to switch the neutral alone.
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Old 11th Oct 2019, 9:41 pm   #14
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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Some manufacturers like Tektronix & Philips got around this issue in their instruments by having the power button connected to a long plastic arm and the mains switch itself nearer the rear of the instrument. Also another trend was that the power button has a passive mechanical optical indicator that changes when pushed.
Yes, in the past I've seen kit that has a push-pull mechanical Bowden-cable type arrangement between a front-panel on/off control and the actual switch/power-supply (this was in telco gear, where you really didn't want to extend the rather thick 48VDC-at-tens-of-amps supply-cables from the rear of the case to a front-panel switch).

Though they got the ergonomics wrong and had 'knob-pushed-in' as off, so you had to be very careful not to nudge the knob while adjusting any other controls.
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Old 12th Oct 2019, 8:56 am   #15
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Default Re: Cretinous idea - The combined volume pot and mains SW.

As we all know, most (UK)Manufacturers of domestic Radios, etc., fitted the combined volume pot with on/off switch, usually of the rotary type, regardless of the very valid objections to this practice mentioned in this thread. No doubt it was for reasons of cost, as things often are.
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Old 12th Oct 2019, 9:22 am   #16
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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Originally Posted by G6Tanuki View Post
Though they got the ergonomics wrong and had 'knob-pushed-in' as off, so you had to be very careful not to nudge the knob while adjusting any other controls.
That might be a safety feature. If you had to turn the thing off in a hurry it's a lot easier to press the knob in than fiddle around to pull it out.
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Old 12th Oct 2019, 9:39 am   #17
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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... No doubt it was for reasons of cost, as things often are.
Another advantage, which applies more in some situations than others, is that by forcing the volume to zero before disconnecting the power the switch-off click/thump can be limited to that which gets into the circuitry after the pot. This can be handy in hifi systems with high power amp stages.

Cheers,

GJ
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Old 12th Oct 2019, 9:41 am   #18
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Default Re: Cretinous idea - The combined volume pot and mains SW.

"I realy do want a new word adding to the English language: "Pessimised"" That's a good word, I'll file that away in my lexicon of cromulent words.

"It is also poor practice to runs mains wiring up to a control panel area with signal control switches on it" Valuable advise, didn't know there was a construction section. I'm left with a bit of a dilemma, IE change the current arrangement and possibly reduce the amps re-sale value and also possibly leave myself open to litigation should something go pearshaped; if I do change it where to put said mains SW as room is tight. This is a two pipe problem.

" No doubt it was for reasons of cost, as things often are." If this was the case then why did manufacturers build radio chassis's out of thicker gauge steel than needed, use complicated tuning dial arrangements and sometimes over engineer old sets? I think part of the use of combined pot and Sw was just lazy design and thinking and cos everyone else did it. Humans act like sheep sometimes.

Andy.
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Old 12th Oct 2019, 10:46 am   #19
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Moving a pot wears out the track, but it also tends to clean the track. Leaving a pot unchanged for a long time seems to lead to a layer of grot appearing everywhere else on the track, which has a similar effect. You cannot win!

I suspect the combined volume and on/off control may have been an attempt to reduce the number of knobs, so that this lovely new piece of furniture they wanted you to buy did not look too much like the control panel at Jodrell Bank.
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Old 12th Oct 2019, 1:06 pm   #20
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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Another advantage, which applies more in some situations than others, is that by forcing the volume to zero before disconnecting the power the switch-off click/thump can be limited to that which gets into the circuitry after the pot. This can be handy in hifi systems with high power amp stages.
This is an analogy of a much more important scenario.

In a TV set, when it is turned off, the scan collapses. This can result in a phosphor burn in the center of the CRT where the beam current gets concentrated, because EHT is still present and the beam current is still significant due to cathode emission. This can damage the phosphor.

Even in popular culture, there was the notion of the white dot in the center of the TV screen, after you turn off the TV set has some meaning.

This has even been the subject of comedy. Take the TV series "The Young Ones". Somebody asked..... that white dot in the center of the screen...what does it mean ? Neil replied: "It means something really heavy, it means there is no more TV"

So if there was ever a case where a rotary control should be connected with an on-off power switch, it would be to put the power switch on the CRT's brightness control, in a TV, so it was zero when the set was switched off to suppress the white spot.

Of course designers wised up to this, and in better designs had "spot killers" that take the CRT's grid very negative at turn off, to prevent the white spot and phosphor burn, regardless of the angular position of the brightness control.
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