powering up marshall avt150 with variac

[movek1]

ok i just replaced output power transistors on my marshall avt150, and i am going to go by enzo's recommendations, by powering it up with a variac and watch it with ammeter.
So here is my two part question, how long should it take me to go from 0 to 120 volts on the variac (1 minute, 5 minutes?) and when i am checking current with my ammeter where should i put my negative leg and positive leg, (sorry if thats a dumb question)

[R ski]

If the transistor circuitry behind the powe3r transistors aren't driving into heavy current conduction, with an ammeter on 10 amp scale and series to the com rail about 5 seconds to line voltage will be good enough to see that emitter/collector currents are low rather then smoking the power transistors.

Remember push pull circuitry class AB check the pos rail semi then the neg rail semiconductor in two trail test. The nasty thing about solid state circuits is. if all is well, nice, if other components cause high current surges, things crap out quickly. On the same note, most discrete solid state amps have current limiting power resistors in low ohmic value, check for continuity per marking and solder connections of your amps front end

[Enzo]

The point of the variac is that we can power the unit at low mains voltage sto see if a problem looks likely. This is nothing like forming caps. We are not trying to ease the mains voltage in over time. We turn it up slow enough we can see if any bad trends appear to be developing.

If there is a dead short in the amp, as you turn the variac up from zero, you would notice current draw immediately starting to rise with the variac setting. That is your clue to turn it right back down. If the current just nudges up a little briefly with each advance of the variac voltage but basically stays low, then continue to turn it up. A normal healthy amp will draw a little current at idle.

You have any other solid state amps? Plug one in and set the variac to 120VAC. What current does the good amp draw? Now turn it off, and start with variac at zero, turn the variac up and waatch what the good amp does.

[movek1]

thank you for your replies.
Ok I understand about the variac's purpose, but where do i hook up my ddm to check current draw, I set it to 20a but where to set ddm leads, before or after transformer, what am i checking.
Obviously, im learning and i thank you for your patience.

[Enzo]

Put them in series with the amp. You want to know how much current is flowing into the amp. If the current starts ramping up right away, back off, there's trouble.

[movek1]

ok, it was the fuse in my dmm that was faulty, but after doing some more research i dont know if my dmm is right for this job.
You can only have it set to 0-400ma or 400ma-20a and know longer than 15 seconds. will this dmm work.
Also, what would be a normal amp reading be for this amp

[movek1]

am i going to have to get a new ammeter

[movek1]

Would this ammeter work for what im trying to do, and should i also get a dc one.
10A AC PANEL METER | AllElectronics.com

[Steve Conner]

That ammeter would be fine, but your DMM should also be fine.

The "15 seconds" thing just means that you can't put 20 amps through the meter for more than 15 seconds. At smaller currents it'll be fine. You should use the 20 amp range, the 400mA one is too small.

[movek1]

So then im wondering what would be normal current draw on this amp (rough estimate) and with my dmm, is it alright to read current from 0-20a, if so then why would there be 2 seperate hook ups, one for 0-400ma and one for 400ma-20a