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Quick dim bulb tester question

bjlefebvre

bjlefebvre

Super Member
Finished wiring up my DIY 2w/ch SET amp and turned it on via a 40w dim bulb tester. The bulb burns only slightly for the first ~10 seconds, which I know is a good sign. But as the amp warms up - as the tubes start to glow - the bulb starts burning more brightly. Is that normal, or a warning sign? For the love of Mike I don't want to see any "magic smoke."

Also, now that I'm thinking of it - is it ok to test voltages on the PCB board while the dim bulb tester is in the circuit? I'm assuming that readings would not come back correctly.
 
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bjlefebvre said:
Finished wiring up my DIY 2w/ch SET amp and turned it on via a 40w dim bulb tester. The bulb burns only slightly for the first ~10 seconds, which I know is a good sign. But as the amp warms up - as the tubes start to glow - the bulb starts burning more brightly. Is that normal, or a warning sign? For the love of Mike I don't want to see any "magic smoke."

Also, now that I'm thinking of it - is it ok to test voltages on the PCB board while the dim bulb tester is in the circuit? I'm assuming that readings would not come back correctly.
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Sounds like you are fine with the bulb 'readings' you've had. When a tube amp first turns on, there is a burst of current that flows through the cold heaters. Then once they calm down , there is a lull until the power tubes have warmed enough to start conducting, then the power rises thus causing the bulb to light brighter.
No, you can't take accurate voltage readings with the lamp in series with the amplifier. The lamp has a voltage drop of its own, which limits the AC voltage to something less than 120v. For my designs, I use a Variac in conjunction with a Kill-A-Watt monitor to watch the power drawn in Watts. If it starts to rise abnormally, I know to shut it down. Plus, I connect a signal generator and scope to it because I want to know that its acting like an amplifier ASAP, even if the waveform looks shitty at first. I generally start with 60 Vac in, just enough to get the tubes going. Similar to using a lamp.
 
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