PLEASE HELP ME troubleshoot my Fender 100T!!

Jhonnybass

Jhonnybass

Aug 22, 2007
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Toronto, Ontario, Canada
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#1
Hey everyone!

I just purchased a Fender 100T bass head from the classifieds here and overall I'm ecstatic with it! I was cranking it at home, running it full blast just earlier today and it was sounding glorious with my Fender 410 Neo cab.

I took it on a gig tonight and the amp shutdown during the soundcheck! The disappointment was unbearable - lol. I was lucky I had my GK MB200 backup with me so I was able to get through the gig just fine.

Right before the 100T stopped working I heard a loud high pitched feedback kind of sound and then the head shutdown altogether. I noticed a burning smell when I got my nose close to the head and upon closer inspection I discovered that the main fuse had blown. When I got home and had a chance to remove the back cover, I noticed that one of the smaller tube fuses was blown as well. The second tube fuse looks like it's still intact but the middle of the wire is a copper color instead of silver which makes me think that it was under some stress as well. Looking at the power tubes I noticed that the V5 (6L6GC) GT logo is blackened compared to the other surrounding tubes which also make me think this tube was the culprit. Was this a power surge and if it was - how come everyone else's gear was fine? Or is this likely a bad power tube?

When I was at home the output tube status was green and solid for all the tubes which is why I find it weird that one of them would fail all of a sudden on a gig. This is my first tube amp and I don't want to stress the head by just replacing the fuses and having the amp go berserk again. Does any of what I described make any sense and can someone think of a diagnostic? I do want to get it up and running for a gig I have tomorrow so any help and advice would be greatly appreciated!!

Thanks in advance!!

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#5
Feedback can really drive an amp crazy.
It is basically an uncontrolled oscillation that can run everything beyond design limits.
Output is feeding input, getting amplified becoming more output feeding more to the input and so on until poof.
It's kind of like a chain reaction in a nuke.
And it sounds like you had it fairly well cranked to start with.

Was this the type if feedback that happens when you hold your instrument up to the speaker cab?
If you do that intentionally, be more Jaco and less Jimi.
 
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#6
The orange paint on Groove Tube products is designed to darken as a sign that the tube is running too hot and acts as a warning. But if a tube fails suddenly, this isn't going to help. These amps are designed to be pushed hard and what you experienced is not common. Pushing it hard revealed an instability and caused the failure. Make sure that the replacement tubes that you buy are burnt in, tested properly, and well matched. Check out www.thetubestore.com - Your online source for audio vacuum tubes., they are close to your location.

Fortunately, this Fender amp is well protected and hopefully it wasn't damaged which will allow the tech to have it running again quickly.
 
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#7
Old Garage-Bander said:
Feedback can really drive an amp crazy.
It is basically an uncontrolled oscillation that can run everything beyond design limits.
Output is feeding input, getting amplified becoming more output feeding more to the input and so on until poof.
It's kind of like a chain reaction in a nuke.
And it sounds like you had it fairly well cranked to start with.

Was this the type if feedback that happens when you hold your instrument up to the speaker cab?
If you do that intentionally, be more Jaco and less Jimi.
Click to expand...

Hahaha I didn't have the head cranked at all when it malfunctioned. It was a soundcheck in a small room so the master was at around 3. I did have it loud at home prior to that when it working perfectly. During the sound check I was really close to the rig because of the small stage but I wasn't rocking out trying to get feedback or anything - neither Jimi or Jaco style. Lol. The feedback just happened and it lasted for like 2 seconds and then the amp was off.
 
#8
Jhonnybass said:
Hahaha I didn't have the head cranked at all when it malfunctioned. It was a soundcheck in a small room so the master was at around 3. I did have it loud at home prior to that when it working perfectly. During the sound check I was really close to the rig because of the small stage but I wasn't rocking out trying to get feedback or anything - neither Jimi or Jaco style. Lol. The feedback just happened and it lasted for like 2 seconds and then the amp was off.
Click to expand...
OK. Just checking. I misunderstood the feedback comment.
I concur with the others then. It's tech time. Hope it's minor.
 
#12
Looks like the tube is dead.
The question is, did the tube fail and take out a resistor
or was the sequence the reverse?
The end result is much the same.
My guess is that moving the amp had something to do with the failure.
The odds of that are higher if you pushed it hard, then packed up while it was still hot.
It's always a good idea to let tube cool a bit before moving the amp.
 
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#13
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#14
Did you get the original warranty off the original owner? If so. ..you may be able to work sumthin out there. ...if it's still under.
Usually those sorta things ain't real pricey to repair. ..ime. good luck.
Oh & ime also. ... on my Super Bassman at least. ..the master vol goes pretty much all the way up to 10. Like it doesn't stop getting louder at 5 or 6... actually it might start getting power tube drive at 6..then just get crunchier.. I dunno re the 100T. ..and rarely do I run my SB up past 6 or 7 on the master.
 
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#17
Downunderwonder said:
so if its a one to 5 year old amp he's still golden?!
Click to expand...
Hopefully for @Jhonnybass.
I went to my local National chain to verify this and they had no clue about it. Apparently it was not on the sales rep's itinerary to let the stores know about this "little" change to the warranty coverage.
 
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#19
Lowendtech said:
Ah cool that's 20 min from my house and I just bought one of these amps myself last week. Good info.
Subbed in to hear the diagnosis on this.
How old is your amp @Jhonnybass ?
Click to expand...

Hey sorry for the delay in replying! I was playing a wedding and just got home right now. The original owner said he bought it back in 2012 when they first came out but he didn't use it much. Maybe the failure occurred because it wasn't being played much? The fact that I moved the amp while it was still warm from playing it at home might also have something to do with it. I mean it was about 15 minutes after I shut down that I actually moved it but I remember that I could still feel heat emanating from the inside when I picked it up and loaded it into my car.

I got the name and # of a well renowned tube amp repair guy in Toronto - John Fletcher. I'll give him a ring in the morning so keep you all updated!
 
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