SLO blew the 1A fuse, power tubes are new.

Rick Lee

Rick Lee

Well-known member
I retubed my SLO with Sovtek wafer bottoms back in Nov. I probably ran it a good seven hours while biasing, checking drift and adjusting until it stayed where I wanted it. Haven't played it much since then, maybe 1-2 hours at most. Last week I turned it on, but with standby off and noticed the blue jewel was flickering. I had forgotten to plug the cab into the head, but it was still on standby. Today I had the OD volume up to 5 and, after about five min. the 1A fuse went. All tubes still glow evenly. I replaced the fuse and fired it back up. Blue jewel still flickered, but even with volume at 5, I played about 10 min (earplugs in) and fuse didn't blow. Do I have a bad power tube?
 
Just played for 10 min. with OD volume on 5, which is pretty loud. No blown fuse. Blue jewel flickered some, but it sounds like an SLO should. WTF? How does a fuse blow intermittently?
 
The fuse doesn't blow intermittently, the tube shorts intermittently. Fuses blow for a reason, popping a new one in without fixing the problem is not a repair. There's a chances of doing more damage each time you stick a new fuse in without fixing what caused the first fuse to blow. It's a power tube. It's going to blow again eventually.
 
Ok, I just put my bias probe in socket, tested each tube after letting it warm up. All worked fine and gave even readings, though the bias had drifted down a lot. How else can I test for an intermittently bad power tube?
 
There is no test for an intermittent tube. Sometimes you can tap on them and make them act up, sometimes you can't. That's the pain with intermittent problems.
 
I called Mike today and he said it really could just be a speck of dust in the tube leftover from the manufacturing process, which briefly shorted the tube and then burned up. He said if I kept blowing the fuse after hitting the standby switch, that'd be a problem, but that, if I was playing it pretty hot for a while with no further issues, I'm probably ok.
 
Rick Lee":1xp28ffr said:
I retubed my SLO with Sovtek wafer bottoms back in Nov. I probably ran it a good seven hours while biasing, checking drift and adjusting until it stayed where I wanted it. Haven't played it much since then, maybe 1-2 hours at most. Last week I turned it on, but with standby off and noticed the blue jewel was flickering. I had forgotten to plug the cab into the head, but it was still on standby. Today I had the OD volume up to 5 and, after about five min. the 1A fuse went. All tubes still glow evenly. I replaced the fuse and fired it back up. Blue jewel still flickered, but even with volume at 5, I played about 10 min (earplugs in) and fuse didn't blow. Do I have a bad power tube?
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With the standby off and the indicator light flickering could mean trouble not related to tubes. The SLO has the power supply feeding the main filter caps prior to the standby switch. The indicator light is fed by the heater supply. Could be just a flaky indicator light. The standby switch feeds the entire B+ including pre-amp tubes. I had a amp do something similar and it was a pre-amp tube shorting and not a power tube. The 1A fuse in the slow is for the entire B+ including pre-amp tubes. Pull the pre-amp tubes and see if the indicator light still flickers.

So it could be a bad power tube and or pre-amp tube.
 
I'll try it again today, as I replaced the indicator bulb yesterday, but didn't have a chance to fire it up. Last week I had it dimed for about 10 min. and it sounded perfect. Wouldn't it not sound perfect if a pre-amp tube were bad or going bad?
 
Actually one of the most common comments before a tube amp goes boom is how great it was sounding....

But joking aside any tube at any time can go bad. What they did before is not an indication on what they will do next.... So the amp could be ripping one day and then just die (tube related) on next power up.
 
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