Light bulb limiter behavior

[jordan86]

Just built a light bulb limiter according to Rob Robinette's specs. Standard wall outlet, with the bulb to outlet adapter. I'm 99% certain I wired it properly, but not sure if its working right, or if I just have too large of a bulb. I am using this 200W bulb I found at Walmart. They only had 75W, 200W, or 300W. Maybe this bulb is just too big for my amps?

https://www.walmart.com/ip/GE-Incandescent-200W-Crystal-Clear-1-Year-Life-E26-Medium-Base-1pk-Light-Bulb/16911775


Steps I took...

- Testing with a multimeter, when the bulb is in the socket, I have proper continuity across hot, common, and ground. From the wall connections to the amp outlet/socket.
- With no bulb, I have no continuity. Is that right?
- When I plug in a working 25w amp (solid state rectifier) the bulb glows very dim, but only when amps is fully engaged. Nothing when the amp is in Bypass.
- I also tried this with a Princeton Rev Reissue (no bypass switch). Amp powers up fine. No response on the bulb. I can feel the bulb gets barely warm, but not visually illuminated at all.

Is this normal? Sadly, I have no other incandescent bulbs lying around. We just bought a new house and everything is LED.

FWIW... I only have 15-25W amps. Making this for an upcoming Princeton Reverb build.

[shooter]

the bulb glows dim when the world is good, very bright when things are bad.

70 -100w should be just fine

[PRR]

> Is this normal?

50 watt demand amps, 200W incandescent.... yes, that's what you expect. Because the lamp does not get HOT, the resistance and voltage stays low, cool. If the amp were to short-out, you'd have a BRIGHT light (probably want to keep it to the side to reduce dazzle) but no house fire.

Yes, amps in that class, 75W-100W may be useful to slow the smoke in a part-short. You can not eliminate all chance of damage, but a lamp "somewhat larger" than the amp limits the stress.

Figure amp demand as 1X-10X the rated audio output. Transistor amps idle real cold. Small tube amps carry a lot of overhead: a Champ sucks 55W whether or not it is delivering its 5.7W audio.

HomeDepot is liable to have more selection but you have to dig in the bottom back of the aisle behind all the "green" lamps.

"Halogen" is incandescent. Better light efficiency at higher cost, but for light you want LED. For power limiting, look at the true consumption not the "equivalent". "Like a 100W!!" probably sucks 85W-90W.

[jjasilli]

After watching some Uncle Doug Youtube video's and similar, I just bought a 300W incandescent bulb @ Home Depot for about $4.  I've been using a 20W bulb, but using the lamp limiter just as a short-tester, only at start-up of the amp under test.   


I now plan to use the lamp limiter with 300W bulb full-time when working on amps. 

[jordan86]

Thanks guys. So I presuming a few things based on your responses:

1) This is normal behavior for a working amp.
2) A 75w bulb may be sufficient for my 15W Princeton build. Though a 200W would offer more protection, but with a less observable reaction if all things are well.

[jordan86]

I did just find a 29W halogen lying around. I tried that and the 200W again and noticed the 29W halogen glows significantly brighter than the 200W. Still what I would consider dim though and not at full brightness. Cool to touch too.

Also noticed the 29W halogen really messes with the amp output. Like it never reaches full power. Volume at 10 is quiet and funky (the kind of reaction I would expect). The 200W bulb doesn't seem to steal any power from the amp at all. Powers up real quick and is full loudness/output. 

Maybe my 200W is not a true incandescent?

[DummyLoad]

i use a GE 150W flood. sadly, not available any longer.


--pete

[PRR]

> 200W would offer more protection

No. It lets the healthy amp work more-normal (as you proved with the 29W), but lats more smoke out of a sick amp.

[Willabe]

1) This is normal behavior for a working amp.

Yes.

Though a 200W would offer more protection, ....

That's backwards. The lower the bulbs wattage, the more protection it gives. The lower the wattage the filament is, the more resistance it has to current being pulled/drawn through it.

I did just find a 29W halogen lying around. I tried that and the 200W again and noticed the 29W halogen glows significantly brighter than the 200W. Still what I would consider dim though and not at full brightness. Cool to touch too.

Also noticed the 29W halogen really messes with the amp output. Like it never reaches full power. Volume at 10 is quiet and funky (the kind of reaction I would expect). The 200W bulb doesn't seem to steal any power from the amp at all. Powers up real quick and is full loudness/output.

Here's what's going on; Smaller filament has more resistance and will heat up easier.

If you take a working 20w amp and plug it into the limiter. Now put in 200w bulb, turn on the amp. You might not even see the bulb glow at all. Take the same 20w amp and go down to a 100w bulb, then a 75w bulb, then a 60w bulb, then a 40w bulb. Each time you go down in wattage, the bulb will glow a little brighter.

If the amp has a short it will draw lots of current. The more current the amp draws the brighter the bulb will glow. If the amp is working right, the bulb will have very little glow.

On a 1st time start up on a new build or on a repair or mod, you use a small bulb to limit the current draw. If it glows brightly, shut the amp off instantly. It can still draw enough current to burn something up.

2) A 75w bulb may be sufficient for my 15W Princeton build.

I use a 60w bulb with a Princeton amp.

[DummyLoad]

these should work well.


--pete

[Latole]

the bulb glows dim when the world is good, very bright when things are bad.

70 -100w should be just fine

I agree and I use 100 watts since 15 years.
200 or 300 watts is too powerfull to easy use.

Easier wiring for a beginner ;

[jjasilli]

FWIW, here's my schematic.

[Latole]

FWIW, here's my schematic.

I can't see it.

[DummyLoad]

FWIW, here's my schematic.

I can't see it.

should be legible now...


-pete

[Latole]

Yes !

Stupid question ; why don't you post like this ?

[sluckey]

Yes !

Stupid question ; why don't you post like this ?

The file was created with a small freeware program called ExpressSCH. Many people use ExpressSCH to create schematics and even layouts. There's even a forum on this board dedicated to ExpressSCH. Most regular contributors on this board have ExpressSCH installed on their computer for the sole purpose of viewing these files. Check it out.

[Latole]

Thank you, I'll look at Express Schem.