Bought a car and this amp was in it(Pyramid Crystal PB781X 2ch MOSFET). it is missing the fuse. what fuse does it require?

need the actual model number to be sure on getting the right one.

Not to disappoint you too much, but the 1000 watts advertised is just marketing hype. If your amp is the PB781X, it should have only a single 15A fuse on it. Your battery provides 12.5 volts when the engine is off, so at most, your amp draws 187.5 watts of power. Draws, not outputs. No amp is 100% efficient due to heat. When your engine is running, assuming your alternator outputs 14.4 volts, your amp's maximum power draw is about 216 watts. Again, this is what your amp draws, not what it puts out. What you should focus on is the RMS (continuous) output, not Max or PMPO. Those numbers are useless, unfortunately.
Your amp is rated to output 50 watts per channel at 4 Ohms, and 75 watts per channel at 2 ohms. This is when wiring your channels separately, not bridged. I did not see true power specs for your amp while bridged, but it's possible you'll just get the full 150 watts to the single bridged output. Do not attempt to apply a 1 ohm load to your amplifier, as you will run the risk of damaging it. It's not designed to handle a 1 ohm load. If you're lucky, you'll blow a fuse before the amp is damaged, but either way, there is nothing to be gained by attempting it.

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how much rms is the pyramid pb3818 5000w amp ?

Professional repair is required.

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about 250 in 2 ch this amp ****

try: a shorter ground cable, a bigger ground cable or sanding the surface where you make your ground. a lot of amps will tell you the ground should be no more than 3 ft and for any the shorter the better

Hello sunshanel,

You did not say what you have the amp connected to and what the impedance load is. It is only stable at 4 ohms and if a lower impedance is connected, say 2 subs in parallel, it would try to produce more power when you turn it up and would most likely blow the fuse. 20 amp sounds right for the amp. The power rating of 800 watts per channel is peak power, not RMS, and many low-end manufacturers tend to state bigger numbers in the ratings.

Hope this helps.

sounds like the Impedance of the speakers does not match that of the amp, therefore the output stage of the amp is trying to compensate, driving the amps too hard and eventually causing the thermal cut out to activate. Check the ratings on everything are compatible before you start sending it off to be fixed.

The protect light is a notification that the amplifier has reached a set temperature and shuts the amp down till it cools off. Look for the thermosister on the heatsink and follow the circuitry back to the control chip. Once you get that fixed the amp should power up until it gets too hot and the protect circuit kicks in. I had one of these amps for a while and the circuit was very touchy. Regards, rcampsr

open it up and check the board and magnets on the side if it smells funny and looks like black resin its burned out