Dim Bulb Tester


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


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


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.  

I will not go into detail on when to use a DBT or include a step by step procedure to build one.  There is plenty of documentation on the web about these topics.  I hope to include a few tips that I use when building my DBT.  Email me if you have any other questions that you feel I may be able to answer. 


See how a Dim Bulb Tester operates.  


Good Radio on DBT

Bad Radio with a short circuit.




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.


IMG_2462.JPG (201300 bytes)Basic parts.  Add a two gang electrical enclosure, two gang cover plate and a power cord.

Use an incandescent  bulb 1.5-2 times the wattage of the test radio. 

IMG_2463.JPG (137023 bytes)I had these parts left over from a remodeling project.   IMG_2464.JPG (127897 bytes)

I purchased several of the plug in lamp sockets to use multiple wattage lamps.  I have this unit attached to the wall with dry wall screws.  

IMG_2476.JPG (149770 bytes)Snap off this strap on the copper colored screws or the HOT side of the outlet (short slits). This puts the outlet into series.  ONLY connect wires to this side of the outlet. One screw to the switch and the other wire to the Neutral side of the power cord.  

This makes for a compact and simple DBT inside a Double Gang box.

IMG_2478.JPG (187371 bytes)Leave this strap on the silver screws or Neutral side of the outlet (Long slits). IMG_2481.JPG (171478 bytes)I used a salvaged power cord.  Connect the thin blade (hot) to the switch.   IMG_2480.JPG (71657 bytes)This is a Variac or adjustable autotransformer.  This and a voltmeter and amp meter replace the DBT. IMG_2496.JPG (195418 bytes)I use a DPST switch in my DBT.  This disconnects both hot and neutral wires to the power cord.  IMG_2497.JPG (209702 bytes)There are 4 screws on the DPST switch. 



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.


This is a picture of how my DBT is wired.  


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.


Hi, I found your diagram for a dim bulb tester linked to one of my favorite web sites, Fun With Tubes, and plan on making one. I work operating a hydroelectric generating plant and have a General Journeyman's license so I am used to working around voltages of all types and am familiar with the dangers. Since the plant I work in was built in 1932 we actually still have tube equipment here so I am familiar with some different tube applications as well. I looked at your wiring diagram for the DBT using a DPST switch and I have a comment. 

I am mainly used to working with Hammond Organ amps and guitar amps, many of which have now been converted to polarized plugs and expect to see the hot on a specified input and the neutral on a specified input. Of course, unpolarized radios don't have that expectation but some of them have been converted and had polarized receptacles and fuses added as well. The way you show the DBT wired the polarization is reversed on the bottom receptacle, where the radio plugs in. 

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. 

To do that, I broke the jumpers on both hot and neutral sides of the receptacle so that both receptacles are independent. Then I wired the top (Lamp) neutral to the bottom (radio) hot slot and the bottom (Radio) neutral to the neutral side of the Line DPST. It's about the same amount of work and the receptacles are now both correctly polarized.


modified dbt.JPG (148917 bytes)

This is Karen's simple circuit modification.

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

 metal DBT.JPG (176630 bytes)

 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. 

One must keep in mind to improve and learn no matter how long or old you are.  

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.  

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.... 


IMG_2484.JPG (208782 bytes) Two basic parts are needed.   IMG_2486.JPG (216357 bytes)Locate and identify the HOT wire.  It is typically in line with the THIN blade. IMG_2488.JPG (193924 bytes)Carefully cut the Zip Cord wires apart.  Do not slice off the insulation or nick the wire.  IMG_2489.JPG (201472 bytes)   IMG_2491.JPG (188312 bytes)Cut the HOT wire and strip back the insulation.  
IMG_2492.JPG (168944 bytes)Find the HOT screw (copper colored).  Connect the wire from the male bladed end of the extension cord here.   IMG_2493.JPG (185050 bytes)Zip tie the wire to the lamp base to provide strain relief. IMG_2494.JPG (185656 bytes) IMG_2495.JPG (164851 bytes)Use an incandescent  bulb 1.5-2 times the wattage of the test radio.   




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. 


IMG_2498.JPG (190537 bytes) Typical TV repair shop isolation transformer.  The bottom two outlets are the isolated outlets.  The top are not. 

There are much simpler isolation transformers with a singe outlet.  

IMG_2499.JPG (101616 bytes)The DBT plugged into the isolated outlets of the isolation transformer.

The Varriac (below the DBT) must also be plugged into the Isolation Transformer to provide electrical isolation.   


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

   Phil also has included a schematic. 


I hope this helps a bit.  


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