I have a QSC PLX3402 that has been in storage for a couple years. I recently powered on another amp that was also in storage and it promptly went "BANG!!" as soon as I turned it on. I'm in the process of troubleshooting that one. I have since learned about Dim Bulb Testers and I'm going to make one. My question is what wattage should I use for the bulb? The manual for the PLX3402 says it uses .9 amps at idle. .99A x 110V = 99 watts. If I use a 100 Watt bulb, wouldn't that allow all the current the amp needs and therefore not offer much protection? Or am I completely wrong on this? I have read that 100W is good for "most amps" but I'm not sure if this amp falls into that category. So what would be the best wattage to use here? Thanks
Your amplifier should play normally without fully lighting a bulb that is roughly 1.5 to 2 times the amp's stated wattage.
More information about bulb wattage choice here: https://www.antiqueradio.org/dimbulb.htm
More information about bulb wattage choice here: https://www.antiqueradio.org/dimbulb.htm
Hi,
Not sure a dim bulb tester will be good for a PLX3402. They have a switching power supply, which generally do not play nice with dim bulb limiters.
A variac might be a better bet.
Good luck if you try anyway!
Cheers,
Ant
Not sure a dim bulb tester will be good for a PLX3402. They have a switching power supply, which generally do not play nice with dim bulb limiters.
A variac might be a better bet.
Good luck if you try anyway!
Cheers,
Ant
No. The lamp and the amp will be in series and share the voltage between them. Neither one will receive the full 110V.
Unfortunately, it is not easy to calculate how the 110V will be divided between the two since neither one is a linear device and they behave differently when they receive less than the full rated mains voltage. Two 100W incandescent bulbs will receive 55V each but the situation is different when you connect different types of appliances in series.
Maybe you should start with a lower wattage bulb. That will limit the current to a lower level and produce a smaller bang if the amp has a serious short circuit. If nothing blows up, you can step up the bub wattage.
Remember that you must use an incandescent bulb, not LED, fluorescent or some other type of device.
EDIT: Just saw Ant Moore's post. I didn't know what the PLX3402 is. He's right. A device with SMPS may not "play nice" with a series bulb. The principle holds with linear supplies.
Last edited:
Thanks for the reply and the link. It's making making much more sense now
This is interesting. I haven't heard this. I'm fairly certain I've seen reputable youtube videos use a dim bulb with switching power supplies. I'll have to double check. I have heard that modern amps aren't good candidates for use with a variac. Not sure which is correct.
Thanks for the reply. It looks like I'll have to dig into this further.
I hope I'm not opening a huge can of worms here, I'm honestly just trying to find a safe way to power this thing on. Here is a video of a couple high power class D amps and he is using the dim bulb tester when powering them on. The link is time stamped: Dim bulb with Class D amps
With a SMPS (Switch Mode Power Supply) neither a Dim Bulb Tester nor a Variac will accomplish much.
With a SMPS, if the line voltage is low it just draws more current, to reach it's steady state value.
If the line voltage is high, it draws less current. It acts some what like a negative resistor.
Note: a big SMPS and a UPS may not be happy together. Each adjusts itself to the line voltage.
With a SMPS, if the line voltage is low it just draws more current, to reach it's steady state value.
If the line voltage is high, it draws less current. It acts some what like a negative resistor.
Note: a big SMPS and a UPS may not be happy together. Each adjusts itself to the line voltage.
PLX amps use unregulated SMPSs. They draw less current at lower line voltage, not more. But they do have undervoltage lockout, so it needs to go high enough before anything happens. 3 100 watt bulbs in parallel works for bringing these up. It might not fully protect against all failure modes, but at least any damage from a fault will be somewhat contained.
the dim lamp tester is used as a quick go/no-go test, once the lamp dims the test is over. It should only take a few seconds. I hear of far to many leaving it in circuit for far to long. Some even think it’s okay to make measurements and adjustments but they should be done using full AC only.
