Bassman RI blowing fuses

[Del Pickup]

I switched on my Bassman yesterday and as soon as I flicked it off standby it blew the fuse. I replaced it and everything seemed fine.

But the same thing happened again today.

I'm using 1.5A normal blow fuses as I was told it was better to run smaller fuses so that, if there is a problem, the fuse will blow before anything happens to the amp. But I notice on the Fender website that the manual for the newer Bassman LTD seems to show the top panel as being marked for a fuse rated at 3A.

Should I change to using higher value fuses or is there a possibility that something internal is causing the fuse to blow and do I need to get it checked out by someone who knows what they're doing - not me!!!

The amp sounds fine when it's going and I don't hear any change in tone from what it was before.

[Mike Dickerson]

Could be a bad tube. If you have an old set try changing them out first. If it still does it proably time to take it to an amp tech.

[sunkidd]

Reminds me, I had a '63 Bassman 6G6-B head and matching 2x12 cab in the late '60's early '70's that when it blew a fuse I wrapped it in aluminum foil and kept on going.... I Would Never ever recommend that....but I was dumb and didn't worry about much back then....
changed power tubes with no rebias, plugged into any speaker cab and extension without checking ohms, used guitar cords instead of speaker wire, etc.
later traded it away, one of my regrets, but somebody is still playing that thing somewhere!

[Scott S]

1.5A is a pretty small fuse. I'd swap in a known good set of tubes, put in a 3A fuse, and see if it still blows. I believe the originals used 3A fuses.

[tazzboy]

Originally Posted by Mike Dickerson

Could be a bad tube. If you have an old set try changing them out first. If it still does it proably time to take it to an amp tech.

That's usually one of the main causes for fuses to blew is bad tubes.

[Del Pickup]

Would that be a power tube or preamp tube? None of the tubes are all that old - maybe 2 years at most but the amp doesn't get used every day or played at high volumes on gigs that often.

[tazzboy]

Power tubes more then likely. The general rule of changing Power tubes is 6 months to 1 year.

[fancychords]

There are many people here that are very knowlegeable...This problem start when you decided to go to a lower value fuse..I would think that's the problem..If the amp was supposed to run with a lower value fuse, Fender would ship them that way..The main thing is that you use a fast blow fuse as opposed to a slo-blow fuse..

[Wally]

The '59RI schematic shows one 3amp fuse and one 4amp fuse. I would suggest installing correct value fuses and firing it up. If the correct value blows, you have a problem somewhere. Running lower value fuses is over-protecting the amp. Running higher value fuses is putting the amp's major components...trannies... at risk. Running correct value fuses is preferred.

[64Strat]

so, I take it no EE's have answered yet.

In NZ you have a mains voltage of 230V, right?

If 3 amp is normal for your Bassman for USA 120V mains, then you would need 1.5A fuse for 230V mains. Simplisticly it is P=EI, if P is constant and you double E, I has to halve.

Now, what the other guys said is possible, you could have a tube drawing too much current or some such but it needs to be diagnosed.

all the best!

[Dacious]

Originally Posted by Del Pickup

I'm using 1.5A normal blow fuses as I was told it was better to run smaller fuses so that, if there is a problem,

Whoever told you this is well intentioned but clueless. When you fire up there is a momentary spike when the amp pulls a lot more than rated power, but it is instantanous and all the components are specc'd to cope.

As your tubes wear they tend to take more power to run. It may be that your amp is developing a problem, but more likely that you need 1.5 amp slow-blows - they allow this momentary surge whereas fast-blows often don't.

Your owners manual should spec the fuse type and rating. Stick to that and I bet you have no further problems. Otherwise, your tubes might be worn out, depending on age and usage.

[Del Pickup]

Just fired the amp up again and everything was OK this time.

I cleaned some of the dust off the top panel it's stamped on there that it should be a 1.6A fuse. So, I'm using 1.5A which is the nearest value. That accords with what 64Strats formula says cos we are on 230V here.

I don't have an owners manual cos I bought the amp secondhand many years ago and the manual has long disappeared. But I've checked the manual I have for my DRRI and that says that it should have a 2A fuse for 100-125V mains voltage and an F1A fuse for 23-240V systems. Presumably that's a1A fuse??

Now that I'm listening for it I think the amp may be a wee bit harsher toned than before so maybe I should change the tubes and see what happens.

[Wayne Alexander]

http://www.geofex.com/ampdbug/fuseblow.htm

Follow that link, everything you need to know is there.

[Dacious]

You could use 1.75 or 2amp fuses with no issue. Half an amp in current rating might make a diff with solidstate gear, it's nothing with tubes. If you're using the solid-state plug in module the amp will also likely pull more current.

Fender amongst others often changes these ratings with experience.

Your issue was probably the fast-blows. Slow blows just cope better with the surge on startup. Another reason to properly use the standby for warmup.

