Dim Bulb Tester

Don’t let the local idiot plug in that old vintage radio

I like to watch the cable shows about pawn brokers and restoration services. Occasionally they get an electrical piece of equipment in, a radio, lamp, fan, etc. They stare at it wondering if it works or not. Next they get the resident idiot to plug it in. That is when I cringe with anticipation of sparks and smoke (cut to commercial). There is a better way to power up that vintage unit and still protect you, the branch circuit and the unit from catastrophic failure. It is called a Dim Bulb Tester.

The Dim Bulb Tester (DBT) is used to limit the current through a Device Under Test (DUT). Unlike a Variac it automatically changes the current to the DUT while providing an immediate visual feedback. No manually adjusted auto-transformers, volt or amp meter is needed. I use one frequently to test vintage tube radios before, during and after restorations.

The Dim Bulb Tester is simple and inexpensive to build. Essentially an incandescent lamp is wired in series with the unit under test. The parts count can be a minimum of a “zip cord” extension cord (Big Lots, a dollar store, etc) and a lamp socket. Or it can be quite elaborate using duplex sockets, switches, device box and a power cord. It all depends on your needs and desires.

The lamp’s wattage is determined by 1.5 to 2 times the power consumption of the DUT. You may have to try different multiples. If the DBT lamp is less watts than the DUT it will glow brightly as though it is plugged directly into the wall. This is a false negative.

When using the DBT if the lamp comes on bright and stays on bright as though it is directly powered by the wall outlet (and the DUT is dead) you have a short in the DUT. If the DBT lamp flashes on bright then dims to an orange glow, wait. The DUT may start to work a little. With the lamp showing a constant dim glow you can now plug the DUT directly into the wall outlet with a minimal chance of catastrophic failure.

So the next time you buy that vintage Hammarlund or Viking transmitter and are looking for the local idiot to plug it in, make up a simple Dim Bulb Tester.

June 5, 2012

My Dim Bulb Tester

The trick to a DBT is the current limiting characteristics of the incandescent lamp. And it provides an intuitive and visual indication of current flow. No interpretation of a voltage and current meter needed! Simple and sweet. I use it religiously before and after each and every restoration. It has saved me time and headaches.

Make A DBT with an Extension Cord and a lamp socket.

I would NOT recommend this DBT for repeated use or regular bench duty. Build a more robust tester with stronger parts like the one above. It is great for the occasional or once or twice use.

The objective is to insert the lamp in the HOT side of the extension cord. This places the current limiter (lamp) in series with the load (radio). Use the radio's power switch in place of the toggle switch in the above unit. Or use the all ready included "Twist on Twist Off" switch. The lamp its self. But don't burn you fingers....

AA5 and HOT chassis radios and the DBT.

AA5 (All American Five) tube radios, "Hot Chassis" (no power transformer) and other radios that have no electrical isolation (i.e., Zenith Transoceanic) from the line (power cord) are dangerous when opened up on the bench.

When a hot chassis radio is open on the bench during servicing present additional hazards. The use of the DBT or a Variac must be incorporated with an Isolation Transformer.

If the hot chassis radio is closed up in "user ready physical status" then I do not use an isolation transformer with the DBT.

Please note. This is homebrew test gear. It meets no safety standards. But has a specific use to me in this hobby.

I periodically receive questions about Dim Bulb Testers (DBT), their application, how to make one. The most difficult thing to explain is how a good radio or a radio with a serious problem should look on the DBT.

See how a Dim Bulb Tester operates.

Good Radio on DBT

Bad Radio with a short circuit.

My Dim Bulb Tester

This is the circuit that I use in my DBT.
The DPST switch completely disconnects the radio and load lamp from the live power outlet.

Karen, a Journeyman electrician, emailed me. She pointed out a modification she made to the wiring of the Dim Bulb Tester to properly accommodate equipment dependant on a polarized plug. See below her email explanation and pictures.

If you are using a polarized receptacle, as shown, it seems to me that you should change the wiring on the bottom receptacle to match polarization, so that it could then be used by equipment with both polarized and unpolarized plugs.

