Building a poor mans variac (Dim Bulb Tester or DBT)

[Morden2004]

With the pending arrival of my new (to me) Kenwood KR-9600, and having lost my previous "load adjuster", I decided to build a new one.

In the process I took several pictures so that others who may also wish to have this very valuable tool, for bringing the power into vintage unit up safely in steps, could see how I built mine.

The parts list is small:

1. Small board to construct on (2X4 about 24" long)
2. Square electrical box
3. Light bulb socket assembly
4. Inexpensive power bar

First, strip the casing off the power bar about 3" back from the unit (we will cut the wires later) - or, alternatively cut the electrical cord about 8-10" back from the power strip.

EDIT: 2007-07-06 -- Because we lost all the pictures in a recent server swap, I have re-built the image links.

Paul

 

[Morden2004]

Slide #2

Screw the electrical box to your base then fit the power cord (plug end) into electrical box thru a strain relief.

<click>

 

[Morden2004]

Slide #3

Pass the wires from the power strip thru the opposite opening in the box. Screw the strip to the base.

Wire the insides as follows:

Both GREEN wires under the grounding screw on the box. Join the two WHITE wires together with a Marr connector or an in-line connector.

<click>

 

[Morden2004]

Slide #4

Now connect the two BLACK wires to the two terminals on the lamp socket.

Secure the socket to the electrical box.

<click>

 

[Morden2004]

Slide #5

And we're done! Now you have a great way to safely start up old vintage equipment by dividing the 115 VAC supply between the light bulb and the unit.

Start with a 40 or 60 watt bulb. Check your unit under test for B+ voltages as the capacitors charge and reform at safer voltages. Step up to 100 watt or 200 watt as your confidence builds.

<click>


Enjoy!

 

[LBPete]

Paul,
Here's mine. If I were to do it again I would use a GFI outlet and add a switch to bypass the lightbulb so the socket gets direct power.

- Pete

 

[outlawmws]

Here is mine, done with a GFI with ful load outputs and it feeds a switched split receptical that feeds the load outlet.

You break the snap off bridge on the "gold" side and the switch goes to the bulb post and the bulb feeds the other side thru the receptical and back to neutral...

 

[skippy_ps]

Cool. Thanks for taking the time to post this, New Scotia. Of course Pete would round the friggin corners off his 1 by x board. Where's the stain?

Interesting to see the different approaches. GF socket is a good idea.

Murray

 

[LBPete]

Stain, I'll add it to the improvement list for version 2.1. When you have a band saw, you just can't leave the corners of a board square. I should have hit it with the router and a round over bit to make it a little more finished.

On a different note, you can get yourself in trouble with these things. They will cause some protection circuits to kick in. A good protection circuit will activate if the operating voltage is too low and that is exactly what the lightbulb circuit does, drops the voltage. I've had it happen on a G-22000 and a 9090DB. Spent days trying to figure out why the G-22000 wouldn't come out of protection before I got brave and powered it up directly. I was smarter when it happened with the 9090DB.

Paul, this is a good thread by the way. Thanks for starting it.

 

[skippy_ps]

I was reading the voltages off my diy variac and was getting about 100v with a 100w bulb while I had a tuner plugged in. Was more than I thought it would be (tuner draws about .25 amp at 117v). 60w bulb was around 85v or so, iirc.

My heart rate goes up about 20% everytime I plug the vom into an outlet as I've blown up more than one along with a few probes when I had it set to amps or ohms and forgot to reset it. :stupid:

Nice work, Pete.

Murray

 

[outlawmws]

I was reading the voltages off my diy variac and was getting about 100v with a 100w bulb while I had a tuner plugged in. Was more than I thought it would be (tuner draws about .25 amp at 117v). 60w bulb was around 85v or so, iirc.

My heart rate goes up about 20% everytime I plug the vom into an outlet as I've blown up more than one along with a few probes when I had it set to amps or ohms and forgot to reset it. :stupid:

Nice work, Pete.

Murray

Here is what I got with a tube amp load:



Load Motorola Mono Tube amp, .62 @ 117VAC rating

bulb wattage - ohms resistance - resulting Voltage @121VAC
40 - 26 - 67
60 - 19 - 81
100 - 10 - 91
150 - 7.5 - 100
240 - 5 - 110

 

[skippy_ps]

Thanks for the numbers. Need to get a couple of 40 watters.

Murray

 

[Morden2004]

Stain, I'll add it to the improvement list for version 2.1. When you have a band saw, you just can't leave the corners of a board square. I should have hit it with the router and a round over bit to make it a little more finished.

On a different note, you can get yourself in trouble with these things. They will cause some protection circuits to kick in. A good protection circuit will activate if the operating voltage is too low and that is exactly what the lightbulb circuit does, drops the voltage. I've had it happen on a G-22000 and a 9090DB. Spent days trying to figure out why the G-22000 wouldn't come out of protection before I got brave and powered it up directly. I was smarter when it happened with the 9090DB.

Paul, this is a good thread by the way. Thanks for starting it.

I'm glad you liked the thread. I'm just a guy with too much time on his hands :yes: .

BTW, with a GFI do you not run the risk of tripping that GFI with a small amount of leakage resistance which may be "normal" for floating ground vintage equipment?

Also, with these home-brew variacs you can apply full VAC and protect the unit by substituting a suitable screw-in FUSE. That's what I do when I'm ready to apply full power.

Paul

 

[Morden2004]

Outlaw:

Thanks for adding that chart!! I will add that to the base of my PMV - (P)oor (M)ans (V)ariac.

Now, just add [15A-Fuse 0 115VAC] to the list and it's complete :thmbsp: .

Paul

 

[Morden2004]

PMV's - Nudie pics please!

Pete, Outlaw:

Thanks for posting pics of your PMV's.

Murray: do you have pics?

Anyone else have one to show us? :scratch2:

Paul

 

[Chad Hauris]

I would not reccomend using a 15 amp fuse on a test device...use something closer to the equipment rating so it blows QUICK if there's a short. We have a test cord with a 4 amp fuse in it.

I made a short test lamp out of an old photoflood fixture and spliced it in series with an extension cord...has been very helpful in troubleshooting equipment with shorted transistors.

 

[Morden2004]

I would not reccomend using a 15 amp fuse on a test device...use something closer to the equipment rating so it blows QUICK if there's a short. We have a test cord with a 4 amp fuse in it.

I made a short test lamp out of an old photoflood fixture and spliced it in series with an extension cord...has been very helpful in troubleshooting equipment with shorted transistors.

Good point. Can you actually get screw-in fuses in the 2-5 AMP range?

Paul

 

[SPL db]

Here is mine, done with a GFI with ful load outputs and it feeds a switched split receptical that feeds the load outlet.

You break the snap off bridge on the "gold" side and the switch goes to the bulb post and the bulb feeds the other side thru the receptical and back to neutral...

Can you show a pic of this? With the cold meds I'm on, I'm having
a hard time picturing how you did the load plug.

Thanks!
Scott

 

[Chad Hauris]

There were screw in fuses in low amp ranges years ago, still see them in jukeboxes...however I would recommend using a small cartridge type fuse holder (AGC type fuse) in the line. We have the series lamp as an unfused device with just the lamp. Then when we are ready for the full power test we hook up another cord which has the fuse in line.

 

[SPL db]

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