# Common Causes For Blowing A Ht Fuse?



## bart221

*Really strange problems with my amp...*

Hello, I have another thread on here describing a problem with my 1982 JCM 800 2204 50W marshall head and I have come to find out that the HT fuse is blown. Can I get another one of these at my local hardware store? Does it go by another name or value? 

Now I am just wondering what caused it to burn out, as I said in my other thread the tube sockets are real loose and maybe some of them werent really connecting, but do you think thats what caused it? What usually causes them to blow?

thanks


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## luekemeyer

It looks like the HT fuse is a T500ma (1/2 amp). (T500mA/250V - Slow Blow 5 x 20mm) This is a standard fuse and most hardware or Radio Shacks should have it. Typically a bad power tube causes the the fust to blow. There are many other components that could also fail and have the same result. If you have loose sockets it could cause issues with intermittent connections which could also result in a blown fuse.


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## steelhorse

luekemeyer said:


> It looks like the HT fuse is a T500ma (1/2 amp). (T500mA/250V - Slow Blow 5 x 20mm) This is a standard fuse and most hardware or Radio Shacks should have it. Typically a bad power tube causes the the fust to blow. There are many other components that could also fail and have the same result. If you have loose sockets it could cause issues with intermittent connections which could also result in a blown fuse.



+1


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## Mase947

I'm pretty sure you cannot buy these at a regular hardware store. I have a 1959SLPX and if you examine the fuses that come with these amps, you will see that the coil inside the fuse is extra thick and wound with extra fillament. Go to a hardware store and all you will find is fuses with the regular hair line filament. These will blow right away. I recommend http://www.guitar-parts.com


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## bart221

ok, thanks for the help guys


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## Shawn Fate

Mase947 said:


> I'm pretty sure you cannot buy these at a regular hardware store. I have a 1959SLPX and if you examine the fuses that come with these amps, you will see that the coil inside the fuse is extra thick and wound with extra fillament. Go to a hardware store and all you will find is fuses with the regular hair line filament. These will blow right away. I recommend Fender Stratocaster Parts,Fender Telecaster Parts,Fender Bass Parts,Marshall Amp Parts, Guitar and Amplifier parts,Gretsch Guitar Parts,Gibson Guitar Parts, Hofner Bass Parts. Parts is Parts has more Guitar parts and amp parts.




Lowes and Home Depot as well as Radio Shack and most hardware stores carry them.


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## bart221

As for an update on this, I changed the HT fuse and the power tubes andwhen I turned it on this time I got sound but it was really quiet and noisy so I tried changeing the two of the 12ax7 tubes since I recieved some spares with the amp, still the same thing. 

I had the amp turned around backwards towards me so I could see what was going on in the back and I played for a minute with the noisy sound and I noticed that the two power tubes started flashing bright blue flashes! Has anyone even heard of this happening before? What was happening in the tubes? Does anyone know what is wrong with it? I'm about ready to send this thing back.


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## steelhorse

Instead of trying to deal with this alone, contact the seller or dealer to ask for assistance. 

Did you get this locally? If they have a service tech I'd definitely take it in.

Let us know what happens and good luck bart!


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## luekemeyer

As a note, fuses that have the filament wrapped around a rod are not that uncommon. I have replaced hundreds of fuses while working with electronics over the last 25 years. Most fuses are easy to find if you know where to look (electronic warehouses, radio shack, etc). I live in California so things are much easier to find here.


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## labiesta

bart221 said:


> As for an update on this, I changed the HT fuse and the power tubes andwhen I turned it on this time I got sound but it was really quiet and noisy so I tried changeing the two of the 12ax7 tubes since I recieved some spares with the amp, still the same thing.
> 
> I had the amp turned around backwards towards me so I could see what was going on in the back and I played for a minute with the noisy sound and I noticed that the two power tubes started flashing bright blue flashes! Has anyone even heard of this happening before? What was happening in the tubes? Does anyone know what is wrong with it? I'm about ready to send this thing back.



Same problem over here. (but the power tubes are heating up and turning red!)


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## RiverRatt

Mase947 said:


> I'm pretty sure you cannot buy these at a regular hardware store. I have a 1959SLPX and if you examine the fuses that come with these amps, you will see that the coil inside the fuse is extra thick and wound with extra fillament. Go to a hardware store and all you will find is fuses with the regular hair line filament. These will blow right away. I recommend Fender Stratocaster Parts,Fender Telecaster Parts,Fender Bass Parts,Marshall Amp Parts, Guitar and Amplifier parts,Gretsch Guitar Parts,Gibson Guitar Parts, Hofner Bass Parts. Parts is Parts has more Guitar parts and amp parts.