To get the most out of a dim bulb tester it should be used with a variac. One can then apply full voltage to the DUT while maintaining a current limiting mechanism in case it suddenly jumps. Often problems simply don’t show up with only 90 or so volts at the plug, and that’s not what you want to set the bias.
But just a DBT will let you re-form the power supply caps if that’s the only concern.
But just a DBT will let you re-form the power supply caps if that’s the only concern.
You don't need a Variac. Just start with a low wattage light bulb.
a 25 Watt 120 Volt bulb will limit current to under 1/4 Amp.
a 25 Watt 120 Volt bulb will limit current to under 1/4 Amp.
Some of these amps draw a lot more than 1/4 amp idling. And these dont like the AC mains to be too low.
25 Watts is for first start. Then use 50 or 100 or more Watt bulb.
This thread is about using a Dim Bulb Tester for a unit that has been on the shelf for a long time or has a questionable history.
Also good for a unit that blows fuses on start-up.
This thread is about using a Dim Bulb Tester for a unit that has been on the shelf for a long time or has a questionable history.
Also good for a unit that blows fuses on start-up.
Starting at 25 on this amp is a non starter. Need to start at 100, but get it running on 300 before trusting it.
Thanks everyone for your replies. This has been very helpful and informative. So much to learn...
I have been looking into this and want to make sure I understand you correctly. Sorry if this is a dumb question, when you say "3 100 watt bulbs in parallel works for bringing these up", do you mean series-parallel, like these? The first pic I found on another thread here, the 2nd is from a Mr Carlson's Lab video.
Later you said "Need to start at 100, but get it running on 300 before trusting it." Just so I'm clear, you would suggest starting with 1 bulb, then if everything is ok, power off, then try 2, then power off and then 3?
Also if I used a variac, how would you recommend using it along with the DBT? Would you have a volt meter in the circuit and start it at 120V and adjust it to ensure the voltage doesn't drop due to the bulbs? I'm fairly sure with one of these amps, you wouldn't want to start it low and slowly bring up the voltage unless I'm mistaken.
Finally,
just out of curiosity, how do you know so much about these PLX amps?
Thanks again for your input.
I have been looking into this and want to make sure I understand you correctly. Sorry if this is a dumb question, when you say "3 100 watt bulbs in parallel works for bringing these up", do you mean series-parallel, like these? The first pic I found on another thread here, the 2nd is from a Mr Carlson's Lab video.
Later you said "Need to start at 100, but get it running on 300 before trusting it." Just so I'm clear, you would suggest starting with 1 bulb, then if everything is ok, power off, then try 2, then power off and then 3?
Also if I used a variac, how would you recommend using it along with the DBT? Would you have a volt meter in the circuit and start it at 120V and adjust it to ensure the voltage doesn't drop due to the bulbs? I'm fairly sure with one of these amps, you wouldn't want to start it low and slowly bring up the voltage unless I'm mistaken.
Finally,
just out of curiosity, how do you know so much about these PLX amps?
Thanks again for your input.
Those 120VAC bulbs are all in parallel. Then the group of bulbs is connected in series with the AC line, as shown.
However, it is best to have a selection of bulbs of various wattages, if you can find them, and only use one at a time.
Start with one 100 watt bulb and if no fireworks, up it to three and it should be able to operate at sane volumes. If you have a variac, you can use it with a dim bulb (or three) to get full voltage while retaining the current fold back feature. I do this to run tube or SS amps under development. Turning up an SMPS slow can be deceiving. If the converter doesn’t START, you might not be able to see a problem downstream. Without a current limit (just a variac with no bulb) it can just snap on when it goes high enough. If you had shorted outputs or bad caps on the secondary side of the converter things go south quickly.
I happen to own a rack full of PLX3102’s for running mid/hi cabs with.
Thanks again for your input on this. It is encouraging that you also own a bunch of PLX amps. I'm currently deciding on what design I'm going to use for the DBT and sourcing parts. It is sounding like I really should have a variac as well and I'm looking at what options are out there. Thanks again