Solid State Amplifiers > Amplifier Discussion

Light Bulb Limiter

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J M Fahey:
Some images:

I don't like the switch bypassing the bulb shown in the lower image, simply because it's too easy to switch it on by mistake.
Suggested bulb: 25W for small SS amps (10/30W) ; 100W for high power ones (60/200W); a tube amp might require a 150W to 200W bulb, simply because it eats too much on idle, because of all those filaments.
Most sites state that the bulb burns if a short is present; it does not; worst case it shines brightly, with full power.
If the amp is fine, the bulb usually lights barely visible red.
It allows to measure many things (do I have power rails? do I have DC on the speaker out?) which a fuse would not let you to, because it burns in a millisecond.
Remember all voltages will be low, the bulb is eating part of the wall voltage, do not aim for "schematic perfect" voltages but a reasonable proportion.
Example: power amp /main PSU states +/-40V DC
If I have a dark red or weak orange bulb, measure +/-25V, have no DC on output jack, amp is probably fine.
If I measure +/- 1 or 2V rails or +/-25V but have 24V on speaker out: bad sign.
In this last case, if I have 49V across the lower output transistor and 0.12V across the upper one, I can be pretty certain that the upper one is shorted, which will be confirmed later with an ohmmeter test with the amp off .
That's the general idea.
Much faster and better than:
replace one transistor -> power on -> blown fuse ... replace another -> power on -> blown fuse ... (repeat 25 times) power on -> nothing burns ... so THIS was the bad transistor !!!!  :grr :grr :grr

Thanks Juan, I think I will rig something like that together for testing purposes.  In all reality I probably should have done this long ago but I've been lazy - time to get my butt into gear :trouble

Alright, I finally built one.  I've attached the pics.  It is an old "spot light" interior lamp that I had while in college and hadn't used in several years.  I disassembled and then reassembled as necessary.  Now the "globe" the bulb used to rotate in to aim it is now a shield to keep the bulb from getting broken by my dog and my future children.  Works really well.  Just barely glows with a soldering iron in, blips for a second when my Vox turns on and shines brightly when shorted with a piece of wire.  I used a 75w bulb since that's what I had on hand.

J M Fahey:
As the cat said: "it's purr-fect"

I built mine into my homemade lab supply. There is a plastic tube that channels the light to the front panel:


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