Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Dec Sat 04, 2021 11:34 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 11 posts ] 
Author Message
 Post subject: 50A2 Ballast Replacement
PostPosted: Sep Sat 18, 2021 11:49 pm 
Member

Joined: Feb Thu 18, 2021 3:09 pm
Posts: 138
I got a radio from the mysterious Plant A in Chicago and there is no schematic for it anywhere. I restored it without it and it plays great but I want to replace the 50A2 ballast that is in it. My question is, whenever I see ballast replacements, people use a diode along with a resistor and I don't know why to use a diode if the original ballast did not. Measuring across the ballast, it's exactly 200ohms. I have a 200ohm 5W resistor but should I put a 1n5408 before the resistor? I tried without the diode and the resistor started to smoke so I cut the power. Is the wattage too low?


Attachments:
Screen Shot 2021-09-18 at 6.46.19 PM.png
Screen Shot 2021-09-18 at 6.46.19 PM.png [ 563.12 KiB | Viewed 687 times ]
IMG_7333.JPG
IMG_7333.JPG [ 915.36 KiB | Viewed 687 times ]
Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Sun 19, 2021 12:34 am 
Member

Joined: Nov Tue 14, 2017 5:09 am
Posts: 4109
Location: Austin, Texas
The power dissipated in a 200 ohm resistor would be: 0.3A x 0.3A x 200 = 18 watts
A 5W resistor should smoke.

Using a diode reduces the effective voltage from 120VAC to 85VAC.
Originally the ballast tube had to reduce 110VAC to 62VAC for the tube filaments.
That takes a resistance of 160 ohms with a power dissipation of 14.4 watts.
Today the line voltage is 120VAC nominal and it needs to be reduced to 62VAC at a current of 0.3A.
The diode (1N4007) takes care of reducing the voltage to 85VAC RMS.
An additional resistor is needed to reduce the 85VAC to 62VAC. That takes a resistor of 77 ohms with a power dissipation of 7 watts.

A proper replacement for the 50A2 ballast would be a 1N4007 diode in series with an 80 ohm, 20 watt, resistor.

Edit: A 1N5408 diode will work fine but 3A is a big overkill on a 0.3A load.
You can also use an 8uF, 250VAC, rated capacitor to replace the ballast tube and that eliminates all the power dissipation for the voltage drop.
The type of capacitor to use is a motor run capacitor for a ceiling fan.

Jay


Last edited by JnTX on Sep Sun 19, 2021 12:43 am, edited 1 time in total.

Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Sun 19, 2021 12:37 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 6404
Location: Minneapolis, MN USA
If the ballast is working, why are you replacing it ?

What is the tube lineup in your radio ?
I presume it does not have a pilot lamp since you shown on one resistance measurement ?

Lets assume a set using four tubes: 6c6, 6d6, 43 and 25z5
This would require a ballast of 191 ohms (200 close enough) at 17 watts dissipation.

5 watts not near enough.

Some folks use a diode or capacitor to reduce the amount of resistance needed in replacing a ballast which can lead to reduced heat dissipation.


Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Sun 19, 2021 1:50 am 
Member

Joined: Feb Thu 18, 2021 3:09 pm
Posts: 138
Ahh thank you for the explanation. I need to learn these equations for myself. I'll probably keep the original ballast in for now. I plan to use this radio frequently so I didn't know if I should replace the ballast now or later. I've heard they are a common failure in sets so that's why I asked.


Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Sun 19, 2021 2:34 am 
Member

Joined: Nov Tue 14, 2017 5:09 am
Posts: 4109
Location: Austin, Texas
Using a capacitor to replace the ballast is a good practice for radios that will have a lot of usage.
The three big advantages of using a capacitor are:
1. The cap eliminates about 15 watts of power loss in the radio so it runs cooler.
2. There is no significant start up surge current through the tube filaments. There is an initial surge current to charge the cap but it is over in milliseconds and doesn't do any damage.
3. The tubes warm up more slowly at almost a constant current and that improves tube life expectancy.

