View Full Version : Tube amp keeps blowing fuses. Help


Durski
12-15-2007, 03:52 PM
Hello,

My Jolida 801 tube amp blows it 6.3A fast-blo fuse every time. I inspect all the components in the system and all the wiring. All looks correct. What might cause the tube amp to continue to blow the correct fuse?

dshoaf
12-15-2007, 04:02 PM
Short answer: Too much current.

Longer answer: You're treating the symptom. What part of the circuit is this fuse in? Pull all your tubes. Inspect each one visually by rolling it around while horizontal. Look for anything that shows up on the glass or rolls around inside. Set any aside that look burned on the plates or has anything rolling around inside. Do not proceed until you have replacements. I _always_ keep a full set of replacements for my tube amps.

Then, using a new fuse, add a tube one at a time starting with the first stage until the fuse blows again or it starts working. That is, install one tube with the power off, power it up, watch for anything that doesn't look right, then power down and proceed to the next stage tube.

Hope that gets you started.

Cheers,

David

clydeselsor
12-15-2007, 04:07 PM
Does your amp have a rectifier tube? One of my console amps was blowing a fuse and replacing the rectifier tube made it work fine.

clydeselsor
12-15-2007, 04:10 PM
Short answer: Too much current.

Longer answer: You're treating the symptom. What part of the circuit is this fuse in? Pull all your tubes. Inspect each one visually by rolling it around while horizontal. Look for anything that shows up on the glass or rolls around inside. Set any aside that look burned on the plates or has anything rolling around inside. Do not proceed until you have replacements. I _always_ keep a full set of replacements for my tube amps.

Then, using a new fuse, add a tube one at a time starting with the first stage until the fuse blows again or it starts working. That is, install one tube with the power off, power it up, watch for anything that doesn't look right, then power down and proceed to the next stage tube.

Hope that gets you started.

Cheers,

David

I didn't think it was safe to power up a tube amp with tubes missing.

jaymanaa
12-15-2007, 04:12 PM
What does it protect (the whole amp maybe)? All my tube amps use slo-blo fuses due to TX inrush.:scratch2:

dshoaf
12-15-2007, 04:15 PM
Its ok to do so but I generally like to make sure that everything else is working ok before installing the output tubes. These generally conduct the most current and it will run through the output transformers, which need to be protected at all costs.

Now, if there's a tube rectifier, that would need to be installed and confirmed to be operating ok before starting with the first stage tube installation.

Remember, I'm assuming our OP doesn't have any test gear or isn't comfortable around high voltage. I'd normally be inside confirming the power supply is ok, then the rest of the stages have proper operating voltages. My recommended procedure is just a logical procedure using the fuse as the only piece of test gear.

Cheers,

David

Durski
12-15-2007, 04:49 PM
None of the tubes appear to be rectifiers. The amp documentation does call for a fast blo fuse.

dshoaf
12-15-2007, 05:30 PM
Ok, so I looked up the 801 online. You've got a 12AX7 in the first stage. Then there are 2 either 6SN7 or a 12AT7 on either side of it. The big tubes should 6550s - a pair per channel. There's no rectifier tube so it will be diodes in the power supply and bias.

BTW, in your trouble shooting, be aware that the output tubes need to be installed into their original positions as they go back in.

Your manual should have a procedure for setting bias on the 6550s. Do you have the tools to do that?

Cheers,

David

avionic
12-15-2007, 05:50 PM
Hello,

My Jolida 801 tube amp blows it 6.3A fast-blo fuse every time. I inspect all the components in the system and all the wiring. All looks correct. What might cause the tube amp to continue to blow the correct fuse?
Shorted tubes,shorted rectifiers,shorted capacitors..see the pattern something is shorted out.

Durski
12-15-2007, 05:59 PM
No problem for me to set bias. I assumed a short some where and a tube was the most likely. Unfortunately, visually all the tubes appear in good shape. Sounds like my amp may have to go a Tech @ $85 per hour. Any other thoughts?

MRX37
12-15-2007, 06:31 PM
Start testing components for a short. An ohmmeter should work fine.

dshoaf
12-15-2007, 07:33 PM
So, does the amp blow the fuse without the tubes installed? This would tend to indicate a power supply problem, if so.

Cheers,

David

Durski
12-15-2007, 09:46 PM
I removed the four KT88 output tubes and tried powering up with only the 3 first stage tubes (two 6SN7'S and a 12ax7eh). Success. The fuse did not blow. This indicates the power supply is probably okay. I then reinstalled three of the output tubes but not the one tube I was suspicious of. The fuse blew again. So, at this point I should be able to figure out which tube is bad. Unfortunately I am now out of fuses. Will complete troubleshooting tomorrow.

A great big THANK YOU for the responses. Nice to learn new tricks. Glad the AudioKarma community is so helpful.

Whaleman
12-17-2007, 09:59 AM
Unfortunately, this could also mean that one of your output transformers is shorted. I learned that from experiene. Let's hope the damage stopped with the tube being shorted. For the sake of the output transformers, you might consider testing the tubes in a tester and not in the amp. Just my 2 cents having been in the same situation and having to find an output tranny. Going through a box of fuses is not good for your amp.

robaut
12-17-2007, 03:31 PM
Hi,

Do you now type of OPT in this amp (Raa)?