Quandary regarding power up with variac & amp meter

[Bigdrums]

Saddened by the loss of Enzo. Whatever he said, you could believe.

Sometimes when I bring up the voltage on my variac, I notice the built-in amp-meter climb higher than expected, Sometimes 1 amp or more without any indication that the filter caps are about to level off or suddenly stop passing current. In the past I have also had a light-bulb current limiter in line as well. My experience is that with most solid state amps, as I slowly crank up the power, the amp-meter will climb a little bit and then start to drop back down. I will then turn up the power just a little at a time as I can see little current is being drawn. But every so often I will hook up an amp that will not turn on completely with the variac up full. Mine only goes to 120v, not higher as some commercial units do. If I remove the limiter and push the power more than I am comfortable with, sometimes it is enough to get the amp to turn on normal, as the current draw drops way down to .2-.3 amps usually. It just scares the hell out of me to do this because I never know if it's safe or about to blow. Any thoughts or comments?

[Helmholtz]

An increase in voltage causes a charging current in the large filter caps until they are charged to the higher voltage.
When the caps have charged to the higher voltage, current drops to the actual circuit draw.

The faster you ramp up the voltage the higher the current surge.

The effect may not be instantly repeatable at the same voltage as the caps hold the charge for some time (and can't discharge via the rectifier diodes and PT.)

[DrGonz78]

Mine only goes to 120v, not higher as some commercial units do. If I remove the limiter and push the power more than I am comfortable with, sometimes it is enough to get the amp to turn on normal, as the current draw drops way down to .2-.3 amps usually. It just scares the hell out of me to do this because I never know if it's safe or about to blow. Any thoughts or comments?

My variac goes up to 150vac and when it is hot in the summer I have to push above 120vac on the dial setting. There is a lot of current draw from AC units and other appliances in the house which ultimately sags the voltage readings on the variac. At first with an amp I have the light bulb limiter connected to see if inrush current is going to be a fuse blowing issue. I might use the variac at that point to slowly bring up the voltage but I also want to flip the power switch, light bulb limiter connected, while monitoring ammeter and light bulb. At that point I ditch the limiter and then slowly bring up voltages with the variac to observe the ammeter. Then I test and monitor current surges when the amp is plugged in and flipped on. Honestly by the time I remove the light bulb limiter I am fairly confident that the current surges are not going to be an issue.

Edit:

It just scares the hell out of me to do this because I never know if it's safe or about to blow. Any thoughts or comments?

How many times has it blown? The answer to the question should make you feel a little better.

[g1]

Some solid state amps will not start up properly if brought up with variac while load is connected.
In these cases:

turn off power
disconnect load
bring up on variac
verify no DC on output

If amp comes up normally with no DC on output, then connect load while amp is still powered up.

[Helmholtz]

Please let me know if I did not satisfactorily answer the original question.

[Bigdrums]

Please let me know if I did not satisfactorily answer the original question.

There is minimal draw without the speaker load. Won't plugging in the load while turned on be risky?

[Helmholtz]

There is minimal draw without the speaker load. Won't plugging in the load while turned on be risky?

The filter cap recharge current surge when ramping up the variac voltage does not depend on the load.
I tried to explain the effect you're describing in your opening post: Fast rising current then drop back. In other words a current overshoot.

[Bigdrums]

Please let me know if I did not satisfactorily answer the original question.

Thank you for the reply. What I'm trying to solve is this amp (a Roland JC120) seems to have all the main issues resolved except I don't see the amp meter indicating the filter caps charging, and therefor the meter just keeps climbing little by little, but not settling down at all. In the past this could mean a minor short, or as with some circuits needing to be pushed past a certain point before dropping down to a more usual idle current.
The outputs all test good, so why does it draw with the load, but nothing without?
When I check for shorts from the supply rails (or before) to ground there is very high resistance.
It's as if some part of the circuit is turning on while ramping up, to a high idle current

Thanks

[Bigdrums]

The filter cap recharge current surge when ramping up the variac voltage does not depend on the load.
I tried to explain the effect you're describing in your opening post: Fast rising current then drop back. In other words a current overshoot.

Thats the thing, it doesn't drop back in this case.

[The Dude]

FWIW: You shouldn't bring a SS amp up slowly on a variac with a speaker load. Supplies almost never come up evenly and there's likely to be DC on the output until the amp stabilizes. Use the variac to bring up the amp while monitoring current to make sure there are no problems. Check the output for DC. If everything looks good, connect a load, power up normally.

[Helmholtz]

The outputs all test good, so why does it draw with the load, but nothing without?
Thanks

That typically means DCV at the load, meaning DC power delivered to load (can damage a speaker).
In this case mains current will continuously rise with variac voltage.

[g1]

Some solid state amps will not come up on variac with both supplies equal, like The Dude mentioned. That can make the output 'stick' to one of the supply rails and draw heavy current through the load.
In these cases, if you then bring it up without the load, and find no DC on output, you will not damage anything by connecting the load while still powered up.
Or just go ahead like The Dude said and power it up with load but no variac.

[Helmholtz]

Some solid state amps will not come up on variac with both supplies equal, like The Dude mentioned.

What could prevent the main rails from coming up equally except for a shorted rectifier diode or filter cap?

[mozz]

Sometimes they tap the positive rail and use that voltage for other things, so I'm guessing that uneven draw causes the rails to swing to one side in the output section if brought up slowly.

[Helmholtz]

Sometimes they tap the positive rail and use that voltage for other things, so I'm guessing that uneven draw causes the rails to swing to one side in the output section if brought up slowly.

That doesn't mean the rails themselves are uneven. It would take huge currents to achieve that.

Of course it's possible that the power amp is not balanced at low voltages because some auxiliary voltage is missing, a zener not conducting yet etc.
In that case some circuits may tend to latch on one rail.