Jay


Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Sun 19, 2021 12:14 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 18916
Location: Utica, NY 13502 (USA)
A seller posted this link in an ad in the Classified forum. It addresses resistance line cords but should be helpful for ballasts as well.

http://www.nostalgiaair.org/References/ ... lash01.htm

Dave


Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Sun 19, 2021 9:08 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 11159
Location: Long Island NY
No reason you cannot use the radio with the original ballast for the time being, but do take care to allow adequate ventilation. Do not push it right up against a wall or backsplash that will block air flow. (Actually that is a good idea for anything electrical). Ballast tubes are made of iron wire wound around a piece of mica, inside a tube shell. Some of them were in glass bulbs with a hydrogen atmosphere, those tend to last a long, long time. The cheaper ones were in metal tube shells and were not sealed so the iron wire eventually corrodes and breaks. Guess they had to ensure themselves a replacement market! Many kinds of ballasts are still replaceable, either with NOS ones or with "universal" types which sometimes required you to either break some lugs off the ballast, or change a wire or two on the ballast socket, to adapt them to particular radios. The complication with any ballast replacement is whether or not they had taps, shunts, or separate sections to run the pilot lamps. If so, these have to be replicated, otherwise the lights won't work.

Putting a diode in series with a 120-VAC source cuts both the voltage and the current during the half cycle the diode doesn't conduct. Hence it is the power that is cut in half, not the voltage. The resulting waveform has the same effective heating power as 87 VAC, which is why that number was mentioned above. If you base your calculations on half of 120 VAC being 60 volts, your tubes are going to run a lot brighter than you expected--for a while.

The big problem with using diodes is, in the event of failure, the diode will likely short out and put full line voltage across the string. The tubes could get considerably hotter than normal before anything burns out. Therefore if a diode is used, fusing of some sort has to be arranged to protect against this. It's not a trivial matter because you want the fuse to blow when the current is 38% higher than normal, which it would be if the diode shorts, but at the same time withstand inrush currents about three times normal when the radio is turned on.

For this reason a series capacitor makes a lot more sense, particularly if you get a motor run (not start) cap or another variety such as an EMI filter or snubber capacitor that is specifically made to be used in connection with the AC line. The good ones carry markings from UL, ETL, or other recognized testing agencies which indicate that if they fail they will do so safely and not start any fires.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Mon 20, 2021 1:57 am 
Member

Joined: Nov Tue 14, 2017 5:09 am
Posts: 4109
Location: Austin, Texas
[quote="Chris108"

The big problem with using diodes is, in the event of failure, the diode will likely short out and put full line voltage across the string. The tubes could get considerably hotter than normal before anything burns out. Therefore if a diode is used, fusing of some sort has to be arranged to protect against this. It's not a trivial matter because you want the fuse to blow when the current is 38% higher than normal, which it would be if the diode shorts, but at the same time withstand inrush currents about three times normal when the radio is turned on.
[/quote]
Actually almost impossible to do fuse protection since the filament resistance will increase when they go over voltage. The current will probably increase about 25% if the diode shorts. Fortunately a 1000V, 1A, diode should be much more reliable than the tubes if it is a good quality diode. Worst case you lose one tube and will probably notice the filaments are very bright when the tube is replaced.

Jay


Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Mon 20, 2021 5:10 am 
Member
User avatar

Joined: Mar Sun 11, 2007 6:55 am
Posts: 12130
Location: Mission Viejo, southern California
Yes, why replace it? However, the capacitor solution works well, and significantly reduces the temperature inside the radio.


Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Mon 20, 2021 8:15 pm 
Member

Joined: Feb Thu 18, 2021 3:09 pm
Posts: 138
I'll look into replacing with a capacitor to reduce the heat. The ballast works but I want to be sure how to replace one if it does eventually fail.


Top
 Profile  
 
 Post subject: Re: 50A2 Ballast Replacement
PostPosted: Sep Tue 21, 2021 3:36 am 
Member

Joined: Apr Fri 20, 2018 6:55 am
Posts: 182
Location: San Rafael, CA. 94903
Another method is to run two diodes in series. But I agree that a 1 amp, 1000 volt diode would fare well.
73,
-marc


Top
 Profile  
 
Post New Topic Post Reply  [ 11 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 16 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB