Question about dim bulb test

[rossm]

Hi
I used my dim bulb and variable transformer to slowly bring up an old amp. After 3 hours I had the voltage up to 100% and the bulb was dark.
I connected an MP3 player and started enjoying the music at minimal volume, I got brave and increased the volume slightly and suddenly the sound started distorting, the light came on full blast and then humming out of both channels. I turned off the amp and dropped the transformer to 50%, slowly increased the voltage back to 100% and amp now works at min volume. Have not tried increasing the volume again. Question, should I not use the amp on the dim bulb? Or something up with power supply?
Thanks

 

[whoaru99]

With the dim bulb tester you are limiting the current available to the amp. If demand of the amp approaches/tries to exceed the current it can pull though the bulb, supply voltage will drop. The amp may be simply reacting to supply low voltage.

If it has passed the DBT, then probably should give it a try connected directly to the wall. Or, at least increase the bulb wattage a couple steps to see if the problem still happens, but perhaps at even higher volume setting.

 

[rossm]

Thanks for the info, makes sense!!!
Guess I was overly cautious

 

[gadget73]

solid state amps draw very little power when not passing sound so this isn't surprising. Bigger bulb or better yet, straight into the wall will tell you whats going on.

 

[Alan0354]

Also, there is possibility that the amp is unstable at lower rail voltages. It is quite common amp oscillates when power down. Sometimes, you can even hear it when you turn off the amp. Like a few seconds after you turn the power off, the amp starts making noise until it finally fade away. Don't rule out this possibility, My Acurus does the same thing, whistle after power off for a few seconds. When I first brought up my own amp, I could see it burst into oscillation when the voltage got very low. I tamed it. Symptom like this indicate the amp does not have enough stability margin, it's better to fix it so it doesn't do that at all. See explanation below if people are interested.

Dim bulb lower the rail voltages when you draw more current as you increase the volume. The amp might not like it. You are not supposed to lower the rail voltage in normal operation.

You should use either dim bulb or variac( variable transformer). If you have variac, don't use dim bulb.

If you use variac, the rail voltage won't change regardless of how loud the music, it would be interesting to lower the voltage of the variac to the point that the amp might burst into oscillation when you play music.( music is just a variable signal to trigger oscillation, it might not start if you don't have input signal to trigger it.)







The reason is the change of parasitic capacitance Cbc and Cce of the big power transistor. These combine to become the input and output capacitance of the transistors. They both increase tremendously when collect to base and collect to emitter voltage decrease below 6 or 7V. At 3V, the capacitance increase over 3 to 5 times. Those big transistors has high capacitance, all of a sudden, you are like adding over 1000pF when you lower the rail voltage. The amp can get unstable with all the extra capacitance at the input and output of the transistors.

That's the same reason why some amps are stable when you play low volume, then when you crank it up, it start doing funny things. The reason is when you play loud, the peak of the signal get close to the rail voltage and the capacitance increase to the point the amp burst into oscillation. Those are hard to detect as the oscillation only ride on the peak of the signal and die as soon as the voltage comes back down!!!!

This is very common, it's the responsibility of the design engineer to fix this before going into production.......in the perfect world.

 

[whoaru99]

@rossm

Results with higher-wattage bulb or direct to wall?

 

[Binkman]

What model and number? or post a link to SM.

If SS use the dim bulb at 100w. Variac auto transformer varies the Volts but current is same not good for SS especially fets. Not need now as problem is with gain.

In short; you could have some tired transistors or just caps have aged to point of replacement. Probably electrolytics is pretty norm if the amp has a lot of hours on it. service the rails and associated drivers in both the pre and output amps stages. i.e. caps hold that ballast that the transistors need to stay open so transistors conduct switching or amplifying etc but caps also filter, too. (balancing act of sorts for short.) cap going the transistor starts making noise.. or the weak transistor, too.

I would presume driver transistors are weak. Been two posts this week for same and many in past. Might post pics .. But you need to do some testing regardless.

need model number make???
bink

 

[Alan0354]

I think I would just plug directly into the wall and try. It passed the dim bulb in my book already. The amp might work perfect in full line voltage and OP can move on.

 

[rossm]

Hi
I have not yet tried bypassing the dim bulb, I have the unit standing on its back right now. I am trying to free up the tuning cap by dripping 3 in 1 oil on the bearings, I have been applying the oil every day for about 1 week with no luck. I might have to byte the bullet and remove the cap from the unit. I sure hate the thought of redoing the dial cord. I have no doubt contaminated the fins with the oil, so it's going to need a good cleanup anyway. I don't dare force anything because I am sure that plastic pulley is very fragile and expensive
This is a solid state fisher 400-tx

 

[slimecity]

I had this with a Technics SU 8080 I was running on the DBT after some work was done.

Bulb was dim, turned it up, bulb grew brighter, then the protect circuit kicked in. So the amp didnt like the reduced voltage.

As others say, if the bulb is dim and you are happy with how the amp is behaving, then chances are you will be all good to take it off the DBT now.

 

[petehall347]

thinking about this if the bulb was rated so it would pass fully operating current of the amp and it then lights up there is a bad problem to look for .