Dim Bulb Tester

[illwood]

I decided that I should build a Dim Bulb Tester since I'm getting more and more into tube amp repair and building.

 

Here is a picture of it.

 

 

It only has a 15W bulb in it because I was testing a small power device at the time of the photo. It cost about $35 to build and is more compact that most other designs I have seen.

 

I built it using a grounded extension cord that I cut, an outdoor metal electrical box with 1/2" NPT theads, and a bulb housing that was for an outdoor light that threads right in to the box. I used a cable gland for the power cord so it won't pull out if the cable gets yanked on or tripped over.

 

For the wiring, I kept the ground wired to the box and the ground of the power jacks and switch (no use disabling a perfectly good ground). The hot (black) wire connects to the switch, then the bulb, and then the neutral side of the bulb wire connects to the "hot" side of the power jack. The neutral side of the power jack is directly connected to the neutral of the plug.

 

Just thought I would share my setup with the community.

[Belva]

For a minute I thought you had maybe built something to check the people @ HCPP:lol:

[illwood]

For a minute I thought you had maybe built something to check the people @ HCPP:lol:

 

Nah, this one only tests rational things like amps.

[Belva]

Nah, this one only tests rational things like amps.

rofl.

[mrbrown49]

Pretty cool! And it's something that can be built from things alot of people just have lying around.

[illwood]

Pretty cool! And it's something that can be built from things alot of people just have lying around.

Agreed. They are pretty useful for making the testing of old equipment (or in unknown condition). I like to use it plugged into a GFI outlet as an extra measure to prevent getting zapped. Especially considering all of the old amps that don't have an earth ground.

[katillac]

Pardon my ignorance, but what is it used for?

[isaac42]

I was wondering the same thing.

[WRGKMC]

Its used for powering up old amps that may have questionable caps or shorts in the power supply.

Leakey caps can be shorted and youy can take out the power transformer the second you flip the on switch.

New caps that havent been powered up abd conditioned will do the same thing.

 

The bulb is placed in series with the AC power to the amp like a fuse

and will take the initial voltage surge and and prevents the transformer from blowing

or if theres a dead short the lamp will come to full brightness vs forcing the transformer to light up and smoke.

 

Its a primative system but it works for many tube amps. A variac is actually a better tool for amps

but they arent cheap. Neither are a substitute for properly troubbleshooting an amp.

Its actually better to remove the load from the power supply and check all caps and components prior to

testing the high voltage components with power applied. Also replacing power caps it should be used.

Most times you're luckey and just installing new caps is no big deal. The one time it does cause a surge

can be the time it takes out the PT. Caps need to be brought up to working condition slowly.

I have first hand knowlege of this myself. I was replacing a leakey cap in my Sound City 50 head

and replaced the 2way cap and just powered it up normally and popped my PT.

Could have kicked myself in the ass for using the light bulb trick.

For a $1 light bulb it would have saved a Partridge transformer that is no longer manufactured.

[illwood]

WRGKM explained it better than I could.

 

Part of the reason I built it is that I have a job coming up where I will be replacing the caps and putting a proper 3-prong grounded cable (and eliminating the ground lift switch) on a '66 Fender Pro Reverb.

 

BTW WRGKMC, is a Variac better because you can constantly increase the power rather than turning on and off while switching bulbs?

[WRGKMC]

The bulb routine was considered to be the poor mans bench tool. still works good but not fail safe.

A trained tech can connect a meter to the transformer outputs or caps and gradually bring the voltage up

and ensure voltage is flowing. I worked on alot of real antique stuff from the 40's 50's and 60s when I did bench work

as well as the newer stuff from the 70's and 80's. (I'm showing my age again) you wouldnt dare bringing them up

to full voltage by switching them on, though many customers did, and thats why they wound up in the shop.

The older stuff was designed to run on 100/110VAC when the power lines carried lower voltages. A variac properly used

to test voltage and current would avoid allot of smoke and lost income through blown parts and estimates customers couldnt afford.

[FredZepplen]

and not just tube amps, especially for a beginner. Sometimes transistor finals fail because of something further up the signal path. A bulb tester can save you from replacing finals twice:thu:

[WRGKMC]

Sure thing. Also, For testing purposes we use to connect large caps to the test speakers as well. If the power transistors short and pump 112vac through the speakers caps would prevent smoking the voice coils. If I remember right they were larger caps that were connected in series with the speakers. They would pass audio frequencies but block high DC current.

I think we had fuses and ceremic resistors to drop the levels down testing the equipment at maximum levels too without having the DB levels too high.Not something you'd do like an attenuator but for stress testing, it worked all right troubbleshooting.

 

We also had a mono block amp with test probes for signal tracing. One lead had an alligator clamp that clipped to chassis ground, the other had a cap in series with the hot probe so it wouldnt take the bias to ground tracing the signal through the various preamp stages.

 

The other equipment I used daily was a signal generator that would sweep from 20 to 20Khz. You could drive a speaker with it or send an attenuated signal throug a circuit.

The there was a frequency analizer, scope, car battery with a charger connected which we used to test car stereo stuff and CB radios. We had an RF generator for testing and tuning radio equipment. What else, Oh a transistor load line tester. You'd plug a transistor in and read its specs and then compare its values to other transistors commonly used in simular circuits. That was handy when you couldnt make out the part number or the circuit diagrams werent available.

 

Then you had decade boxes for transistors and caps. You could select a valuse and use the boxes caps or resistors in the circuit as temp parts in finding the best valuse in a circuit.

Then you had cap testers, normal 3 way transistor testers, multi meters, and plenty of cross reference manuals for finding generic part matches.

 

There was probibly a buttload of other handy tools you'd use in a shop. A heavt duty hair dryer to heat a circuit and find faults, canned freeze spray to freeze parts, parts cabinets, tube testers, demagnitizers, you name it.