Amp head keeps blowing fuse... what's wrong?

[darkshadow54321]

Hiya! It's been a while since I've logged on, as I don't have internet at home anymore, but a friend has been having some problems with his amp and I know one of you will probably be able to help!

He's using an amp head with a 2x12 cabinet. He has linked them up with a guitar cable (not a speaker cable like I advised him to). He's connected the two together properly, making sure they're both 8 ohms. The amp was bought in the USA, so he is using a transformer to change the voltage from US to UK voltage.

The problem he has is that everytime he turns on the amp (to standby) there is a humming noise for around 3 seconds and then the fuse blows. There are no loud bangs and he has had a look at the electronics inside and there are no obvious circuit burns, etc. All the tubes are in the right places.

He's tried it with two different transformers... one of 300 watts and another of 1000 watts. Both should be fine and they say they're for American electrical appliances.

What could be causing the problem? Could it be the fact that he's using a guitar cable instead of a speaker cable? (doubt it). He's tried plugging in the amp in different places, in case of bad grounding, and he's tried two transformers so it's unlikely to be that... Any ideas?

Thanks for the help!:dance:

 

[McLaughysSN]

Re: Amp head keeps blowing fuse... what's wrong?

Had the same problem with my old sovtek head, can't remember the exact problem. I THINK it was either bad power tubes or a bad power transistor. Try the tubes first, take them out and check em out

 

[ES350]

Re: Amp head keeps blowing fuse... what's wrong?

Could be a few things; bad PT/OT, bad filter caps, screen resistor, rectifier circuit, etc...

BTW, do him a favor and get him a speaker cable...using a guitar cable for power out is just dumb.

 

[kmcguitars]

Re: Amp head keeps blowing fuse... what's wrong?

Because he used a guitar cable to his speakers.( THIS IS A MAJOR NO-NO) I'm Inclined to think he baked his OT. Get the proper speaker cable and a different set of Power tubes. If it still pops a fuse? Hopefully it's only the OT.

 

[Scott_F]

Re: Amp head keeps blowing fuse... what's wrong?

baked transformer? mmm... sounds delicious. can I get that with a side order of "I told you so, you stupid wanker!"?

I'd suspect the tubes first. In the mean time, get Einstein to go buy a $2 replacement lamp cord and solder some 1/4 connectors on teh end and make a proper speaker cable.

 

[greco]

Re: Amp head keeps blowing fuse... what's wrong?

baked transformer? mmm... sounds delicious. can I get that with a side order of "I told you so, you stupid wanker!"?

I'd suspect the tubes first. In the mean time, get Einstein to go buy a $2 replacement lamp cord and solder some 1/4 connectors on teh end and make a proper speaker cable.

Scott....Please try not to (emotionally) hold back so much when you respond :laugh2: :laugh2: :laugh2: :laugh2:

You know I'm joking here...right !!

 

[Scott_F]

Re: Amp head keeps blowing fuse... what's wrong?

No problems. I have a hard time feeling sorry for someone that knows they're doing something they shouldn't but they continue to do it anyway. Friends just don't let friends ruin tube amps.

 

[darkshadow54321]

Re: Amp head keeps blowing fuse... what's wrong?

Thanks for the help guys.

I'm not very savvy when it comes to amps... OT = transformer, right? and PT = power transistor?

I told my friend what you said but he's still being stubborn. He says that the only difference between a speaker and guitar cable is different insulation methods to counteract hum, interference and things like that, so it shouldn't make a difference. I'm sure there must be a difference in construction somewhere...

Can someone please give me an explanation of the real differences between cable types and how using a speaker cable could contribute to the blown transformer/transistor, etc? Something clear that I can read out to him...

 

[Gr8Scott]

Re: Amp head keeps blowing fuse... what's wrong?

There's a huge difference between the two cable types. One uses 12-16guage wire akin to the stuff you use to plug your lamp into 220 AC volts (though I suspect the voltage is much more like 450 volts coming out of the amp head into the speakers) and that powers the speakers. Even small tube amps have enough juice in them to kill you. The shielded guitar cable uses wire that only has to be capable of carrying a signal, which equals maybe half a volt at most if that (24 gauge). BIG DIFFERENCE!!! You're friend is beyond ignorant. There is a difference in shielding of course as the guitar cable is shielded and the speaker cable isn't, but the shielding is not the only difference. The amount of current these different cables can handle are extremely different. Tell your friend that he needs to take up skydiving and pack his own chute with the same care that he treats his amp. If your friend takes this advice, he'll be dead on his first solo jump.

Speaker cable = 2 wires that are both roughly 12-16 gauge (smaller gauge number equals thicker cable and larger gauge number equals thinner cable)

Guitar/instrument cable = one 16 gauge wire (ground/shield wire) twisted around a super thin 24 guage wire.

24 gauge wire isn't capable of carrying large currents for long. It will take a higher current than it's rated at for short durations, but as it heats up it starts to fail. Once the wire reaches it's melting point or the solder's melting point, you suddenly have blown output transformer. Your friend is probably running it much higher than it's rated for and is literally treading on the thinnest ice you can imagine every time he uses an instrument cable for a speaker cable. His amp may already be severely damaged as a result. Tell your cheap friend to spring the $12 or so for a cheap speaker cable or sell the amp to someone with a brain and take up skydiving instead. If he won't listen to the words of an amp tech, he can't be very smart.

PS - Maybe you shouldn't hang out with this guy. Stupid people can get you in trouble and will even possibly get you killed.