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 Post subject: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 2:43 am 
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Hi all. this is a follow-up to my topic "WTB: Resistor Line Cord AKA Curtain Burner" in the classifieds. I followed the advice to upgrade to a 12A8, 12K7, 12Q7, 35L6GT, 50Y6GT line-up, and now the radio plays. But the inrush current smoked a wirewound resistor that is in circuit with the dial bulb. It currently reads 40 ohm but not sure what it should be since I can't find a schematic. I've tried several different values as well as bulbs and they go off like flashbulbs when turned on. If I use the variac I can bring everything up slow and they are plenty bright but don't really want to have to do that. So far I've tried #47,49 and 51 types. I know the discussion settled on 150ma tube heaters being the best route. I think the bulbs I'm using are something like 220mA. Should I be looking for a 150mA bulb? I'm sure ohms law would sort this out but I don't know what angle to attack from.


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 3:12 am 
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viewtopic.php?f=15&t=363443&hilit=burner

With only the information provided, it seems that with the original resistive line cord and that resistor provided enough inrush limiting to protect the bulb. But with the modification to the tube line up, that extra cushion is not there anymore.

If you're convinced nothing else is wrong, maybe the fix is to install an additional inrush limiter (some people conveniently call it "CL-90").


Last edited by AJJ on Nov Wed 06, 2019 3:32 am, edited 2 times in total.

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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 3:27 am 
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http://rtellason.com/tubedata/50Y7-GT.PDF

Make sure there are no other wires on pin 6 of the 50Y7 socket. The only connections to the lamp should be one wire from it going to pin 6, the other wire going to pin 7 which will have at least one other wire on it. Use only a #44, #47 or #51 lamp.

Try that and report back.

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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 3:48 am 
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The series string tubes draw several times their rated 0.15A current when cold, long enough to blow a small bulb. The AA5 circuit solved this by putting the bulb across part of the 35Z5 heater, which carried the initial surge. None of your tubes have heater taps. Two ways I can think of to limit bulb current : first is to put the bulb in series the heater string and shunt the bulb with a pair of 6.8V or 7.5V zener diodes. 1W would be marginal; 5W is better. Second to provide a separate current source for the bulb - a 3.3 uF AC cap in series with 120V would generate no heat and limit current to 0.15A. You could use a lower current bulb and smaller capacitor, but the #47 or 51 bulb is universal .

Two 5W zeners will cost a buck at Mouser, AC cap is $2.

OK, a third way - change the rectifier to a 50Y7 and wire the bulb across the heater tap. I've never seen a 50Y7 but they seem to be available.

(The 120 Ohm CL-90 inrush limiter may do the trick too!)

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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 4:35 am 
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Why not use a 50L6 power tube and a 35Z5 rectifier? That would make more sense, and those tubes are the most common probably because they work the best.

Then use the standard tap on the 35Z5 to power your bulb.

I have some 120 volt mini bulbs that I pulled from a corpse years ago, good stand alone pilot lights.

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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 3:23 pm 
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Tom Bavis wrote:
first is to put the bulb in series the heater string and shunt the bulb with a pair of 6.8V or 7.5V zener diodes. 1W would be marginal; 5W is better.


The zeners clamp AC voltage at .707 of RMS... For full lamp brightness a 9.1v zener should be used, even values up to 12-15v reduce flair to almost nil.

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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 3:56 pm 
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I favor using a 3uF, 250VAC, cap to drive the lamp. That makes it independent of the tube filaments so you don't have to be concerned about what happens if the lamp burns out.

Jay


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 5:50 pm 
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With the zeners it makes no difference if lamp or even 35Z5 tap is burned out, voltage drop across them is same. The best selection will be 8.2v which gives around 5.8v drop. The normal drop for 35Z5 & #47 lamp is approx 5.2v.

Using cap, a quick off on before cap discharges will blow the bulb.

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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 7:49 pm 
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Didn't the original circuit have a resistor across the lamp? You need a resistor across the lamp to prevent burnout when first turned on.

Zener diode or series cap will work but wasn't used when this radio was built.

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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Nov Wed 06, 2019 10:35 pm 
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I don't know if this might apply to this case or not. I bought some nice looking brand new #47's at a swap meet awhile back. I think they were 50 cents apiece. They pop in 20 seconds or less. Finally, after a lot of messing around, I found some #47's at a hardware store for something $2.48 each. Expensive, but they work great. Their filaments are much heavier than the 50 cent ones.

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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Feb Fri 14, 2020 2:55 am 
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Location: Orlando, FL
Finally have a chance to mess with this again. I'd rather not modify any more than what I already did with the tube line-up. I subbed in a 47, 51 and a 46 in place of a 44 (pilot lights). None of them blow like the 49, but are all far too dim to be seen through the dial. The 47 does the best out of the group. Really bright with the inrush but settles down. I'm going to play around with bulbs but it would be nice to understand the math first so I can make an informed choice. I'm trying to balance standing up to the inrush with usability. Not looking for super bright but at least glow in anything other than total darkness. I was looking at trying next, a #45, #43 and #1850
specs are as follows:
#43, 2.5v/.5A/1.25W
#45, 3.2v/.35A/1.12W
#1850, 5v/.09A/0.45W
#47, 6.3v/.15A/0.95W

Ideally if I can be taught how to calculate for the proper bulb all the better. What makes it harder is I've never been able to find a schematic.
"Give a man a fish he eats for a day, teach a man to fish he eats fore a lifetime." or something like that.
Thanks all.


