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Old 21st Feb 2018, 8:22 pm   #1
stevehertz
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Default Panel & dial illumination bulbs - specs

This post is regarding miniature incandescent bulbs used for illuminating panels, dials and function lamps on vintage hifi equipment.

Typical Japanese specifications and parts lists identify them with a voltage and a current rating. Firstly, is this an AC or a DC rating with regards to replacements ie the actual spec of the bulb, NOT what it's used on? Because in some instances they are fed from either type of current and this appears to create anomalies. Here's an actual example, bulbs specified at 6V, 30mA are being fed by 12.5V DC? Hence the need for clarification. On the face of that example, it's suggesting to me that they are AC specified/rated, but can be run on DC voltages of up to double the AC one (kind of thing). There is one thing however, I measured the 12.5VDC with O/C bulbs fitted, perhaps the DC voltage is dragged down with good bulbs? dunno.
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Old 21st Feb 2018, 8:29 pm   #2
Dave Moll
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Default Re: Panel & dial illumination bulbs - specs

Quote:
I measured the 12.5VDC with O/C bulbs fitted, perhaps the DC voltage is dragged down with good bulbs? dunno.
I imagine that measuring the voltage with a 200Ω resistor across the bulb holder should answer that question.
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Old 21st Feb 2018, 8:59 pm   #3
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Default Re: Panel & dial illumination bulbs - specs

Well, I've actually got bulbs in there now, so if I went out to my workshop I could just measure it. That point was kind of anecdotal to the main question and came out as I started the thread. The question being, how are these bulbs normally specified etc?
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Old 21st Feb 2018, 9:09 pm   #4
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Default Re: Panel & dial illumination bulbs - specs

It won't make any difference, eg: 6 volts AC (RMS) and 6 volts DC (non varying) are the same so far as power goes for the same current.

Lawrence.

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Old 22nd Feb 2018, 6:34 am   #5
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Default Re: Panel & dial illumination bulbs - specs

Ok. I thought perhaps the inductive component of the element may play a part.
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Old 22nd Feb 2018, 11:43 am   #6
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Default Re: Panel & dial illumination bulbs - specs

Only if the bulb feed is UHF!

Depending on the mindset of the designer, panel lamps may be deliberately underrun for longevity which could lead to strangely high voltage ratings being specified. Another way to do this is to feed a lower voltage bulb via a series resistor which will give the opposite- strangely low voltage ratings.
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Old 22nd Feb 2018, 11:58 am   #7
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Default Re: Panel & dial illumination bulbs - specs

Another trick to lengthen the life of the bulbs is to wire a diode in series so they're fed with 1/2-wave rectified AC. This can confuse people when measuring the voltage across the bulbs, just as happens with diode-fed series-heater chains as used in some 1960s TVs.
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Old 22nd Feb 2018, 12:27 pm   #8
G8HQP Dave
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Default Re: Panel & dial illumination bulbs - specs

A 6V bulb fed directly from 12V will have a very short but happy life. You need to check again.

The voltage spec for an incandescent bulb is RMS, which is the same as DC.
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Old 22nd Feb 2018, 1:39 pm   #9
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Default Re: Panel & dial illumination bulbs - specs

I have put incandescent bulbs in to radio dials and meter dials that were never there in the first place. Have a look at page 18 of this article. I mounted the bulbs on a strip of pcb material etched down the middle with segment of tracks so I could put them inside the lower part of the dial escutcheon in a series string. Also I added them inside the meters.

http://worldphaco.com/uploads/WORLDFETRON.pdf

The key point using these small bulbs is never run them at their rated DC (or RMS) voltage, they should be run at about 75% to 85% of that. This way the bulb life is greatly increased and you won't find yourself having to replace them. Which is important if they are a hard wired in bulb.

However, there are other factors which can destroy the incandescent lamp, even if they are run at a modest filament temperature.

For example in early VHS video machines a small lamp was used to operate the optical sensors that detected the beginning and end of the tape in the cassette, due to the fact there was an optically transparent tape leader at each end.

The video cassette of course was ejected in the old machines by a mechanism that lifted the cassette tape up above the top of the machine's deck. It had a strong spring. Without some kind of damping it would slam the mechanism up to a sharp stop.

So the designers of the cassette eject mechanism used a damping design taken from 19th century music boxes, with a paddle wheel spinning in air, to regulate the speed of the upthrust of the cassette carrier. When this mechanism fails the cassette carrier mechanism slams to a stop at the end of the eject process and the mechanical shock wave often breaks the filament in the cassette lamp, even though the filament is being run conservatively.
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Old 22nd Feb 2018, 3:04 pm   #10
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Default Re: Panel & dial illumination bulbs - specs

Interesting stuff Argus, thanks.
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Old 22nd Feb 2018, 3:42 pm   #11
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Default Re: Panel & dial illumination bulbs - specs

Yes, as others post, the voltage rating is exactly the same for AC or DC, presuming that the AC figure is for RMS voltage.

Although lamps for household lighting are much bigger they work in the same way, remember that back in the old days, that some mains supplies were AC and others were DC and it mattered not for filament lamps.

Torch bulbs though often superficially similar to pilot lamps are designed to very different specifications.
Torch bulbs are designed to extract the greatest possible light from the very limited and very expensive energy in batteries. As a result the life varies from a few hours to a few dozen hours.

