kudzurunner
2341 posts
Feb 23, 2011
10:18 AM
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My HG2 has developed a big problem. It blows fuses. When I plug it in--having replaced the fuse with an equivalent--it plays beautifully. That's when I switch it on once. The second time I switch it on, the fuse blows.
This has happened three times. It happened twice two weeks ago, so I called Brian Purdy and we had a conversation. He suspected that the 5Y3 GT tube was leaking. I swapped it out with another tube from a working amp. Exact same thing. Switch it on once, it works. Switch it off and back on, the fuse blows.
I put the amp away for a couple of weeks. Today I pulled it out and tried again. Same thing. Switch it on and the amp plays beautifully, with full power. No noises. Nothing out of the ordinary. I played hard, with the amp dialed up, for about 5 minutes. Switched it off. The fuse blew.
Is it possible that the amp is fine but I'm simply using the wrong kind of fuses? They're Cooper Bussman 1 amp fast-acting. BP/GMA-1A. They are NOT slow-blow. Is that the problem? Or is something else going on?
Last Edited by on Feb 23, 2011 10:23 AM
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Kingley
1458 posts
Feb 23, 2011
10:25 AM
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I had a similar problem with an amp once and once I changed to slow-blow fuses it stopped. I'd try some and then if it still happens get Brian Purdy to have a look at it.
Last Edited by on Feb 23, 2011 10:25 AM
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belfast_harper
222 posts
Feb 23, 2011
10:45 AM
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I have the same problem with my HG 2. I have just read Kingley's post and realised that the problem started after I bought my last box of fuses.
I have just checked the fuse box and I am using SkyTronic Quick blow glass fuses, I will buy some slow blow fuses tomorrow see if that fixes the problem.
Last Edited by on Feb 23, 2011 12:12 PM
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tookatooka
2169 posts
Feb 23, 2011
12:16 PM
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The techs on here will put you right but in my opinion you shouldn't really need to use a special slow-blow fuse.
When you switch it on the second time (when the fuse blows) has it cooled right down or still warm from the previous use.
I'm trying to ascertain whether it fires up perfectly well from cold but the problem occurs only when the components are warm.
I'm afraid valves are not my forte, digital electronics is my speciality but the fault finding steps are much the same.
However it sounds like something is drawing excessive power when it's warm. Power Supply components?? Main power transformer??
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Last Edited by on Feb 23, 2011 12:25 PM
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Barry C.
183 posts
Feb 23, 2011
12:22 PM
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i will check which fuse i have in my HG2 - it's whatever it came with from brian - never had this issue (or any problems) in 3 years of ownership.
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~Banned in Boston!
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MJ
253 posts
Feb 23, 2011
3:53 PM
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Some info I keep handy. I found it somewhere on line.
"The amp blows fuses
This is a common problem. Usually it's a shorted rectifier or power tube. Take all the tubes out of the amp, replace the fuse and turn on both switches. If the fuse blows then you probably have a shorted filter cap or in extreme circumstances, a blown power transformer. This is not going to be fixable at the gig. If the light stays on when you take all the tubes out, shut the amp down and install the rectifier tube (if it has one), then the power tubes, then the preamp tubes one by one turning the amp on every time you install a new tube. When the fuse blows, you've found the bad tube. Sometimes the fuse will blow if you simply flip both switches on at the same time (which you should never do! Shame on you if you do this! Let the tubes warm up about 30 seconds before you turn on the second switch!). If you find that the fuse blows after replacing one of the power tubes, and still blows when you install a known good power tube, you could possibly have a blown output transformer."
Last Edited by on Feb 23, 2011 3:54 PM
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kudzurunner
2342 posts
Feb 23, 2011
7:47 PM
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MJ: Do you really think that my amp could have a blown power transformer and yet--as I note--put out full power with perfect tone the first time I turn it on, every time? That's doesn't make sense. Maybe I'm just stupid, but that doesn't make sense.