[Del Pickup]

I'm going to take the amp into the tech today and just get him to check it out. Hopefully it'll be OK but better to be safe than sorry.

Touch wood, it's been fine in the last day or so but he'll be able to put my mind at rest that nothing internally is starting to crap out and I'll maybe get a new set of tubes anyway.

I'll be asking him about the fuse rating anyway to clarify that point and make sure that I have the right ones as spares.

Thanks for all the input.

[Del Pickup]

Got my amp back today. You guys with the electronics knowledge were right. I'd been using fast blow fuses instead of slow blow ones. I didn't know the difference! They were the correct rating at 1.5A for the 230v mains but of the fast blow variety they weren't coping with the power spike at start up.

The tech checked everything out and tells me that it's all fine - including tubes. It was $47 well spent to have the peace of mind that my amp isn't about to die on me!

[Dacious]

That's great news - it probably not bad to have him check it out. Better to err on the side of caution.

[tazzboy]

Lesson to be learn it pays to have the right kind of fuses if you want to make your amp work right.

[Del Pickup]

Absolutely right, Tazzboy, but I thought I had the right fuses. When I got a pack of fuses as spares I took the original one out of the amp into an electrical wholesaler and asked them for more of the same and ended up with the wrong thing!!

Until now I had no idea that there were such things as 'slow blow' and 'fast blow' fuses.

You live and learn............

[fancychords]

Were you talking about the fuse on the control panel or the fuse inside the control panel???

[Stewart Ward]

Originally Posted by Del Pickup

I switched on my Bassman yesterday and as soon as I flicked it off standby it blew the fuse. I replaced it and everything seemed fine.

But the same thing happened again today.

I'm using 1.5A normal blow fuses as I was told it was better to run smaller fuses so that, if there is a problem, the fuse will blow before anything happens to the amp. But I notice on the Fender website that the manual for the newer Bassman LTD seems to show the top panel as being marked for a fuse rated at 3A.

Should I change to using higher value fuses or is there a possibility that something internal is causing the fuse to blow and do I need to get it checked out by someone who knows what they're doing - not me!!!

The amp sounds fine when it's going and I don't hear any change in tone from what it was before.

Hi,

It is a good rule to disregard 'what you hear' when it comes to fuses and general safety! There are just too many 'semi-skilled brain surgeons around', especially on the net!!

It is normal for fuses to blow for no real reason after a few years of use... they get tired and fail. But for a fuse to blow that frequently suggest there is either a fault or the fuse is too small.

Use ONLY the fuse rating and type specified by the amplifier manufacturer. Have your amplifier checked out by a qualified electronics technician who knows what he's doing. Your life may depend on it!!

Sorry to sound sturn... but I hear of 'bad advice' all the time and this kinda thing scares me to hell when I see it.

Live long and prosper!

[Del Pickup]

I hear what you're saying Stewart Ward but the original advice on fuses came from someone who seemed to know what he was talking about - not just a hobbyist electronics geek. This guy thinks nothing of pulling things apart just to see how they work - he's an engineer by trade and been in the business for 30+ years!! So, I assumed that he knew what he was talking about.

Similarly, when I went into the electrical wholesalers, it was an older guy who served me and I gave him the original fuse to match the new ones to. So, again, there was no reason for me (who knows virtually nothing about electrics) to doubt the info and items I was given.

Hopefully now I've had it all checked by a reputable tech everything will be fine - and I'll go get some slow blow fuses today for my spares box!

[tazzboy]

I have the service manual if you would like to have copy send me private email.

[Dacious]

Originally Posted by Del Pickup

I hear what you're saying Stewart Ward but the original advice on fuses came from someone who seemed to know what he was talking about - not just a hobbyist electronics geek. This guy thinks nothing of pulling things apart just to see how they work - he's an engineer by trade and been in the business for 30+ years!! So, I assumed that he knew what he was talking about.

Similarly, when I went into the electrical wholesalers, it was an older guy who served me and I gave him the original fuse to match the new ones to. So, again, there was no reason for me (who knows virtually nothing about electrics) to doubt the info and items I was given.

Hopefully now I've had it all checked by a reputable tech everything will be fine - and I'll go get some slow blow fuses today for my spares box!

Someone on the main board has a saying: 'when I say anything, ask yourself - what does this guy know?'

Unfortunately an 'electronics geek' or a guy at the counter of a trade counter probably have zip experience of vacuum tube power supplies, only about 0.001& of the population today does - guitarist and tube hi-fi anorak wearers and some ham radio dudes. Solidstate gear doesn't use slo-blos - not much does apart from VT gear with transformers. A lot or them wouldn't even know slow-blows existed as it' pretty arcane technology.

If you don't know, one 1.5amp fuse is pretty much like another. Just be thankful you only suffered annoyance and not loss.......

Not saying you shouldn't listen to people, but don't think unless they do exactly what you do, they know more about it. At least this time it's a painless lesson.

Remember, last resort - RTFM. I'd download a copy from Fender's site.