Hi:

I heard when working on old sets it would be wise to install a fuse for extra protection. What type of fuse should be installed? Also is this a good suggestion or just a waste of time. Why didn't they do that back then. I don't think I ever came across and old set with this feature. Thank you.

:Hi:
:
: I heard when working on old sets it would be wise to install a fuse for extra protection. What type of fuse should be installed? Also is this a good suggestion or just a waste of time. Why didn't they do that back then. I don't think I ever came across and old set with this feature. Thank you.

John: installing a fuse is well worth the time and effort to prevent damage to other valuable components in case of a failure in the radio. A 1-amp fast blow fuse will work fine for most radios. It can be installed on the leg of the power cord that switches the radio off/on. Old radios lack many of the safety features newer radios have, and back then it may have been a cost factor. Correcting a problem after a fuse blows is a lot cheaper than having to replace a number of expensive tubes and/or transformers. Example: many power transformers have been ruined by shorts in rectifier tubes (such as the 6X5). A fast blow fuse would avoid this problem. An in-rush current limiter is also a good idea. When a tube radio is first turned on, the tube filaments have a rather low resistance, which causes a rather large current draw. The in-rush limiter gives the radio a softer start and helps protect the tubes for longer life.
Steve

You may want a 1 ampere slow-blow fuse if you have troubles with frequent blowing. When the set is warm, turning it off and on will create a surge due to the electrolytics taking an instant charge (rectifier warm). When cold, cold filaments draw a lot of current. Fast blow fuses protect better, but might not work in certain cases.

Thomas

There are many different preferences here.

What I've settled on is installing a CL-90 inrush limiter (sold by Mouser), and then installing the lowest-amp fast-blo fuse that will hold at turn-on.

But a slo-blo fuse should provide mucho protection, as well.

:You may want a 1 ampere slow-blow fuse if you have troubles with frequent blowing. When the set is warm, turning it off and on will create a surge due to the electrolytics taking an instant charge (rectifier warm). When cold, cold filaments draw a lot of current. Fast blow fuses protect better, but might not work in certain cases.
:
:
:Thomas

Hi John

You may need a slo-blo fuse for a radio using power transformer? At turn on there will be a surge that can blow a fast blow fuse of the proper rating.

Norm

:There are many different preferences here.
:
:What I've settled on is installing a CL-90 inrush limiter (sold by Mouser), and then installing the lowest-amp fast-blo fuse that will hold at turn-on.
:
:But a slo-blo fuse should provide mucho protection, as well.
:
:
::You may want a 1 ampere slow-blow fuse if you have troubles with frequent blowing. When the set is warm, turning it off and on will create a surge due to the electrolytics taking an instant charge (rectifier warm). When cold, cold filaments draw a lot of current. Fast blow fuses protect better, but might not work in certain cases.
::
::
::Thomas

Yeah, this is true. Sometimes inductors pass a lot of current when first connected to the source. I'm not exactly sure why this is so, but it has something to do with it being an inductor. I know that sometimes my tube tester will flash the overload light really brightly when first turned on. This is a split second thing. I used to have a very large theatre grade variac (for lighting) that would do the same thing on occasion, or if plugged in and unplugged rapidly (which is guaranteed to give you one of these inductive surges). I have a feeling that this has something to do with phasing and internal voltage build-ups. You know how when you disconnect an inductor from a DC source it creates an arc, right? Well, I'm guessing that sometimes when an inductor is connected to an AC source, a magnetic surge might take place that'll make it pass perhaps twice its normal current. Normally the magnetic waves that are set up in the inductor cancel out the current passing through it, which is why an inductor doesn't pass AC readily. Normally the voltages generated within an inductor by magnetic build-up and collapes are out of phase with the current passing through it from the source. If, for some strange reason (unknown unless you were to analyze switch activity....perfect connection, bounce, whatever) an initial surge creates an induction voltage that is in phase with the source voltage, the two won't cancel, but will instead multiply. I have a feeling that my theories here are right on (minus a little extra technical explaining that an expert might be able to give), but I may be wrong. At any rate, what Norm is talking about really does happen with all inductors (fans, transformers, variacs (a type of transformer), etc.). It will happen at random, whenever the device is turned on. It can be warm, cold, or anywhere in between. The same phenomenon also occurs when the device is disconnected, but since it is being disconnected, it no longer causes a doubling (or whatever the multiple) of current draw. Instead you simply see a larger than normal arc at the switch contacts. Again, it happens at random, because we are dealing with alternating current. If you were to pass direct current through the inductor, you'd get a larger than normal (normal for a non-inductive device) arc every time.

Thomas

Thomas:
The inductor initially passes current as does any regular wire ....but because it is a coil it builds up a large magnetic field. Then ...when the AC sine wave is reversing polarity the built up charge tries to collapse creating an additional induced voltage opposing the next incoming cycle.
This voltage opposition all looks a bit like a resistor to the incoming voltge

That's called "Back E M F" or back electro motive force.

http://www.newton.dep.anl.gov/askasci/phy00/phy00351.htm

http://www.fas.harvard.edu/~scdiroff/lds/ElectricityMagnetism/BackEMF/BackEMF.html

Peter, your comments on "back" EMF throw me back to DC Motor Generator course in high school. We called it CEMF (counter electro motive force). With a coil when voltage is applied a magnetic field is built up from the instant of application till it reaches peak, that magnetic field passes over the very coils the voltage is travleing through but it induces a voltage in the coil counter to the applied voltage. The thing to remember is that you can generate electricity by moving a wire through a magnetic field OR moving a magnetic field through the wire. So as the magnetic field builds up it passes the coil wires and induce the CEMF. Conversely, when the voltage is disconnected, the magnetic field falls right across the wires of the coil and induces a voltage. All short lived but thats what happens. PL

Makes sense. I'd like to know why it only happens sometimes. I suppose it might have to do with whether you turn on the coil mid-cycle (at zero volts) or if it's somewhere positive or negative of mid-cycle. If the current is at mid-cycle or at a rather low voltage, I suppose that it wouldn't create as much of an initial surge as it would if it was at a full + or - 120 volts. Makes sense, though, that the initial current would pass right through the coil because there's no reverse EMF created yet.

