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Post by brianscheffer on Jul 26, 2013 23:09:00 GMT -6
Who is our resident amp expert? The studio's 1982 JCM800 2203 is blowing HT fuses occasionally and I'm trying to figure out why. My understanding is that this is often indicative of power tubes going bad. The first time it happened, I changed the power tubes. It's done it twice since, including today. I swapped in another fuse and it got us through the session, but I need to get to the bottom of this situation so I'm not worried that the amp will ruin a take at any second. What else caused the HT fuses to pop?
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Post by matt on Jul 26, 2013 23:41:05 GMT -6
Who is our resident amp expert? The studio's 1982 JCM800 2203 is blowing HT fuses occasionally and I'm trying to figure out why. My understanding is that this is often indicative of power tubes going bad. The first time it happened, I changed the power tubes. It's done it twice since, including today. I swapped in another fuse and it got us through the session, but I need to get to the bottom of this situation so I'm not worried that the amp will ruin a take at any second. What else caused the HT fuses to pop? Unless you feel confident about working inside the chassis, I would find a local amp tech to put it on his bench and test it for you. The voltage/amperage involved is not trivial. I have two heads that do this right now, including my prized Friedman BE100. I will not touch them beyond simple tube swaps. And even then, replacement power tubes typically require biasing, which any good tech will do as part of the service work. |
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Post by brianscheffer on Jul 27, 2013 10:45:19 GMT -6
I'm confident working inside that thing. I know enough to be dangerous! I know how to properly drain the filter caps and to keep one hand behind my back when I'm digging around in there, so I shouldn't kill myself. I did bias the power tubes when I swapped them out last. I also repaired an improper conversion to EL34s. In some poking around the internet, I heard a few people mention that bad filter caps can cause the HT fuse to blow. I'm almost certain these are the originals, so I need to change them anyway. Again, I know enough to be dangerous and can follow instructions, but my theoretical and diagnostic knowledge is a little light. Is there anything else I can check to try to chase down the problem before I get new capacitors in there?
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Post by matt on Jul 27, 2013 11:31:16 GMT -6
I'm confident working inside that thing. I know enough to be dangerous! I know how to properly drain the filter caps and to keep one hand behind my back when I'm digging around in there, so I shouldn't kill myself. I did bias the power tubes when I swapped them out last. I also repaired an improper conversion to EL34s. In some poking around the internet, I heard a few people mention that bad filter caps can cause the HT fuse to blow. I'm almost certain these are the originals, so I need to change them anyway. Again, I know enough to be dangerous and can follow instructions, but my theoretical and diagnostic knowledge is a little light. Is there anything else I can check to try to chase down the problem before I get new capacitors in there? I can't add much since I would have a tech look at it if I owned this amp. Hopefully somebody else can offer some sage advice. Best of luck! |
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Post by bobbritt on Aug 5, 2013 19:33:06 GMT -6
Are you using a slow blow fuse?
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Post by sozocaps on Aug 5, 2013 20:58:04 GMT -6
Are the tubes glowing red ?
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Post by matt on Aug 6, 2013 11:28:32 GMT -6
Are the tubes glowing red ? If you mean super-bright "sunrise" orange, I've seen that on two of my heads; both have suffered blown HT fuses. I assume this indicates something specific, like bad biasing. Or just a bad tube. |
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Post by sozocaps on Aug 6, 2013 11:55:12 GMT -6
yes... exactly, could be either bad bias, bad tube, or very unmatched tubes.... I have set 3 tubes to bias at 35ma, and the 4th output biased at 48 for example.
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Post by brianscheffer on Aug 15, 2013 19:03:01 GMT -6
I've been away for a bit, so I'm catching up...
I'd double check if they were slow blows, but the last in the pack was the last one in there. I'm not certain. I assume what you're getting at is that it should be a slow blow and may have had a fast blow in it, right?
The tubes are not red-plating.
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Post by bobbritt on Aug 15, 2013 19:06:03 GMT -6
I've been away for a bit, so I'm catching up... I'd double check if they were slow blows, but the last in the pack was the last one in there. I'm not certain. I assume what you're getting at is that it should be a slow blow and may have had a fast blow in it, right? The tubes are not red-plating. Yes... should be slow-blow. |
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