The filament size and shape doesn't matter. Like luekemeyer said in the first reply, if it's a T500mA 250v. slow blow fuse, it will be fine. The "T" in the part number means slow blow.


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## Wilder Amplification

labiesta said:


> Same problem over here. (but the power tubes are heating up and turning red!)



This means your bias is too hot. Could mean that the bias isn't set up for that batch of valves or it could point to a problem with the valves themselves, problem with the bias supply, etc etc. Best to have a tech look at it.


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## Davies047

Hi Guys,

Im new to the forum, Iv joined as I noticed you were discussing problems with the 6100.

I have this amp, and a few days ago I was using it in the studio when the power suddenly cutt out. It appears to be the Output fuse which is the T500mA. I know this is not a serious problem (or is it?), however when I was reading through this thread (aswell as others) I couldnt help noticing that everyone else's amp seems to be powering up, however, mine doesn't. 

The amp was a gift from my uncle who has had it since 1993, Iv had it for around a year, and I can remember him saying he had the valves replaced before he gave it to me.

Will it just be a case of replacing the fuse, or is there more to it? 

Would really appreciate some feedback asap. 
Thank you very much

Dan


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## Wilder Amplification

Davies047 said:


> Hi Guys,
> 
> Im new to the forum, Iv joined as I noticed you were discussing problems with the 6100.
> 
> I have this amp, and a few days ago I was using it in the studio when the power suddenly cutt out. It appears to be the Output fuse which is the T500mA. I know this is not a serious problem (or is it?), however when I was reading through this thread (aswell as others) I couldnt help noticing that everyone else's amp seems to be powering up, however, mine doesn't.
> 
> The amp was a gift from my uncle who has had it since 1993, Iv had it for around a year, and I can remember him saying he had the valves replaced before he gave it to me.
> 
> Will it just be a case of replacing the fuse, or is there more to it?
> 
> Would really appreciate some feedback asap.
> Thank you very much
> 
> Dan



There's typically more to it. Usually when an HT Fuse blows that means you either shorted a power valve, or you could've shorted a filter cap, OT, etc etc. 

The amp should still power on though as the HT Fuse only interrupts the high voltage supply if it blows. If your amp isn't powering on, this is typically indicative of your mains fuse blowing as well, which could mean more serious problems.

I suggest taking it to a tech and having it gone through.


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## Davies047

Thanks very much for the quick reply. Not the really the news I wanted to hear, but these things happen. 

I took the mains fuse out and it seemed fine incomparison to the Output fuse. I didnt look like it had blown. 
I shall take it get it looked at. 

If it is any of the problems you stated, is it expensive to get repaired?


Thanks


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## HOT TUBES 70

These amps are a real maze inside , its worth taking it to a qualified tech for a repair .
If you have not tubed this amp in a while , there's a good chance its tube related IMO .


welcome to the forum !!!


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## MartyStrat54

...and a 6100 has a whole lot of tubes, right Hot Tubes?

Davies047-If you got that nice of an amp as a gift, then you shouldn't feel bad having to put new tubes in it. It really should be taken to a Marshall authorized shop, because that amp is not a normal design.

Also, if you are new to tube amps, I suggest you do a little studying about how to hook an amp up to a cab and what type of cable do you use and knowing that a 16 ohm cab is plugged into the 16 ohm output of the amp. 

There are some basic things you must know, or else you will find yourself taking the amp to the service center and shelling out dough. These are not like solid state amps. They require some knowledge of simple electronic theory to ensure they run properly.


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## Davies047

It was the main Fuse that had gone. 
Turns out the one of the power valves had gone, I have ordered a fresh set of JJ EL34 tubes.

I took the dud valve out and powered the amp with only the 2 outside power valves fitted, that was fine, powered up as normal. However, the channel switching lights were not lighting up what so ever when switched on and there was no sound coming from the cab. There was the indication that some form of signal was going to the cab as I could here the hiss/hum coming from the cab.

Could this mean there is a problem with the electronics/channel switching board with in the amp?

Thanks


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## Wilder Amplification

Davies047 said:


> It was the main Fuse that had gone.
> Turns out the one of the power valves had gone, I have ordered a fresh set of JJ EL34 tubes.
> 
> I took the dud valve out and powered the amp with only the 2 outside power valves fitted, that was fine, powered up as normal. However, the channel switching lights were not lighting up what so ever when switched on and there was no sound coming from the cab. There was the indication that some form of signal was going to the cab as I could here the hiss/hum coming from the cab.
> 
> Could this mean there is a problem with the electronics/channel switching board with in the amp?
> 
> Thanks



Very well could be...or the low voltage supply winding on the power transformer is dead, in which case you'd need a new power transformer.

Either way, have a tech go through it and you shall find the problem. Everything is speculation at this point until you have a competent tech go through and find the real cause of the issue.


