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 Post subject: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sat 03, 2018 6:45 pm 
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I've read threads here about replacing incandescent bulbs with LEDs. I successfully did this on my Marantz 2250 with the dial lights. But, I don't use the Marantz for DXing. For all the "All American Five" radios I've rebuilt, I want to replace the incandescent bulbs also. But I use these old radios for DXing. :-)

So my question is, does converting an incandescent bulb to an LED (with resistor in series) in an old radio create any interfering spurious signals across the longwave, mediumwave, and shortwave spectrum? Won't the diode switching at 60 cycles per second produce harmonics all over the spectrum?


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sat 03, 2018 7:02 pm 
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The AC power frequency is irrelevant- it is the rate at which the switching transition takes place that can cause EMI. I'd be inclined to first try it to see if there actually is a problem. If there is, it can be solved by shunting the LED with an appropriately-sized capacitor.

Since most transformerless sets require that the dial lamp consume a specific amount of current in order to ensure the correct voltage across it, you may find it necessary to also shunt your LED with a resistor that in combination with the current draw of the LED gives the proper voltage across the pair.

At one time it was standard design practice to bypass power supply silicon diodes with caps, presumably in order to prevent EMI; but this seems to have been mostly abandoned.


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sat 03, 2018 7:06 pm 
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I don't think you will have any noise. The "switching" is really quite slow and harmonic energy will be minimal.

Caution: an LED is a diode, but it is not a RECTIFIER. The max reverse voltage on most LEDs is spec'd at about 4 volts peak. If you connect it to 6.3 V rms, you will have a peak voltage around 9 Volts. Sometimes a LED will survive this, but it is not good practice. Put a 1N4004 or better in series (or in parallel with the LED) so it does not see reverse voltage.

Image

You would use a different series resistor on 6.3 V rms, but the idea is the same.

If the old bulb is in the filament string of an AA5, you will need an appropriate shunt resistor. LEDs do not survive 150 mA!

Rich


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sat 03, 2018 8:17 pm 
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Lorenz and Rich:

Yes, it is in series with the tube filaments, so I will use an appropriate resistor in series. And I will try it to see if it needs a shunt cap. Many thanks for the tips and the quick replies!

I'm also considering adding LEDs in old radios that don't have any lamps at all, just to light up some dials that could use it, and your tips will be very useful.


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sat 03, 2018 11:00 pm 
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https://www.pinballlife.com/index.php?p=product&id=2561

https://www.pinballlife.com/index.php?p=product&id=3108

NP with interference, they are plug and play, so no need for a Diode or resistor. Here's two of my radio's using them; the Zenith has one on each side of the dial.


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sat 03, 2018 11:06 pm 
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I just replaced a 47 bulb in an AA5 with an LED circuit. Installed a 47 ohm (2 watt) resistor in place of the 47 bulb. Then added 2 LED's in parallel but inverse (anode to cathode) with a current limiting resistor in series (I believe it was 100 ohms). Looks very nice.


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sun 04, 2018 12:32 am 
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NP with interference, they are plug and play, so no need for a Diode or resistor. Here's two of my radio's using them; the Zenith has one on each side of the dial.


Is one of those an AA5? If so, you are the luckiest person in the world. No current sharing or limiting resistor? I bet those LEDs are almost incandescent?

Rich


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sun 04, 2018 1:38 am 
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Fred M wrote:
Lorenz and Rich:

Yes, it is in series with the tube filaments, so I will use an appropriate resistor in series. And I will try it to see if it needs a shunt cap. Many thanks for the tips and the quick replies!

I'm also considering adding LEDs in old radios that don't have any lamps at all, just to light up some dials that could use it, and your tips will be very useful.

Biggest issue here is the 35Z5 MUST have a parallel load/shunt across the heater tap or it will be drawing excessive current... Without it tube life will be shortened, same as operating radio with blown dial bulb...

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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Sun 04, 2018 1:57 am 
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Rich, W3HWJ wrote:
Quote:
NP with interference, they are plug and play, so no need for a Diode or resistor. Here's two of my radio's using them; the Zenith has one on each side of the dial.


Is one of those an AA5? If so, you are the luckiest person in the world. No current sharing or limiting resistor? I bet those LEDs are almost incandescent?

Rich

Here's the print for the Coronado;
http://www.nostalgiaair.org/pagesbymode ... 007007.pdf

Your recommendation for making it better -with my LED replacement installed- will be followed.

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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Mon 05, 2018 5:15 am 
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Well after torturing a 6v LED lamp I dunno if they need anything other than maybe a parallel resistor... To start with they are bipolar, tried one at both polarities on DC supply with as much as 20v and it just kept on operating... Current starts zipping upward around 8v, was over 125 ma at 20v... I only tried this for maybe five second bursts...

In a AA6 that has a factory 100 ohm across tap/bulb, I measured 6.65v without dial lamp and 3.4v with conventional #47... With the LED it was 5.9v... Removing lamp & clipping resistor, tap voltage increased to 10.6v, which shows why its important to replace burned out dial lamp(or at least add a parallel resistor) ... With #47 voltage was 4.25v & 8.05 with LED...

So there should be at least a parallel resistor, a pair of 5.6v zeners back to back or cathode to cathode would probably be best solution...

BTW the LEDs I have only begin to light at 3v and then just barely visible, by 5.5v they are almost normal, increasing only slightly at 6.3v..

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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Mon 05, 2018 9:23 am 
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Quote:
Yes, it is in series with the tube filaments, so I will use an appropriate resistor in series.


You still need to preserve the current through the filament string but limit the current through the LED to perhaps 20mA each. You need a series resistor to replace the dial lights, find how much voltage there is available across that resistor, then parallel all the LEDS in series with another resistor to establish the current and yet another rectifier diode to block the reverse voltage.

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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Tue 06, 2018 12:27 am 
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Joined: Jan Tue 16, 2007 7:02 am
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Location: Lexington, KY USA
Two different sorts of "LED"s are being discussed here, usually without reference to which sort a given post is about.

The Pinball Life, and other bulb replacement LED lamps, include some sort of series current limiter in order to work at the rated voltage. Usually no series resistor is needed for these. They are usually designed to work properly on AC, as well as DC.

A simple LED requires a series resistor to limit the current, and is itself DC only.

Either one will likely operate at a lower current than the original incandescent bulb. This does not matter in a transformer set, but in an AC/DC, AA5, sort of radio, the original bulb is usually part of the circuit, so a parallel resistor will be needed to avoid problems.

When measuring the voltage across the lamp in an AC/DC radio, you must use a meter that is accurate with the waveform found across the lamp. The sort of meter that will work in all cases is often called "AC+DC True RMS" responding.

Lacking such a meter, it is usually close enough to measure the voltage across the original bulb, then adjust the shunt resistor so the voltage measures no higher across the replacement LED circuit. ( For the AC/DC radio.)

The replacement LED lamps are nice because they can fit the original socket and require at most a shunt resistor for operation in a radio. The
suppliers offer a warm white color that will be "cooler" than the original bulb, but probably close enough. Especially through a yellowed dial and dial cover.

Ted


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Tue 06, 2018 1:24 am 
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35Z5 wrote:
Fred M wrote:
Lorenz and Rich:

Yes, it is in series with the tube filaments, so I will use an appropriate resistor in series. And I will try it to see if it needs a shunt cap. Many thanks for the tips and the quick replies!

I'm also considering adding LEDs in old radios that don't have any lamps at all, just to light up some dials that could use it, and your tips will be very useful.

Biggest issue here is the 35Z5 MUST have a parallel load/shunt across the heater tap or it will be drawing excessive current... Without it tube life will be shortened, same as operating radio with blown dial bulb...


Thanks for the info. The one I just rebuilt does have a 35Z5.


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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Tue 06, 2018 3:10 am 
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Usually Lurking wrote:
The replacement LED lamps are nice because they can fit the original socket and require at most a shunt resistor for operation in a radio. The suppliers offer a warm white color that will be "cooler" than the original bulb, but probably close enough. Especially through a yellowed dial and dial cover.

Ted

Thanks for setting record straight...

I have pinball type a friend sent me as thanks for a couple tubes...

I'd been planning the switch, just needed this little push to activate the process... I've now installed a 68 ohm across lamp tap which puts voltage on LED at 5.5v, no other changes... Going to use it as daily for a while to test longevity of(almost) direct replacement type in this application...

These must be cold white as the light is frigid, gave me chills just looking at it... Sooo in a attempt to warm things up I used a yellow Sharpie to paint lamp, green as a gourd, yech... Cleaned that and used orange, now appears to be a neon lamp, not in love with that either... Probably will try another marker or maybe transparent cellophane, for time being it's going to be orange...

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 Post subject: Re: LED replacement for 6.3v incandescent bulbs
PostPosted: Mar Tue 06, 2018 5:32 am 
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My Pinball LED's are warm white.

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