Mesa Boogie M9 Carbine

[lowell]

I have replaced the whole MOSFET block in this thing. It's a later version that has all the service updates. After installing the block, I plugged it up with my bulb limiter. It's a 43watt halogen bulb. The thing glows bright and does not get dim over time. I disconnected the power supply from the MOSFET block and the bulb is dim. I'm worried about blowing the new block right out of the box. I'm not sure if this thing is safe to plug into the mains yet or not. I did check the power supply voltages, and while on the limiter I have ~ +/-24v. I'm curious if maybe my bulb is too low of wattage, and that the amp is fine.

[MarkusBass]

If you are able to disconnect the MOSFET block from the power supply, it means that you can also disconnect one wire (e.g. positive rail) and measure the current. And then decide whether the current is too high, or not.
How do you know that it has all the service updates? Do you have schematic for the amp?
In case of doubts, it always makes sense to check the power amp without output transistors (also with a signal but without a load), measure few voltages, and then add the MOSFETs. Now it's maybe too late for this but think about it next time you get similar amp.
I'm also not sure whether the bulb that you use is correct. Why do you use 43W halogen bulb? I would try classic 100W bulb.

Mark

[lowell]

Mesa sent me the MOSFET block and said it's the updated version. It's installed. I power it up with no load. Was biased way too hot, the heat sink got hot quick and there was 85mv across the .2ohm source resistors. I quickly adjusted it to the specified 10mv across the resistors. Amp is working great. I guess my 43watt bulb is just too low of wattage. I will get some 100watt bulbs.

"it means that you can also disconnect one wire (e.g. positive rail) and measure the current. And then decide whether the current is too high, or not."
I would think that there would be no current if I did this. Could you explain a bit further on why this helps?

"In case of doubts, it always makes sense to check the power amp without output transistors (also with a signal but without a load), measure few voltages, and then add the MOSFETs. "
Again here, is this so that the drivers can be confirmed as OK first?

[MarkusBass]

"it means that you can also disconnect one wire (e.g. positive rail) and measure the current. And then decide whether the current is too high, or not."
I would think that there would be no current if I did this. Could you explain a bit further on why this helps?

To measure the current, you break the wire, connect a meter set to Amperes in place of the broken wire and measure the current. I didn't write about the meter but, hey, this is basic method to measure current. I hope you know it .

"In case of doubts, it always makes sense to check the power amp without output transistors (also with a signal but without a load), measure few voltages, and then add the MOSFETs. "
Again here, is this so that the drivers can be confirmed as OK first?

Yes, I thought this this was exactly what I had written. The input stage of the amp should work correctly, of course without providing output current and driving the speaker. Not all amps will allow for this should work with Mesa. It works with many Ampeg, Eden and many, many other amps.

Mark

[lowell]

Ha...I thought you meant disconnect the positive rail...then measure current through the MOSFETs...with NO POSITVE RAIL connected at all. But now I get it. Meter in series to measure current.

Good call on checking without the MOSFETs installed. Makes sense that the drivers will still function and that the Gates will still have the required bias voltage.

[Jazz P Bass]

Question: if Mesa sent you this unit & it is not working, why don't you call them and express your concerns?

[MarkusBass]

The amp is working correctly now. This is written in post #3. The only problem was that the quiescent current was set way too high. I'm not sure whether this is set in the MOSFET block, or in the voltage amplifier.

Mark

[lowell]

I did and they said it was biased by their tech and that I should see 10mv across the source resistors. They won't let me have an open discussion with the tech, but I can email a question and get a response. I posted here because I learn a lot from everyone on here and can have a true open discussion about things. Thanks for the help!

[lowell]

In post #3 I stated:

Mesa sent me the MOSFET block and said it's the updated version. It's installed. I power it up with no load. Was biased way too hot, the heat sink got hot quick and there was 85mv across the .2ohm source resistors. I quickly adjusted it to the specified 10mv across the resistors. Amp is working great. I guess my 43watt bulb is just too low of wattage. I will get some 100watt bulbs.

I biased it correctly. There is a pot on the MOSFET block. Thanks Markus!

[clintronics]

Carbine

I had an interesting conversation with mesa today. Evidently the carbine had a redesign and the later version uses a transformer with a 55VAC secondary which is a lower voltage than the original.

update: This was for a Carbine M6 not the M9 (my mistake, sorry)
might want to contact Mesa to check for upgrades