Dim bulb tester or limiter

[Phoenix1]

Just curious as the the factors affecting which wattage bulb is best ?

 

[FleshOnGear]

I believe twice the wattage of the amp is the rule of thumb. [But you should trust what Jon Snell says below.]

 

[Jon Snell]

A 40W lamp covers all valve amplifiers with the valves unplugged ; checks for short circuits.
A 100W lamp covers most amplifiers.
A 150W lamp is for larger power amplifiers.
Bearing in mind, the amplifiers cannot be run up at high power.

If running up a Class D or Switch Mode Power Supply, the light bulb limiter cannot be used as damage will occur.

Disclaimer: I do not and have never found the need for this system as I have a Variac, an isolated Current limiting power outlet, (multi-voltage eg 120v, 240v and three phase 16Amp for the bigger stuff All isolated) and a voltage conditioner. (DC - AC supply).

 

[Chris-in-LA]

For us mere mortals, a light bulb limiter is good to have.

 

[Phoenix1]

Actually I have one I built but expanding it to a three bulb piece with a 40, 60 and 100 watt bulbs. It came in really handy as I am still learning every day but building the JTM45+ a bad diode under the rectifier allowed me to at least turn it on and off and when I found the bad diode and replaced it I knew I had it when the bulb barley glowed. I do plan on getting a Variac, but down the road !

 

[StingRay85]

I would never engage in troubleshooting an amp without variac plugged into LBL

 

[paul-e-mann]

Actually I have one I built but expanding it to a three bulb piece with a 40, 60 and 100 watt bulbs. It came in really handy as I am still learning every day but building the JTM45+ a bad diode under the rectifier allowed me to at least turn it on and off and when I found the bad diode and replaced it I knew I had it when the bulb barley glowed. I do plan on getting a Variac, but down the road !

You can get a cheap variac for like $50 off Amazon or eBay.

 

[Purgasound]

I use a 300W bulb. They're not expensive and you only have to buy it once.

 

[Pete Farrington]

I use a 300W bulb. They're not expensive and you only have to buy it once.

Why such a high wattage?
As a ‘special’ for use with a SVT, yes, they’re handy.
But for general purpose use it’s not going to provide much of a limit on current flow, and the instantaneous initial current peak could be enormous.
Such a high wattage could allow common newbie errors eg reversed ecaps / diodes to still result in things being damaged, even if it’s only a fuse.
I suggest to have a range of bulb wattages available, start as low as is feasible for the application, and work up.

I think it may have been me that put the ‘bulb wattage twice amp wattage’ guideline out there. It should workout ok for most typical class AB amps.

Any bulb used in a LBL will have an easy life, I’ve never had one fail on me.

I’ve got a variac but pretty much only ever use it to tweak mains voltage to 240V.

I would never engage in troubleshooting an amp without variac plugged into LBL

Surely that’s wall outlet then variac then LBL then ‘equipment under test’?

 

[PelliX]

You can get a cheap variac for like $50 off Amazon or eBay.

Yes, though always be careful with these, especially if you're intending to use a scope or other grounded equipment. Some are labeled as "variable isolation transformers", but they're not actually isolated.

 

[paul-e-mann]

Yes, though always be careful with these, especially if you're intending to use a scope or other grounded equipment. Some are labeled as "variable isolation transformers", but they're not actually isolated.

How do I check my $50 variac if its isolated?

 

[PelliX]

How do I check my $50 variac if its isolated?

Measure the resistance from the primary L and N contacts to the secondaries, so "input to output". You should get near infinite resistance in all combinations. Then check the resistance between the secondaries and chassis ground and/or the earth pin on the outlet(s), which should also be nearly infinite. Just for the record, you need to break the ground (earth) wire between the isolation transformer and the device in question to use it with a scope.

