I just bought a 1940s' 5 tube superhet. It works great. When I turn it on, one of the tubes, I think the 35Z5, filament glows very bright for about 1 sec, then it goes to normal brightness. I mean really bright. The pilot light is in place and is ok.

I read someplace that this is the LightHouse effect but I can't find where I read this or what the solution is.

Any help would be appreciated.

Thanks
Gary / ke9zm

Gary, pull the tube and check the resistance across the heater, pins 2 and 7. Probably open, in which case replace the 35Z5. The heater is tapped for the lamp.

If the heater tests normal, which I bet it does, it is possible that the filament is of a certain type which does this. I am surprised that Doug did not think of this.

Does the filament get really bright just at the bottom and/or top? Some companies wound the heater filament in a very small helix and then coated it with porcelain. The porcelain, however, does not cover the extreme ends of the coil which go to the terminals, and also when the coiled heater makes a bend at the top of the cathode, sometimes the porcelain breaks away and exposes some turns of heater wire. The exposed wire will heat up much more quickly than the wire which is covered with porcelain. This is because the porcelain absorbs some of the heat. Also, the fact that the wire is coiled close to itself causes it to heat rapidly--the heat from each turn of wire aids the wire next to it. This is also true to some extent within the porcelain insulation, but the porcelain absorbs a lot of heat, so the exposed wire heats up first.

As we all know, too, hot wire has a higher resistance than cold wire, so the small part which heats up first is dramatically heated even more so because the rest of the heater in the 35Z5 as well as all of the other tubes has not heated up much yet.

Common tubes with this type of filament are the 117Z6 G and many European made tubes. Occasionally you will find the 117Z6 tube with an ordinary zig-zag type heater. If you get any 1940s era Zenith radio that has its original 117Z6 tube in place, you will see this phenomenon when you turn on the radio cold.

Thomas

Here is some more info.

The radio is a Temple E512 from the 1940's. The schematic is here.

http://www.nostalgiaair.org/Resources/download.asp?FN=\M0021797.pdf

I did a few experiments. It only flashes brightly when it has been off for about 30 min. If I turn it on and then turn it off for 5 min and turn it back on then the the tube filament lights normally - no flash. So I'm thinking that it has to do with a discharging cap.

I opened up the radio. The tube in question is the 50L6GT. The radio is from the 1940s and all original. It has not been recapped. Under the chassis one of the caps connected to pin 3 (plate that shares a connection with output transformer) is leaking. So I don't know what that has to do with the flashing (filament voltage?) but thats what I see.

I use the radio all the time and it works really well. So I wonder if I recap this thing it will fix this filament flashing problem... and if I don't whether I will eventually blow the tube.

gary / ke9zm

Hi Gary

This is caused by a thin spot in the filament away from cathode that heats fast. Like Thomas mentioned this happens in some high voltage filament & many European tubes.

Although I haven't seen tubes burn out I don't prefer using ones with the bright flash at turn on. As you noticed once the filament has heated it doesn't happen again. Replacing the 50L6 will stop the flash.

Norm

:Here is some more info.
:
:The radio is a Temple E512 from the 1940's. The schematic is here.
:
:http://www.nostalgiaair.org/Resources/download.asp?FN=\M0021797.pdf
:
:I did a few experiments. It only flashes brightly when it has been off for about 30 min. If I turn it on and then turn it off for 5 min and turn it back on then the the tube filament lights normally - no flash. So I'm thinking that it has to do with a discharging cap.
:
:I opened up the radio. The tube in question is the 50L6GT. The radio is from the 1940s and all original. It has not been recapped. Under the chassis one of the caps connected to pin 3 (plate that shares a connection with output transformer) is leaking. So I don't know what that has to do with the flashing (filament voltage?) but thats what I see.
:
:I use the radio all the time and it works really well. So I wonder if I recap this thing it will fix this filament flashing problem... and if I don't whether I will eventually blow the tube.
:
:gary / ke9zm

NO, THE TUBE WILL ONLY FLASH LIKE THAT WHEN IT IS COLD, AS YOU HAVE ALREADY DISCOVERED. THERE IS NOTHING WRONG WITH THE TUBE. THAT IS HOW THE FILAMENT IS DESIGNED. IT HAS ABSOLUTELY NOTHING TO DO WITH CONDENSERS. PLACE THE TUBE IN ANY RADIO AND IT WILL DO THAT.

Thomas

OK, Gary, everybody's got their money down, and the betting is now closed. You may now reveal the true cause of the flash.

Actually, I'm reneging, because you first said it was a 35Z5, not a 50L6.

If Norm's theory (that there is a thin spot in the filament) is correct, then maybe just keep using the tube until it burns out?

It isn't a thin spot in the filament. The filament did that when it was new. It is simply that the filament is wound in a helix, so that the coils help heat eachother. The very ends are not covered in porcelain like the rest, so they heat faster, and since the coils are right next to eachother, they incourage even more rapid heating. The tube won't do this after the set has been on, because all of the tubes' filaments have increased in resistance because they are all warm. You may notice the tube do the bright thing just a little when the set is warm--start glowing at the bottom and work its way up.

There is nothing wrong with the tube. It will last just about as long as any other tube. There are tubes of all types that have this heater design. As I've said before, usually you find this in only some U.S. tubes and in a lot of European tubes. I have a Mullard 6AU6 tube that does this. I have several Mullard and Blaupunkt 12AU7 tubes that do this. I have several Zenith 117Z6 tubes that do this, along with two Sylvania 117Z6 tubes which I bought brand new and unused, which do this. I have a 12SA7 tube that does this. It is made by Fapesa in Argentina. All of these tubes have the same helix wound filament design, and all have this "fault" because the very end coils of heater wire are not covered in porcelain.

Thomas

Thomas

Hi

Agree with Thomas. Ends of the filament aren't covered with porcelain so they heat faster. Although I haven't seen tubes burn out from this prefer to not use them. A lot of European lower voltage tubes do the same thing. Although preferred by the audio guys I don't use these tubes for replacement.

Norm

:It isn't a thin spot in the filament. The filament did that when it was new. It is simply that the filament is wound in a helix, so that the coils help heat eachother. The very ends are not covered in porcelain like the rest, so they heat faster, and since the coils are right next to eachother, they incourage even more rapid heating. The tube won't do this after the set has been on, because all of the tubes' filaments have increased in resistance because they are all warm. You may notice the tube do the bright thing just a little when the set is warm--start glowing at the bottom and work its way up.
:
:There is nothing wrong with the tube. It will last just about as long as any other tube. There are tubes of all types that have this heater design. As I've said before, usually you find this in only some U.S. tubes and in a lot of European tubes. I have a Mullard 6AU6 tube that does this. I have several Mullard and Blaupunkt 12AU7 tubes that do this. I have several Zenith 117Z6 tubes that do this, along with two Sylvania 117Z6 tubes which I bought brand new and unused, which do this. I have a 12SA7 tube that does this. It is made by Fapesa in Argentina. All of these tubes have the same helix wound filament design, and all have this "fault" because the very end coils of heater wire are not covered in porcelain.
:
:Thomas
:
:
:
:Thomas