Too, if one of the tubes was blown, would the amp really have full power with great tone, the first time I switch it on? That doesn't make sense.
Brian initially said that the problem could be caused by a leaking rectifier tube, not a blown or shorted rectifier tube. He suggested that I replace the 5Y3 with a brand new one. (The one that was in the amp still looks brand new and has no off-sounds when I shake it.) I replaced it with the 5Y3 in my '54 Deluxe. Exactly the same thing happened.
The HG2 has only one switch, not two. It has an on/off switch. I assume the info you found on line is about amps that have on/off switches plus standby/on switches. The HG2 only has one switch.
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kudzurunner
2343 posts
Feb 23, 2011
7:55 PM
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Brian's response: "It should be slo-blo. That will probably fix it."
Home Depot, please stock slo-blo fuses!
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Joe_L
1097 posts
Feb 23, 2011
8:12 PM
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You could probably buy them online.
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The Blues Photo Gallery
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NiteCrawler .
111 posts
Feb 24, 2011
4:36 AM
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I would try Radio Shack or a NAPA auto store for the fuses,maybe try calling before making the trip.If not go with Joe L,s advise.I recently had a problem with a fuse in an amp,went to Radio Shack with no avail,then NAPA and they did have what I needed.I hope it works out,theres nothing worse than having one of your go to amps take a sh.....Good Luck
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tookatooka
2172 posts
Feb 24, 2011
5:42 AM
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OK I stand corrected. It may be the fuse needs to be a special slow blow type. Thinking about it, there would be some large value capacitors in the circuit which would cause a sudden surge when first switched on thereby knocking out an ordinary fuse. I'm still not sure about the reason why there are problems when switching on after it has been used though.
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jawbone
407 posts
Feb 24, 2011
5:48 AM
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Do you suppose the fuse just barely survives the first start up but has been weakened and the second surge burns thru?? Just a thought.
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If it ain't got harp - it ain't really blues!!!!
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AirMojo
117 posts
Feb 24, 2011
6:08 AM
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I was going to check the fuse in my HG2 to see what it looked like and buy some spares... took me a few minutes to figure out where it was, as I was looking for a black screw down button, but there wasn't one.
Mine is located in a little slide out "drawer" as part of the plug assembly. I attempted to try and pull the fuse out, but there's little room to grab onto it or pry it out with a little screwdriver.
I was afraid to try and pull the fuse-drawer out without breaking it...
Does anyone know if this fuse-drawer comes out ?
P.S. The HG2 manual says that it takes a 1-AMP slo-blo fuse.
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5F6H
541 posts
Feb 24, 2011
6:21 AM
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A Slo Blo fuse will withstand the current inrush as the amp is switched on when hot, a fast blo won't. If a slo blo is specified you must use a slo blo. In the UK Fenders ship with fast blo fuses, careless use of the standby on RI Bassmans often results in blown fuses (but is not normally indicative of a deeper issue, though failing rectifiers do kill fuses, very rarely power tubes do the same).
When the amp is turned on cold, the tubes are drawing no current (no stress on the fuse). When the amp has been played & the tubes are warm, they expect to see the same amount of current as they did when it was last on, instantly, after you throw the power switch...the fast blo fuse can't cope with this. This is why the fuse blows on warm start up.
Adam, you don't have a blown PT, power tube is most likely fine, old rectifier was perhaps dodgy, you just need the right fuses.
Last Edited by on Feb 24, 2011 7:43 AM
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MJ
254 posts
Feb 24, 2011
8:37 AM
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I also doubt it would be a bad PT Adam. A blown PT was the last option given, which is usually a worst case scenario. As far as the Rectifier tube, I believe the article suggests checking all of the tubes in a particular order. Bad tubes can sometimes work until they get to a certain temp. The first check is to see if a fuse blows when there are no tubes inn the amp. I agree with Brians suggestion of putting in a so blo. I worked on a Bassman 20 from someone who said that it blew fuses all of the time. There were regular fuses installed. I put in slo blo and the problem was solved. Did you change one of the original fuses? I always use slo blo in the amplifiers I build.