EDIT: I feel this explanation is a little incomplete without briefly touching on what an isolation transformer actually is or can be. Effectively, there are two purposes, and this is where the ground comes in to play. The first usage would be touchy (for example medical) equipment that does not take well to (or somehow causes) interference on the AC mains. These *typically* have the ground intact and often tie the neutral line to ground on the secondary side. Doctors are picky about when they want to shock people or get shocked, it seems. The other purpose which is the one we're interested in here, is ensuring you can use a scope safely on the circuit (amp or whatnot) in question, and for that you want it "floating", i.e. completely electrically isolated from the outside world *including* earth ground reference. Of course this means that the chassis is no longer ground referenced and thus could potentially go hot without a fuse, breaker or GFI popping anywhere [big warning exclamation mark].

 

[Phoenix1]

Why such a high wattage?
As a ‘special’ for use with a SVT, yes, they’re handy.
But for general purpose use it’s not going to provide much of a limit on current flow, and the instantaneous initial current peak could be enormous.
Such a high wattage could allow common newbie errors eg reversed ecaps / diodes to still result in things being damaged, even if it’s only a fuse.
I suggest to have a range of bulb wattages available, start as low as is feasible for the application, and work up.

I think it may have been me that put the ‘bulb wattage twice amp wattage’ guideline out there. It should workout ok for most typical class AB amps.

Any bulb used in a LBL will have an easy life, I’ve never had one fail on me.

I’ve got a variac but pretty much only ever use it to tweak mains voltage to 240V.



Surely that’s wall outlet then variac then LBL then ‘equipment under test’?

just things I have learned in here from you and others made me want to expand mine to a three bulb limiter of 40, 60 and 100 watt ! I scrounged around the house nd found some older bulbs and walmart still carries incandescent 40 watt oven and fridge lights!!!! I am getting tooled up for a little bigger build and try learning a bit more ! Thank you !

 

[Max Gahne]

I use a 100W bulb in my LBL. It's gone 7 or 8 years now. My amps only go up to around 25w though. I use it every time I get into an amp. I'm sure it's saved me a few transformers.

 

[Phoenix1]

I use a 100W bulb in my LBL. It's gone 7 or 8 years now. My amps only go up to around 25w though. I use it every time I get into an amp. I'm sure it's saved me a few transformers.

All mine are 100 watt except the JTM45+ I just built ! All I ever had was 100 watt bulbs until now !

 

[PelliX]

I scrounged around the house nd found some older bulbs and walmart still carries incandescent 40 watt oven and fridge lights!!!!

Not sure about the US, but in the EU you can still buy all the 'forbidden' lightbulbs, just you have buy them from toolstores or DIY places. You pay a little more and they're labeled "Professional" or even "Not for domestic use" and so on, but they're there at least. I have a stockpile of literally hundreds of lightbulbs, so I should be covered.

 

[Ronquest]

Not sure about the US, but in the EU you can still buy all the 'forbidden' lightbulbs, just you have buy them from toolstores or DIY places. You pay a little more and they're labeled "Professional" or even "Not for domestic use" and so on, but they're there at least. I have a stockpile of literally hundreds of lightbulbs, so I should be covered.

'strawberries', the show "Last Man Standing" had light bulb episode.

 

[PelliX]

If running up a Class D or Switch Mode Power Supply, the light bulb limiter cannot be used as damage will occur.

Why would an LBL cause damage with an SMPS? Not only do I use this combo for initial testing of some kit, I'm having a hard time seeing what would get damaged. The inrush to the filter caps will be a bit delayed and it's well possible that the power supply will not function at all with a low(er) wattage bulb, but I don't see how that hurts it. Perhaps some voltage regulators would be stressed a little harder due to the situation, is that what you're getting at?

 

[Phoenix1]

The only 100 watt bulb I found i a specialty bulb dont Laugh it looks like a mad scientist !

 

[FleshOnGear]

I bought some 100, 150, and 200W bulbs awhile ago. I had been mainly using the 150. I hope the batch I got lasts me a long time. I fear incandescents soon will be worse than vacuum tubes as far as availability.

[edit: looks like they’re still available on Amazon.]