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OTL, Output Transformerless Amplifier User Group.

Sure sounds like a bad power tube to me.

If you have the power supply boost option the rear panel fuse is often 7 Amps.

If it blows that says nothing about the other fuses, they are just fine.

I would be looking for a bad tube; I would start by replacing the fuse (make sure its a slow blow type). You didn't mention how long it took to blow the fuse or what kind of MA-1 this is. If the fuse blew instantaneously I would be suspicious that the fuse was a fast blow type or that there is a malfunction in the amp.

Older MA-1s with 12 power tubes used a power transformer that had a large inrush current so there are 7A fuses used on those as well. If you have one of these, try removing all the power tubes and see if the fuse still blows (of course you can do this on newer amps too). If the fuse does blow it indicates a problem in the power supply. Some of those 12-tube MA-1s are old enough that filter caps can be failing in the power supply.

Please keep me informed of your progress.


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