GK MB500 red protection ring blinking and doesn't turn on

[Misanthropic]

So my Gallien Krueger MB500 has (yet another) problem. Or if I'm to believe what I've read, it's a "feature" and not a problem, i.e. a safety shutdown due to overcurrent/bad cables etc...

Scenario:

• I tried to use the MB500 with a cab belonging to a local venue's backline.
• The MB500 wouldn't turn on, but instead the power button ring was red for a couple of seconds, and when it tried to turn blue, the cab made a quiet "puff" sound, and then repeated indefinitely... red-blue-puff-red-blue-puff-red-blue-puff...
• The cab is a rather crappy 8x10 (I think Behringer), which has a custom made switch installed for turning off half of the speakers and switching between 4 and 8 ohms. It didn't matter in which position the switch was.
• The same cab happened to be borrowed to another venue at another gig and the same thing happened, so it's not a local power issue.
• The head worked normally before and after trying the 8x10, using my own cabs (two EBS 1x12's) as well as a friend's Ashdown 4x10.
• I tried two different cables, both of which work fine normally. I also tried both the plug and speakon cab inputs.
• The 8x10 that didn't work with the MB500 has been played by hundreds of other bands with different heads, and also worked fine with the Hartke HA-something and Markbass Little Mark 250 heads I had to borrow in order to play the shows.

Could someone shed some light on what's happening?

 

[agedhorse]

If the amp works properly with other cabinets, I would certainly suspect that there is an illegal load being presented to the amp by that speaker cabinet and you shouldn't continue using it until it's properly diagnosed.

A defective crossover or open tweeter (if it has a tweeter) can cause this symptom for example.

Why would anybody want to shut off 4 speakers in such a cabinet? If the cabinet does not have separate air spaces for each set of 4 drivers, the tuning will be greatly affected as well as the power handling being de-rated because the speakers are not properly loaded by the necessary air mass. This is a poor choice on many levels. Makes me suspect other things that might have been changed (and causing your problems).

 

[Misanthropic]

Yes, I also think the switch is a stupid idea. But that doesn't change the fact that the cab seems to work with every other head except mine. So even if it's an issue with the mod, it doesn't seem to break any bass heads... The Hartke head is the house head and it hasn't had a problem.

Can't remember if the cab had a tweeter, don't think so.

 

[agedhorse]

Yes, I also think the switch is a stupid idea. But that doesn't change the fact that the cab seems to work with every other head except mine. So even if it's an issue with the mod, it doesn't seem to break any bass heads... The Hartke head is the house head and it hasn't had a problem.

Can't remember if the cab had a tweeter, don't think so.

Your MB-500 head (and others using this output section) has additional protection features that your other heads may not have. You just don't know this, which is a flaw in your troubleshooting logic.

That protection is an essential feature of the power amp, which improves reliability.

 

[Misanthropic]

Thanks, I didn't mean to sound arrogant about it, I just find it strange, not being experienced at all with this stuff.

But good to know it's most likely the cab. Since this head has had all kinds of reliability issues before, my initial thought was "not again!!".

 

[agedhorse]

Thanks, I didn't mean to sound arrogant about it, I just find it strange, not being experienced at all with this stuff.

But good to know it's most likely the cab. Since this head has had all kinds of reliability issues before, my initial thought was "not again!!".

Based on what you described, I don't think this has anything to do with reliability issues.