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In Reply to: RE: MA-1 blew 7A fuse posted by ChasLieb on July 20, 2014 at 18:42:17
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.
Follow Ups:
Hi Ralph,
This is an MA-1 Deluxe with 12 tubes but was brought up to 3.1 spec by you a few years ago. It has two very big power transformers and has the large blue oil caps situated between. It may well be a bad tube as I could not leave well enough alone and replaced the "old" tubes on one amp with a bunch of NOS tubes I picked up and decided to try out to see the difference between the amps was noticeable. It wasn't. I then decided to keep playing the new tubes amd only noticed a problem when I tried,some days later to turn the amp on and the third LED,did not light. I conditioned the tubes for three days and all tubes were tested before insertion but who knows. I can try retesting the tubes but will certainly try what you recommend and see if it is the power supply. If a tube went bad during burn in will it show on the tester or is there a different criteria to see if a tube is likely to take out a fuse? The wires to the tube bases seemed intact but I'll have to check again. I suppose the new Chinese tubes are better than RCAs finest.
If the top LED won't light you have a blown fuse by the power cord.
Generally speaking we have had bad luck with the American power tubes, the exception being the Sylvanias.
If a tube does have a problem it will show up on a tester. I would do a visual examination first, see the link below.
I pulled the tubes and subbed the 7A fuse from the other amp to see if it was the power supply- it wasn't. I then checked the tubes and reinstalled them without the speaker cables and powered up. All three lights are good and strong. If it were a bad tube I think it would have taken out the fuse right away, but so far so good. I use a set of old Z-Transformers and was trying a different wiring which I believe shorted the amp and probably caused all my issues. I will see how it goes and report back if any issues but I think it was a case of fooling with something that should probably have been left alone.
Thanks,
Charles
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