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Scott 299B Blown Fuse

Zack Sainz

Zack Sainz

Active Member
Hello all!

I love my Scott Amp, it sounds so good and was working perfectly for a year. A few months ago the fuse blew. It was a quick blo fuse so I tried another as I thought maybe it was a bad fuse, but same results. I let it chill for awhile. Now since I have time I thought I’d look into it more.

I just order slow blo fuses, as on the unit itself recommends slow blo. Same result fuse blown. I opened up the unit, I see no visible scorching.

My friend, rebuilt the power supple a few years ago. And it looks like it was also recapped.

I wish I knew more about this stuff I’ve been studying and doing my best to learn as much as possible.

Any thoughts?

thank you again!
 

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Zack,
Definitely use slow blow fuses (keep a few on hand). Question: is the fuse blowing right when you turn it on or after the tubes warm up? This is an important indicator in the troubleshooting.

If it's blowing right away, first pull the 5AR4 rectifier tube and see if the fuse still blows. If the fuse blows without the rectifier tube operating, you likely have a problem with the power transformer. If that's the case, stop there and report back.

Next would be to test existing or try a new 5AR4 rectifier tube. If routine operation after than, it was the rectifier tube. They do fail (especially some of the new production types) and will blow the fuse if they do.

If that doesn't do it, you may have to look elsewhere and we can guide you there. Other possible causes to rule out would be shorts in the B+/High voltage supply (one or more of the capacitor sections) or the output tubes. If you have a shorted 7189 output tube, I wouldn't expect the fuse to blow until after the 5AR4 rectifier has warmed up.

Just some initial thoughts. Let us know and good luck with it!
Dave
 
Dave451 said:
Zack,
Definitely use slow blow fuses (keep a few on hand). Question: is the fuse blowing right when you turn it on or after the tubes warm up? This is an important indicator in the troubleshooting.

If it's blowing right away, first pull the 5AR4 rectifier tube and see if the fuse still blows. If the fuse blows without the rectifier tube operating, you likely have a problem with the power transformer. If that's the case, stop there and report back.

Next would be to test existing or try a new 5AR4 rectifier tube. If routine operation after than, it was the rectifier tube. They do fail (especially some of the new production types) and will blow the fuse if they do.

If that doesn't do it, you may have to look elsewhere and we can guide you there. Other possible causes to rule out would be shorts in the B+/High voltage supply (one or more of the capacitor sections) or the output tubes. If you have a shorted 7189 output tube, I wouldn't expect the fuse to blow until after the 5AR4 rectifier has warmed up.

Just some initial thoughts. Let us know and good luck with it!
Dave
Click to expand...
Thank you so much! It blows about 5 seconds after I turn it on.
 
Dave451 said:
Zack,
Definitely use slow blow fuses (keep a few on hand). Question: is the fuse blowing right when you turn it on or after the tubes warm up? This is an important indicator in the troubleshooting.

If it's blowing right away, first pull the 5AR4 rectifier tube and see if the fuse still blows. If the fuse blows without the rectifier tube operating, you likely have a problem with the power transformer. If that's the case, stop there and report back.

Next would be to test existing or try a new 5AR4 rectifier tube. If routine operation after than, it was the rectifier tube. They do fail (especially some of the new production types) and will blow the fuse if they do.

If that doesn't do it, you may have to look elsewhere and we can guide you there. Other possible causes to rule out would be shorts in the B+/High voltage supply (one or more of the capacitor sections) or the output tubes. If you have a shorted 7189 output tube, I wouldn't expect the fuse to blow until after the 5AR4 rectifier has warmed up.

Just some initial thoughts. Let us know and good luck with it!
Dave
Click to expand...
Reporting back! Something worked I had pulled all the tubes a few months ago and reseated the tubes, and just did it again. We have lift off!

What ever you did, you scared the gremlins out of Scott!

thanks!!!!!
 
Glad to hear it! You are welcome for whatever mojo the post provided (if any!). If it's a tube contact issue, consider cleaning the socket and tube pins with a cleaner like DeOxit D5.
Dave
 
I love when sound advice is followed and the problem is solved in a few hours. Congrats!
 
Hello All!

@Dave451 You might have been right, I might be having issues with my rectifier tube.

After 20+ min playing I get a loud bassy “pop” or pops randomly throughout listening. Obviously I power down after these pops. And with the previous blown fuses, I think I might need to get a new rectifier tube.

The current rectifier is Sovtek, stay in the same family or something better? I believe my current tube is one of the new production tubes, which could be the culprit.

Good news is the fuse is intact, so those gremlins are gone! :)

cool gloves huh?
 

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Definitely try a new one...now! I've had good luck with Sovteks, but lately have been trying Tube Amp Doctor 5AR4's, but the jury is still out on those. Is the popping both channels or just one?
 
Dave451 said:
Definitely try a new one...now! I've had good luck with Sovteks, but lately have been trying Tube Amp Doctor 5AR4's, but the jury is still out on those. Is the popping both channels or just one?
Click to expand...

I've had good luck with modern TAD 5AR4-STR "selected" rectifiers.
 
Purchased the TAD!

I just read a review that stated over 4 years of use and still going!

Fingers crossed!

thank you everyone!
 
TAD installed!

So the biggest problem is fixed! Loud pops on both channels have been solved! I have listened with the new rectifier tube for a few hours now. No LOUD pops, but now that one issue has been fixed I am noticing very quiet infrequent pops only on the left channel, maybe like quiet 1 pop in the last hour and a half. I already deoxed everything, I'll swap some tubes around, I'll double check the internal speaker connection wire on the amp after it cools down, and my speaker cables are snug.

I wish I had a tube tester right about now!

Thanks again everyone.
 
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