Fender Twin Reverb 65 RI keeps blowing 8 amp filament fuse

[tim]

Folks,

I got this amp in the other day and customer complained about the amp had blown the mains external fuse. When he replaced it, still nothing. He replaced the power tubes. He didn't realize that there were three fuses inside as well.

I found the 8a filament fuse and the internal mains fuse blown. I determined that one of the 1N4001 diodes in the bridge had shorted so replaced that and replaced both blown fuses. Turned amp on and all was well.....until the customer got it back. He went to turn it on and no lights and tubes didn't light up.

I got it back, and found that the 8a filament fuse had once again blown.

I measured surge current as best I could (digital meters are pretty slow) and it kicked to almost 11 amps when turned on cold and then came to rest around 5.2 amps. I did the math and 5amps is about right for six 12AX7's and four 6L6's, I have a 10 amp fuse in there right now and when I turn it on from cold, I can see the filament in the fuse glow red for a few tenths of a second.

Is it possible that one or more of these new power tube's cold filaments could hog this much current?

[g1]

I'm not sure if maybe there is a new version of the 65 Twin RI, but the usual schematics don't show any such fuses.
From your description of the fuse turning red at surge, it does not sound like a slow-blow (time delay) fuse. Isn't it supposed to be?

Edit: I see in the parts list some fuses that are not shown on the schematic. 8AT fuse, so time-delay aka slow-blow fuse is required here.

[Jazz P Bass]

I have never seen a filament fuse on a Fender Twin RI.

Although the service manual does specify one for 220 volt operation.

Maybe measure TH1 thermistor cold & see what you get.
It may be duff.

Any one of the tubes could be trying to pull that initial 11 amps.
It does sound excessive though.
It does call for a slo blo fuse.

[tim]

I have never seen a filament fuse on a Fender Twin RI.

Although the service manual does specify one for 220 volt operation.

Maybe measure TH1 thermistor cold & see what you get.
It may be duff.

Any one of the tubes could be trying to pull that initial 11 amps.
It does sound excessive though.
It does call for a slo blo fuse.

There is definitely a filament fuse in this amp, designated XF4 rated at 8AT @ 250v. I did check the fuse I installed and a quick acting fuse must've snuck into my slo blow fuse bin. I don't have the original so who knows what was in there. I put a slow acting fuse in there and it seems to be holding it's own. I told the owner I want to keep it a few days if possible and do a bunch of on/off cycles and he's cool with that.

[Jazz P Bass]

[QUOTE=tim;385049]. I did check the fuse I installed and a quick acting fuse must've snuck into my slo blow fuse bin. QUOTE]

The Gremlin strikes again.

[tim]

[QUOTE=Jazz P Bass;385051]

. I did check the fuse I installed and a quick acting fuse must've snuck into my slo blow fuse bin. QUOTE]

The Gremlin strikes again.

indeed!

[tim]

well....hmmm. Amp is back. Still blowing the filament fuse. I had replaced the 8 amp fuse with a 10 amp slow blow before I let him take it when it was in a few weeks back because it was really stressing the 8 amp slo blow fuse that I had installed and asked him to take it for a week or so to see if it survived. It didn't.

You can see the 10a slow blow fuse filament stress when turned on cold. It lasted about two weeks and then opened. I have replaced all the power tubes and replaced the preamp tubes one at a time...still does it. This isn't rocket science...it's a darned AC filament circuit. Is it possible the in rush thermistor might be dicey? It seems to be working ok.

EDIT: I checked the NTC thermistor and it is 10 ohms cold and drops to less than an ohm when I heat it with a soldering iron so it seems to be working as advertised.

[The Dude]

(My original post here deleted. I missed that you already tried different tubes)

I wouldn't over fuse the filament circuit. You could end up burning up the filament winding of the PT. I might, in this oddball situation, remove all of the tubes and the lamp, unhook the filament connections from transformer to board, and check resistance across the pads to see if there's an unexpected load somewhere. You could even remove the hum balance pot for extra measure. with the pot in, you shouldn't measure less than the pot value. With the pot out, you should see infinite resistance. I'm wondering if there isn't a blob of solder somewhere or some lead touching a trace it shouldn't be that is enough to push you "over the edge". It sure sounds like your dealing with a real PITA.