T.

:Hi John
:
: You may need a slo-blo fuse for a radio using power transformer? At turn on there will be a surge that can blow a fast blow fuse of the proper rating.
:
:Norm
:
::There are many different preferences here.
::
::What I've settled on is installing a CL-90 inrush limiter (sold by Mouser), and then installing the lowest-amp fast-blo fuse that will hold at turn-on.
::
::But a slo-blo fuse should provide mucho protection, as well.
::
::
:::You may want a 1 ampere slow-blow fuse if you have troubles with frequent blowing. When the set is warm, turning it off and on will create a surge due to the electrolytics taking an instant charge (rectifier warm). When cold, cold filaments draw a lot of current. Fast blow fuses protect better, but might not work in certain cases.
:::
:::
:::Thomas

Hi All,
I hate to break the unaninimity of opinion, but I haven't considered fuses to be really necessary. My concern is that it could blow during a short power surge creating a nuisance repair.
Last month there was a lively discussion about the problems with 6X5G tubes. Many suggestions were good and did not require a fuse, even for that problem.
The surge supressor is a good idea, but I am skeptical about the fuse.

All the Best,

Bill g.

Well, there are fuse holders that you can mount under the chassis, which would eliminate the need for a soldering iron. Also, attractive fuse sockets are sold at many places including Radio Shack. Most look just about like any fuse holder from 50 or 60 years ago. Of course drilling a hole will ruin originality, but I kind of like the idea of fusing. I haven't ever fused one of my radios, but I own all of them, and noone else uses them. I do intend to fuse the kit radio that I am very slowly (at a stand-still right now) building for a relative. I do believe that fuses (or similar devices, such as a #47 pilot lamp for the 6X5 B supply) afford the best protection. If chosen properly, they are pretty much fail-safe. They will always burn through because they're designed to do that. Current limiters and in-rush limiters might not save a radio. I have little experience with these, so maybe they will. They just don't seem as straight forward, though, as a fuse. I do have a slow-start device on my 1950s RCA color bar/dot generator. I wonder what this thing would do if there was a short in the unit. Actually, this thing decreases resistance as it increases in temperature, so it might not help at all, though it might eventually burn through if loaded enough. I do believe that the unit also has a fuse, though, being professional (so-called) grade.

Thomas

:Well, there are fuse holders that you can mount under the chassis, which would eliminate the need for a soldering iron. Also, attractive fuse sockets are sold at many places including Radio Shack. Most look just about like any fuse holder from 50 or 60 years ago. Of course drilling a hole will ruin originality, but I kind of like the idea of fusing. I haven't ever fused one of my radios, but I own all of them, and noone else uses them. I do intend to fuse the kit radio that I am very slowly (at a stand-still right now) building for a relative. I do believe that fuses (or similar devices, such as a #47 pilot lamp for the 6X5 B supply) afford the best protection. If chosen properly, they are pretty much fail-safe. They will always burn through because they're designed to do that. Current limiters and in-rush limiters might not save a radio. I have little experience with these, so maybe they will. They just don't seem as straight forward, though, as a fuse. I do have a slow-start device on my 1950s RCA color bar/dot generator. I wonder what this thing would do if there was a short in the unit. Actually, this thing decreases resistance as it increases in temperature, so it might not help at all, though it might eventually burn through if loaded enough. I do believe that the unit also has a fuse, though, being professional (so-called) grade.
:
:Thomas

Fellows,

What I would like to have is a large box of the fused linecord plugs of yesteryear. The plug was a bit larger than the average plug and had space for a couple of standard fuses inside its shell.

Radiodoc

You can clip them off of old Noma Christmas lights, though I don't recommend it because these sell well on eBay, and I like them. Noma lights of the mid to late 1940s had plugs with prongs that you could twist to remove. Inside were two fuses.

Thomas

http://cgi.ebay.com/VINTAGE-NOMA-SAFETY-PLUG-CHRISTMAS-LIGHTS-GREAT-BOX-WOW_W0QQitemZ120036932250QQihZ002QQcategoryZ33842QQrdZ1QQcmdZViewItem

Well, guys, that was some very heavy discussion generated by John's question! Just wanted to say that I own only one radio that includes a fuse - a Westinghouse WR-8. I had an amusing experience with it when the fuse blew on it. I kept trying to replace it with a fast-blow fuse and watched each one go! Finally talked to someone that mentioned the slow-blow fuses and problem solved. The fuse on this radio is mounted under a small cover on top the chassis and is easy to get to. Mark from Kalamazoo

:Hi:
:
: I heard when working on old sets it would be wise to install a fuse for extra protection. What type of fuse should be installed? Also is this a good suggestion or just a waste of time. Why didn't they do that back then. I don't think I ever came across and old set with this feature. Thank you.