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Feb Fri 14, 2020 3:20 am 
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Location: Lexington, KY USA
I think the original circuit worked without blowing the lamp because there was a large dropping resistance to limit the current until the tubes warmed up.

The present tube line up was chosen to allow plug-in replacement of the original lower voltage 300mA series string tubes.

Has anyone suggested trying an LED replacement bulb? The pinball parts guys have these in different colors, including yellow and warm white. Different versions beam the light in different directions. We are getting reports of good results with these in radios.

The zener diodes, as suggested, are a good solution. Not a big modification, or hard to reverse.

Ted


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Feb Fri 14, 2020 3:45 am 
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Location: Austin, Texas
ff4312 wrote:

Ideally if I can be taught how to calculate for the proper bulb all the better. What makes it harder is I've never been able to find a schematic.
"Give a man a fish he eats for a day, teach a man to fish he eats fore a lifetime." or something like that.
Thanks all.


It's not just a calculation to select the right bulb. When the tubes are hot, the filament string current is 150mA so a #47 bulb should be ideal.
However, when the tubes are cold, the resistance of 150mA tubes is about the same as their voltage rating. For example, the 50Y6 will have a cold filament resistance of about 50 ohms. The total cold resistance of the tube string will be around 120 ohms so the initial current will be 120 volts/120 ohms or about 1000mA. The tubes can take that much over current but the tiny pilot lamp goes off like a flash tube.

One way to reduce the initial current surge is to add a NTC resistor in series with the tube string. The NTC has a high resistance when cold and then goes to a low resistance when it gets hot. However adding 90 ohms does not reduce the inrush current all that much and you may still blow the bulb if you do an off/on too quickly.

Another suggestion was to add two zener diodes in back-to-back series across the bulb. That is a way to bypass most of the surge current around the bulb so the over voltage is not enough to blow the filament.

You can try different resistor values across the bulb but you may not be able to find a value that keeps the bulb alive while still providing enough light.

My suggestion is still the same as above. Put a 3uF, AC voltage rated, capacitor in series with a #47 bulb and run it directly from the 120VAC power. No tricks are necessary to compensate for the cold resistance of the tubes since the bulb is independent of the tube string.

Jay


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Feb Fri 14, 2020 4:58 am 
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Back in the day, rather than spending money on a capacitor, they would use a tapped heater to provide a place to connect the bulb where it might survive, at least for awhile. To make the bulb brighter while the radio was playing, the rectifier's plate current was usually routed through the bulb and heater tap, as well. The plate current would not develop until the heater was already pretty hot, and the turn on surge was almost over. Thus the common really bright, then dims a lot, then back to sort-of bright sequence on AC power on. Not a perfect solution, but very inexpensive and good enough.

Radios were built with 120V rated bulbs, and also with separate ballast tubes just for a low voltage pilot lamp.

Ted


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Feb Fri 14, 2020 6:38 am 
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Usually Lurking wrote:
To make the bulb brighter while the radio was playing, the rectifier's plate current was usually routed through the bulb and heater tap, as well.
Ted

Ted,
I was using a circuit analysis program today to check the ripple currents in a typical 35W4/35Z5 rectifier B+ circuit. The RMS plate current of the rectifier is typically in the 120mA to 150mA range. It would appear that a #47 bulb could be lighted by putting it is series with the rectifier. The B+ would be a bit lower but not by a significant amount. The radio would stop playing if the bulb burned out and there might be some audio modulation of the brightness at high volume levels.

Anyway, it might be a good solution for lighting the bulb in this situation. Definitely no current surge problem with the slow rise of the B+ current and the bulb should have good brightness.

Note that a bulb failure often ended the operation of a radio using the tapped filament circuit. It just took a little longer until the overloaded section of the rectifier filament failed.

Jay


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Feb Fri 14, 2020 7:17 am 
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Location: Lexington, KY USA
I do not know why, but the AC/DC radios of the period seem to have never used this scheme. It is pretty likely there is some problem that was well known at the time. After everyone had time to copy whoever figured out the "best" way to light the lamp, they mostly all used the scheme with both the tapped heater and rectifier plate current. involved. If B+ current ran high, a parallel resistor was added.

The quick off then back on again problem may have been worse.

A single zener across the lamp should make this scheme work OK.

Note that as the tubes and filter cap age, the RMS rectifier current falls. This may have been a consideration.

Ted


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Feb Fri 14, 2020 6:17 pm 
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[quote="Usually Lurking"
The quick off then back on again problem may have been worse.

A single zener across the lamp should make this scheme work OK.

Note that as the tubes and filter cap age, the RMS rectifier current falls. This may have been a consideration.

Ted[/quote]
I also thought about the rapid on/off/on problem after posting. I'll probably take a look at that on a real radio since it will depend on how fast the rectifier cools compared to the filter cap discharge. The zener may still be needed to save the lamp under some conditions. I'll use a resistor instead of a lamp. Even though I have a lot of #47 bulbs, I don't like blowing them out.

Aging is probably not as much concern with modern caps and the amount that most restored radios get played.

Jay


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 Post subject: Re: Inrush current blowing bulbs
PostPosted: Feb Fri 14, 2020 6:44 pm 
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Not sure how the lamp is wired? An 800 ohm in series with #47 lamp will work but resistor would need to be 20 watts. Could use a cap in place of the resistor.

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