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## Jethro

*Re: Crazy HT fuse or its just me?*

I have a 4212 JCM800. While biasing the power tubes I have noted that the output power was varying alot at one time like some playing with the volume knob. When I went to to measure the the voltage and touched the hightension fuse connection the amp became steady and powerful. I have also noticed a hum but when I touched this fuse it sudenly went. Could this be a loose connection of the hightension fuse or does not make any sense?


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## Micky

Yes, it could very well be a cold solder joint.

Best to have a tech look at it and reflow the joints.


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## medicjg

Dirty jacks in the effects loop of my dsl when i had it caused the ht fuse to blow..


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## Guitar-Rocker

Davies047 said:


> It was the main Fuse that had gone.
> Turns out the one of the power valves had gone, I have ordered a fresh set of JJ EL34 tubes.
> 
> I took the dud valve out and powered the amp with only the 2 outside power valves fitted, that was fine, powered up as normal. However, the channel switching lights were not lighting up what so ever when switched on and there was no sound coming from the cab. There was the indication that some form of signal was going to the cab as I could here the hiss/hum coming from the cab.
> 
> Could this mean there is a problem with the electronics/channel switching board with in the amp?
> 
> Thanks


 You have a problem in your B+ rail somewhere, most likely a bad capacitor. Have a tech check it out.


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## lowinput

what is it when there is a HUM when standby is switched on, play a few notes and NO sound. HT fuse blown. valves still lighting.
any suggestions?
4 power Valves were put in by marshall (1 year ago) due to faulty components when purchased. (valves not marked, lots of 'bad sound' noises comming from speakers, all valves replaced) One power valve blew within 6 months (turned white inside and a crack in the glass). Now the H.T. fuse blows. 
the amp condition is as new, untouched.
any help is apreciated!


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## greenghost1964

I have a Marshall 6100 LM..just blew a fuse about an hour after hooking up to a Eleven Rack processor.....I don't think that had anything to do with it, but the light on the read panel was on ( the light next to the speaker jack). So I take it now I need to replace naturally the HT fuse, but some output tubes as well? This amp has the 5881's.........anything else anyone is aware I need to do? thanks


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## mickeydg5

It is most likely an issue with a power tube (on the side of the fuse) or the power amplifier.


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## Rick Lee

lowinput said:


> what is it when there is a HUM when standby is switched on, play a few notes and NO sound. HT fuse blown. valves still lighting.
> any suggestions?
> 4 power Valves were put in by marshall (1 year ago) due to faulty components when purchased. (valves not marked, lots of 'bad sound' noises comming from speakers, all valves replaced) One power valve blew within 6 months (turned white inside and a crack in the glass). Now the H.T. fuse blows.
> the amp condition is as new, untouched.
> any help is apreciated!



Precisely this same thing happened today with my '68 12000 clone. Has always run like a top in the past, power tubes are lightly used ValveArts. But I just replaced the HT fuse and it's fine now. WTF?


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## anitoli

greenghost1964 said:


> I have a Marshall 6100 LM..just blew a fuse about an hour after hooking up to a Eleven Rack processor.....I don't think that had anything to do with it, but the light on the read panel was on ( the light next to the speaker jack). So I take it now I need to replace naturally the HT fuse, but some output tubes as well? This amp has the 5881's.........anything else anyone is aware I need to do? thanks


The output fail fuse should be blown if the fail indicator LED is on. This signifies a blown tube. If the HT fuse blew and the output fail did not make sure you have a 500ma fuse in those two positions. And yes replace with a new quad if you have no way to test them.


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## ampmadscientist

bart221 said:


> *Really strange problems with my amp...*
> 
> Hello, I have another thread on here describing a problem with my 1982 JCM 800 2204 50W marshall head and I have come to find out that the HT fuse is blown. Can I get another one of these at my local hardware store? Does it go by another name or value?
> 
> Now I am just wondering what caused it to burn out, as I said in my other thread the tube sockets are real loose and maybe some of them werent really connecting, but do you think thats what caused it? What usually causes them to blow?
> 
> thanks



It's not a strange problem.
Most of the time, it's a bad speaker load, bad speaker cable, bad speaker connection.
And this will eventually cause the tubes to go bad.

When the fuse blows it's usually speaker related. Remember this. 
If the fuse blows - it blows for a reason.

Fuses don't blow by themselves - i*f the fuse blows, there IS something causing it to blow.*

1. Don't ever assume that there could be nothing wrong. If the fuse blows check everything. Especially the speaker cables and jacks.

2. A bad connection inside a speaker plug can make the fuse blow.
3. A corroded jack in the speaker cabinet can make the fuse blow.

4. DID you measure the speaker with an ohm meter?

It turns out statistically that when he fuse blows it's speaker related most of the time.


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