[tim]

(My original post here deleted. I missed that you already tried different tubes)

I wouldn't over fuse the filament circuit. You could end up burning up the filament winding of the PT. I might, in this oddball situation, remove all of the tubes and the lamp, unhook the filament connections from transformer to board, and check resistance across the pads to see if there's an unexpected load somewhere. You could even remove the hum balance pot for extra measure. with the pot in, you shouldn't measure less than the pot value. With the pot out, you should see infinite resistance. I'm wondering if there isn't a blob of solder somewhere or some lead touching a trace it shouldn't be that is enough to push you "over the edge". It sure sounds like your dealing with a real PITA.

Thanks for the post...unfortunately, I've done all that. The balance pot was replaced with 2 two watt 50Ω resistors, either side of the filament to ground through those resistors so it's essentially the same as if the pot was in there and set to center. With all tubes, pilot lamps and fuse removed and one filament wire lifted that went to one of the two resistors mentioned above, resistance is infinity. I'm stumped...

Thinking aloud here, what if I increased the value of the two balancing resistors to 100 Ω each? I think the old SF twin reverbs used that value for hum balancing. It would be drawing less current through them.

[The Dude]

Do you have a scope? If so, monitor the filament line at startup and see that there are no sudden spikes. Possibly a transformer arc? Sorry, grasping at straws a bit here, but it seems like you've covered your bases pretty well.

[tim]

Do you have a scope? If so, monitor the filament line at startup and see that there are no sudden spikes. Possibly a transformer arc? Sorry, grasping at straws a bit here, but it seems like you've covered your bases pretty well.

Yup...I've done that too...no voltage spikes that I can see but there is a definite current spike. I have a current probe and it goes well over the 8 amps, almost to 12 amps with cold filaments then settles to about 5.5ish amps after a few seconds. If I turn it off for a few seconds, 10-15, and turn it back on, very little, if any current spike. It seems to be the inrush current into cold filaments is just too much for the 8 amp fuse.

I did the math on what effect increasing the resistance of the hum balancing resistors...almost negligible. There's only 126ma through each 50 ohm resistor. Total load would be just a tad over 1/4 amp

BTW, I see you're from Sioux Falls...I have a brother that lives in Mitchell, SD. He was an RN at the Sioux Falls medical center for a few years. I think he's an RN now at some large nursing home west of Mitchell...my mom and dad live in Lake Norden...we're all originally from NH but my parents moved out there in the 90s and my brother moved out there after getting his nursing degree. It's a nice place...I've been there several times visiting...still have yet to see the badlands though.

[The Dude]

You've probably seen this, and it's not much help, but it does look like they went to a T10A in later revisions, so you're probably ok there.

http://music-electronics-forum.com/t30062/

Edit: If you're ever in the neighborhood, gimme a heads up!

[tim]

You've probably seen this, and it's not much help, but it does look like they went to a T10A in later revisions, so you're probably ok there.

http://music-electronics-forum.com/t30062/

I did not see that. Thanks...you may have solved my problem...the PCB has T8A silkscreened on it but I wonder if Fender was having a lot of nuisance fuse problems and changed it. The fuse I have is a slowblow 10a but not sure how long the delay is on it....maybe it's not a looooong delay (as said on the other thread).

I'll deal with this again tomorrow night...I'm beat. It's bed time.

[The Dude]

......I wonder if Fender was having a lot of nuisance fuse problems and changed it. The fuse I have is a slowblow 10a but not sure how long the delay is on it....maybe it's not a looooong delay (as said on the other thread)......

That's what I was thinking. Be sure to update us with your results.

[Enzo]

Try being scientific. Pull the fuse and clip in your current meter. What is the draw? Pull all the tubes, now how much current is there? Your two 50 ohm resistors are 100 ohms across the voltage, so 63ma there. Or is there more than that? Oopss, wait, the pilot light draws a little, or pull the bulb.