LSC blowing 4A fuses help

[egkor]

Hi, any help appreciated!

I was playing my 2005 (version 1) 1x12 50/100 LSC combo, both channels in 100w mode. Suddenly the volume level reduced, and the amp started sounding bad. I realized it might be a power tube going out. By the time I looked behind the amp to see if I could tell which 6L6 tube, the amp started to hum, and then the amp's AC power went out.

It did blow the 4 amp rear panel fuse. And I did see the 6L6 tube next to the rectifier tube was glowing red before the fuse blew.

So I replaced the fuse, installed a quad of known good Mesa EL34's, and switched the rear panel bias switch from 6L6 to EL34.

Now, when I switch the power on from the front panel, it blows the fuse immediately.

Question: In reviewing previous posts, I see that this might be a "burned screen resistor" problem (each power tube socket has one). Does this sound like a screen resistor issue? Would this cause a new fuse to blow?

If so, does the power tube going bad burn the resistor, or does the resistor going bad blow the tube?

Thanks,
-Gary K

 

[Octavarius]

Hi, any help appreciated!

I was playing my 2005 (version 1) 1x12 50/100 LSC combo in 100w mode (both channels). Suddenly the volume level reduced, and started sounding bad. I realized it may a power tube going out. By the time I looked behind the amp to see if I could tell which 6L6 tube, the amp started to hum, and then the amp's AC power went out.

It did blow the 4 amp rear panel fuse. And I did see the 6L6 tube next to the rectifier tube was glowing red before the fuse blew.

So I replaced the fuse, installed a quad of known good Mesa EL34's, and switched the rear panel bias switch from 6L6 to EL34.

When I switch the power on from the front panel, it blows the fuse immediately.

Question: In reviewing previous posts, I see that this might be a "burned screen resistor" problem (each power tube socket has one). Does this sound like a screen resistor issue? Would this cause a new fuse to blow?

If so, does the power tube going bad burn the resistor, or does the resistor going bad blow the tube?

Thanks,
-Gary K

Most of the time it's the bad red-plating power tube that blows a screen grid resistor, if it's not caught in time by the fuse. The fuse is supposedly there to prevent blown screen grid resistors, amongst many other things. I'm not an expert on this, so someone else with more knowledge should chime in, but I'd try replacing the fuse again. Sometimes they go for no reason, even though it's somewhat unlikely here. If the tubes are known to be good, it might be something internal indeed.

I'd perhaps take it to be diagnosed by a known good technician if another fuse fails. :wink:

 

[egkor]

I'm not an expert on this, so someone else with more knowledge should chime in, but I'd try replacing the fuse again. Sometimes they go for no reason, even though it's somewhat unlikely here.

Thank goodness, I replaced the fuse again with a new 4A slo-blo, and it works!

I did pull the amp (chassis?) from the combo box, to take a look. Each of the four screen resistors measured about the same resistance, so it looked like those were all ok.

Put it all back together, put in the new fuse, and presto, I have my pig again!

Thanks for your advice, it worked!

-Gary K

 

[Octavarius]

I'm not an expert on this, so someone else with more knowledge should chime in, but I'd try replacing the fuse again. Sometimes they go for no reason, even though it's somewhat unlikely here.

Thank goodness, I replaced the fuse again with a new 4A slo-blo, and it works!

I did pull the amp (chassis?) from the combo box, to take a look. Each of the four screen resistors measured about the same resistance, so it looked like those were all ok.

Put it all back together, put in the new fuse, and presto, I have my pig again!

Thanks for your advice, it worked!

-Gary K

I'm happy everything worked out for ya! Sometimes these things happen. If the screen grid resistores measured fine, I'm sure there aren't really any issues internally. The fuse itself may have been a bad one, or perhaps just some "leftover" irregularities among the internal components caused by the power tube failure earlier was the reason the fuse went. I'd always carry spare power tubes and fuses with me, though!