I use this page to solve many problems while working on my various problem. It is a second source to my original post.
username
An exerpt from the part on fuses.
~Fuses blow quickly for massive overloads, like AC shorts to a grounded chassis. They blow with a some time lag - maybe only a second or two, up to minutes, as the overcurrent gets closer to the fuse's actual rating. Anything that uses enough power to cause the AC line current to exceed the rating of the fuse will eventually cause it to blow.
It is important to remember that a fuse NEVER blows without something else being wrong. It could be that:
* fuse is the wrong rating - replace it with the correct rating
* power tube shorted
* rectifier tube shorted
* power supply filter cap failing
* Carbon trails on the output tube sockets between the plate lug and the other electrodes, especially the heater electrodes.
* power tubes have lost bias or biased incorrectly
* power section of the amp is oscillating at too high a frequency to hear
* there is an ac wiring short or high leakage
* power transformer is faulty
* choke (if present) is shorted/leaking to chassis
* output transformer is faulty
* AC power wiring or B+ power wiring is faulty/shorted~
Last Edited by on Feb 24, 2011 8:48 AM
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chromaticblues
633 posts
Feb 24, 2011
9:15 AM
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As MJ stated always use slo-blo fuses. Adam said it played fine. That should be enough information for someone that works on amplifiers to know its not a bad tube, transformer, global warming or declining realestate prices. His initial post stated it was not a slow blow fuse. Start with the easy and obvious. Then work your way to the difficult and expensive!
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barbequebob
1570 posts
Feb 24, 2011
9:49 AM
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Most tube amps tend to use slow-blo fuses and boutique amps (yes, harp only amps come under this category) are often based on either the '59 Bassman or a Vox AC30 most often and none of those ever used quick blow fuses at all.
I always, as a rule of thumb, make sure that I always have spare fuses handy.
I wouldn't be surprised if damage may be caused by using quick blow fuses.
One bass player I remember seeing do when he blew a fuse at a gig one night was to wrap silver foil around the fuse, and got the amp working again, but that is something that is kinda dangerous and playing with fire as far as I'm concerned.
For some years, there were tons of 5AR4/GZ34 rectifiers that were actually just relabeled 5Y3's and they are VERY different and that can do some damage to an amp not properly biased for those.
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Sincerely,
Barbeque Bob Maglinte
Boston, MA
http://www.barbequebob.com
CD available at http://www.cdbaby.com/cd/bbmaglinte
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belfast_harper
223 posts
Feb 24, 2011
12:19 PM
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I bought some time-lag fuses today and that has solved the fuse problem with my HG2.
I am glad that this topic came up because I thought that there might have been a a problem with my amp.
Last Edited by on Feb 24, 2011 12:22 PM
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5F6H
542 posts
Feb 24, 2011
12:29 PM
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BBQ Bob wrote:"One bass player I remember seeing do when he blew a fuse at a gig one night was to wrap silver foil around the fuse, and got the amp working again, but that is something that is kinda dangerous and playing with fire as far as I'm concerned." I know a guy who did this once, it worked for whole seconds, then in a puff of smoke, his AC plug was shot out of the wall & the venues lights went down! Both his transformers were burned up, repair was only marginally less than replacing the amp.
Never use a fuse with a higher "A" rating, the fuse is designed to blow to save everything else. Carry spares. An HG2/Champ style amp only has a rectifier & power tube that can blow a fuse (12A#7 can't), so if the fuse blows pull these, replace rectifier first, still blows fit a new one, then repeat with the power tube.
Bob ALL RI Bassmans (& some other Fenders) sold in the UK were fitted with fast blow fuses. Most of the time they're OK.
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