You could plug the lamp, which doesn't care about polarization, into the bottom receptacle and leave the top receptacle, correctly polarized, to plug the equipment under test into. However my preference is to have both receptacles correctly polarized.

This is Karen's simple circuit modification.

This is Karen's DBT in a metal 4x4.

The power cord is not shown. I would bet it is a three wire grounded cable with the ground properly connected to the metal box.

This is such a simple change I believe that I will do this to my DBT detailed above. I use a two wire unpolarized (the one show above is polarized. The one I used is not) line cord that I would replace with a three wire grounded.

Thank you to Karen. Keep us posted on your Hammond and Guitar amp repairs.

Make A DBT with an Extension Cord and a lamp socket.

I would NOT recommend this DBT for repeated use or regular bench duty. Build a more robust tester with stronger parts like the one above. It is great for the occasional or once or twice use.

AA5 and HOT chassis radios and the DBT.

AA5 (All American Five) tube radios, "Hot Chassis" (no power transformer) and other radios that have no electrical isolation (i.e., Zenith Transoceanic) from the line (power cord) are dangerous when opened up on the bench.

When a hot chassis radio is open on the bench during servicing present additional hazards. The use of the DBT or a Variac must be incorporated with an Isolation Transformer.

If the hot chassis radio is closed up in "user ready physical status" then I do not use an isolation transformer with the DBT.

Click on the picture to see "Phil's Old Radio's" example of a DBT:

Phil also has included a schematic.

Please note. This is homebrew test gear. It meets no safety standards. But has a specific use to me in this hobby.

I periodically receive questions about Dim Bulb Testers (DBT), their application, how to make one. The most difficult thing to explain is how a good radio or a radio with a serious problem should look on the DBT.

See how a Dim Bulb Tester operates.

Good Radio on DBT

Bad Radio with a short circuit.

My Dim Bulb Tester

This is the circuit that I use in my DBT. The DPST switch completely disconnects the radio and load lamp from the live power outlet.

Karen, a Journeyman electrician, emailed me. She pointed out a modification she made to the wiring of the Dim Bulb Tester to properly accommodate equipment dependant on a polarized plug. See below her email explanation and pictures.

If you are using a polarized receptacle, as shown, it seems to me that you should change the wiring on the bottom receptacle to match polarization, so that it could then be used by equipment with both polarized and unpolarized plugs.

You could plug the lamp, which doesn't care about polarization, into the bottom receptacle and leave the top receptacle, correctly polarized, to plug the equipment under test into. However my preference is to have both receptacles correctly polarized.

This is Karen's simple circuit modification.

This is Karen's DBT in a metal 4x4.

The power cord is not shown. I would bet it is a three wire grounded cable with the ground properly connected to the metal box.

This is such a simple change I believe that I will do this to my DBT detailed above. I use a two wire unpolarized (the one show above is polarized. The one I used is not) line cord that I would replace with a three wire grounded.

Thank you to Karen. Keep us posted on your Hammond and Guitar amp repairs.

Make A DBT with an Extension Cord and a lamp socket.

I would NOT recommend this DBT for repeated use or regular bench duty. Build a more robust tester with stronger parts like the one above. It is great for the occasional or once or twice use.

AA5 and HOT chassis radios and the DBT.

AA5 (All American Five) tube radios, "Hot Chassis" (no power transformer) and other radios that have no electrical isolation (i.e., Zenith Transoceanic) from the line (power cord) are dangerous when opened up on the bench.

When a hot chassis radio is open on the bench during servicing present additional hazards. The use of the DBT or a Variac must be incorporated with an Isolation Transformer.

If the hot chassis radio is closed up in "user ready physical status" then I do not use an isolation transformer with the DBT.

Click on the picture to see "Phil's Old Radio's" example of a DBT:

Phil also has included a schematic.

This is the circuit that I use in my DBT.
The DPST switch completely disconnects the radio and load lamp from the live power outlet.