Pilot lamps are designed primarily for very long life from ample and cheap electricity from mains supplies or vehicle electrical systems.
Lives vary from a thousand hours up to tens of thousands of hours.
1000 hours is only about 6 weeks of continual burning on say an industrial control panel. A 10,000 hour life would be a sensible minimum in such cases.

Unfortunately many vendors of miniature lamps do not differentiate between pilot lamp and torch bulbs so it is left to the purchaser to guess, based upon the most likely rating.
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Old 22nd Feb 2018, 4:37 pm   #12
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Default Re: Panel & dial illumination bulbs - specs

Quote:
Originally Posted by G6Tanuki View Post
Another trick to lengthen the life of the bulbs is to wire a diode in series so they're fed with 1/2-wave rectified AC. This can confuse people when measuring the voltage across the bulbs, just as happens with diode-fed series-heater chains as used in some 1960s TVs.
I'm not too convinced that a diode is beneficial in the way you suggest. Some models of Gulbransen electronic organs were fitted with console illuminating lamps (24v, about 6 or 7 in parallel) with a diode as you describe, and I seemed to be changing these lamps at least, if not more often than I might if no diode was fitted. The lamp filament temperature would be cycling up and down in sympathy with the mains frequency, and so will the lamp current. The only (dubious) benefit I could see was that this gave a "strobe" effect which might have made the player's hands appear to be more dextrous than in reality!
An A.C. supply keeps filament migration low, and if at about 80% or less of the rated voltage of the lamp, increases lifespan very noticeably. Tony.
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Old 22nd Feb 2018, 6:15 pm   #13
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Default Re: Panel & dial illumination bulbs - specs

A 6V bulb can be run on 12V so long as it is connected in series with another similar bulb. Measuring the voltage when the bulb is removed, will indicate 12V but this is not the running voltage.

I use 8V bulbs on 6.3V for dial lights.

I was given the task of repairing old christmas tree lights and decided to put a diode in series to avoid further work. When the light strings were broght out of dtorage, the first string went pop with a bright flash. The user then went LED.
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Old 22nd Feb 2018, 6:52 pm   #14
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Default Re: Panel & dial illumination bulbs - specs

Many years ago I found that a 6V 60mA bulb could be modulated with audio. Although somewhat muffled this suggests that a small bulb with a thin filament could have a small enough thermal time constant to be sensitive to the difference between 100Hz heating (normal AC) and 50Hz heating (half-wave rectified AC). Temperature cycling is not good for metals.
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Old 22nd Feb 2018, 8:16 pm   #15
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Default Re: Panel & dial illumination bulbs - specs

Yes, very low current bulbs can flicker at line frequency or even low audio frequencies. I suspect that attempts to under run such lamps via a series diode would not be entirely satisfactory.
Note that this is related to lamp CURRENT and not to wattage, since it is low current lamps that have thinner and therefore more responsive filaments.

The lowest current lamp that is readily available is 6 volts, 40ma. Still lower current lamps do exist but are not exactly common or readily available.
I have some 5 volt 16ma lamps, and also 240 volt 10ma.
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Old 22nd Feb 2018, 8:39 pm   #16
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Default Re: Panel & dial illumination bulbs - specs

On the subject of availability I often find that the values in vintage hifi units are the ones no longer made or stocked! And yes, I am an expert 'ferriter out' of hard to find parts! You just have to adapt others to suit or go down the LEDs route. But they also require mods and sometimes filters (glass paint) as they can be too bright and 'bluey'. In short, bulbs are the bain of the vintage hifi restorer.
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Old 22nd Feb 2018, 9:03 pm   #17
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Default Re: Panel & dial illumination bulbs - specs

It’s quite common for 7 or 8 volt bulbs in Japanese amps running on 6.5 volt rails in valve amps back in the 60’s. Gave enough brightness and a good long life.
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Old 23rd Feb 2018, 3:42 am   #18
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Default Re: Panel & dial illumination bulbs - specs

Rapid Electronics have a reasonable of miniature lamps including some very odd ones, but as with many suppliers they do not seem to know if these are torch bulbs, that will have a very short life in continual use, or pilot lamps that will be disappointingly dim in a torch.

International lamps can get various odd lamps to order, but you may need to order 100.
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Old 23rd Feb 2018, 4:00 am   #19
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Default Re: Panel & dial illumination bulbs - specs

Quote:
Originally Posted by broadgage View Post
Yes, very low current bulbs can flicker at line frequency or even low audio frequencies.
This can also apply to mains light bulbs such as the 240V 15W variety which have a fairly thin long filament.

Back in the 1970's my brother made a party strobe light pulsing an array of these with a brief 400V DC pulse, derived from some KT88's. It was very effective, as good as a gas discharge tube and the filaments heated & cooled more than rapidly enough to get a good stroboscopic effect. It was surprising how long they lasted, I can't recall what the pulse duration was.
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Old 23rd Feb 2018, 5:35 pm   #20
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Default Re: Panel & dial illumination bulbs - specs

Quote:
Originally Posted by G8HQP Dave View Post
..the difference between 100Hz heating (normal AC) and 50Hz heating (half-wave rectified AC).
Are you sure about that? I was under the impression that normal AC in the UK was 50Hz. If this is half-wave rectified, you still get 50Hz I thought. Full-wave rectification results in 100Hz, surely?
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