Setting Quiescent Current Whilst Using Lightbulb Trick...?

Thread Starter


Joined Jan 15, 2014
Hi folks,

I am struggling with an amplifier I have replace the output transistors in.
I states that it is critical to set the quiescent current when any components have been replaced.
Just incase I turn the unit on normally and they all blew again, I used the lightbulb trick and all was fine.

The light bulb lit and then dimmed immediately. I also put a known good unit onto the light bulb and it behaved the exact same way - so I deduced that all was well.

When checking the quiescent current I had an imbalance. One side read zero across the resistors (R's 4, 14, 44, 45)... the other read -76mV.
I could not set them and assumed it was because of the lightbulb.

So I removed the lightbulb - after all, it didn't "light" brightly and behaved in the same way as the known good unit...
I got sparks (from R42) and blew the output transistors again...!

My question is - should I be able to set the quiescent current whilst using the lightbulb trick...?

Appreciate any help,



Joined Oct 2, 2009
What is the "lightbulb trick"?
Do you mean using a lightbulb as a load, from output to GND? It depends on the resistance of the lightbulb.
If the resistance is too low the amp is going to blow.
The lightbulb should not light up, ever, unless you are over driving the load. The purpose of the lightbulb is to save the speaker, not the amp.

You can test the amp in stages.
1) Without any load, measure the output voltage with respect to GND. Voltage should be 0V.

2) You can test the amp output with a live speaker, putting light-bulb + 4700μF 100V capacitor + 8Ω speaker all in series from output to GND. Measure voltage at output of amp with respect to GND. Voltage should be 0V.

3) You can measure the output current by putting lightbulb + ammeter + 8-10Ω load in series. Current reading should be 0A.

Thread Starter


Joined Jan 15, 2014
No - I meant putting a lightbulb in series with the mains feed.
I thought it was a widely know 'trick' to help prevent newly installed components from blowing in case there was still a fault present...!?