I have a 1979 Marshall Super Lead 100-watt head that started blowing the 2amp fuse after 5 minutes on. I replaced all tubes (it was time) and it still burns a fuse, but after about 15 minutes. The impedance is properly matched with the speaker. Ihave some experience having built a couple of amp kits. Nothing looks burned or damaged inside. Any suggestions before I bring it in? Thanks in advance.
1979 Marshall Super Lead 100-watt head-blowing fuse
- Thread starter plarche
- Start date
This site may earn a commission from merchant affiliate links like Ebay, Amazon, and others.
2A fuse so I'm assuming you're on 230/240V mains?
If the mains fuse is going then that indicates a problem with the power supply or the heaters. Generally the HT fuse will pop if it's something in the B+ rail. Likely causes are bad tubes shorting internally but could be something else. Shorted diodes, bad filter caps maybe. The fact it is happening after a short duration of time and not immediately would make me think tubes. Once it warms up to a certain point it may be shorting and drawing excess current or shorting out the heaters.
Do you have a spare set of output tubes? It's helpful to try the easy stuff first if you don't have access to diagnostic equipment.
If the mains fuse is going then that indicates a problem with the power supply or the heaters. Generally the HT fuse will pop if it's something in the B+ rail. Likely causes are bad tubes shorting internally but could be something else. Shorted diodes, bad filter caps maybe. The fact it is happening after a short duration of time and not immediately would make me think tubes. Once it warms up to a certain point it may be shorting and drawing excess current or shorting out the heaters.
Do you have a spare set of output tubes? It's helpful to try the easy stuff first if you don't have access to diagnostic equipment.
21fret
Active Member
- Joined
- Mar 29, 2018
- Messages
- 195
- Reaction score
- 209
hey nice amp @plarche. I hope that issue gets resolved so you can rock that amp again so all your neighbors can enjoy it with you.
after powering down, patiently check each of the big cans with a meter to see if they are dissipating their charge normally.
a cracked resister will heat up and let go allowing, or letting current pass to places it shouldn’t, so look for any discoloration in the resister color bands that heat would have affected.
after powering down, patiently check each of the big cans with a meter to see if they are dissipating their charge normally.
a cracked resister will heat up and let go allowing, or letting current pass to places it shouldn’t, so look for any discoloration in the resister color bands that heat would have affected.
dtier
Well-Known Member
Assuming your HT fuse is correct (1 amp), and your wall voltage is 230/240, the mains fuse blowing could be the reservoir capacitor failing. They are typically before the HT fuse in your amp model. If your wall voltage is 120 you need a 4 amp mains fuse. If your HT fuse is larger than 1 amp (too big), the problem could be anywhere. I have heard of some running a 3 amp mains fuse at the higher wall voltages but don't know that I would recommend that.
Thanks for all the feedback and suggestions. I replaced all the tubes to ensure a bad tube was not causing the issue. The fuse that is blowing is the T2A, not the T1A. I am in Canada with 120 volts, not 240. The Mains Select is set to 120 on the back of the head. I can leave the head on and out of standby with no problem. The T2A fuse fuse will only blow after 10-15 of playing through the at moderate volume (4-6). Not sure if this info can help.
Oh damn, well, if you're in Canada and running 120V mains then you have too low of a fuse value. It's 4A for 120V mains. The HT fuse value does not change.
A lot of those older heads didn't have separate screen printing for North American distributed amps. They may have had a sticker or label to denote the proper fuse type which has likely fallen off. If the amp is working properly with no faults you should be ok.
I was repairing a 2203 last night and at low volume it was drawing about 1.5 amps so that's fairly close to burning out a 2A fuse. Running a wide open healthy Superlead would surely break 2A.
I was repairing a 2203 last night and at low volume it was drawing about 1.5 amps so that's fairly close to burning out a 2A fuse. Running a wide open healthy Superlead would surely break 2A.
El Gringo
Well-Known Member
Don't mess around with this . Take it to a authorized Marshall Service center and get this properly diagnosed please !
Well, if it blows the correct fuse type, then yes. In the thread we have established the OP is using too low of a mains fuse value. Half the required rating, to be exact. A healthy 100W Superlead in 120V will absolutely pop a 2A mains fuse. It's supposed to be 4A. No cause for alarm yet.
El Gringo
Well-Known Member
Don't mess around with this . Take it to a authorized Marshall Service center and get this properly diagnosed please !
Sounds good
jgab
Well-Known Member
Yeah no need for alarm yet. That’s why we have this form, so people can troubleshoot these things and learn something along the way. Don’t need to run to a tech for $100 dollars an hour (or whatever they charge these days?)
Also, make sure you are using a slow blo fuse and not a thin wire type.
Also, make sure you are using a slow blo fuse and not a thin wire type.
Similar threads
