GK MB212 MK1 Red ring at moderate volume

[beanondabass]

Hi all,

Yes, I know this is stupid, but I'm a tinkerer its what I do. I have taken the amp head out if my GK MB212 combo and made it a seperate head and cab. Just used thick cable to wire in 2 6.35 jacks and away we go!

It works... sorta. Trying it at home it was ok, but if I attack too hard, or turn the Gain/boost/master knobs up too high, on note attack the sound will cut out, the power ring goes red and after 2 seconds it all comes back (but will happen again as soon as I play).

This happens when plugged into the cab or even just on headphones from the phones jack on my desk, so I'm not sure but at least thats isolated the issue to the amp head.

The manual says: "Should the amp experience a fault (over heat or over current) the amp will mute and the ring will light red until the condition is corrected."
This leads me to believe its a overcurrent issue as heat wouldn't fluctuate this fast and precisely.

Has anyone got any ideas where to start or any solutions to this issue? Googling hasn't got me far, and I don't want to take this to a tech I want to learn. Anyone words of wisdom from the wizards of talkbass?

Thanks in advance and I can provide pictures if needed. Cheers!

 

[agedhorse]

You are lucky that you haven't destroyed the amp.

This amp uses a BTL configured power stage which means that NEITHER of the speaker lines are ground referenced.

You used a grounded sleeve jack which is shorting 1/2 of the bridge to safety ground, which is bonded to the power supply ground.

If you plan on continuing your tinkering career, you will be better served if you understand what you are tinkering with before you blow stuff up or worse, hurt yourself.

The speaker jack MUST be isolated from the chassis, there are specific parts for this purpose.

 

[beanondabass]

Thank you for the reply! I'm not sure I fully understand what you mean but I can offer more info. That cable is homemade, its two regular patch cable jacks, but i removed all the cable and replaced it with 14awg copper parallel cables. So it is just two thick wires from one end to the other, one is sleeve one it tip, and they are isolated from eachother, as far as I'm aware there is no ground in that connection. There were two wires from the board to the speakers, and I just re-paired them up again.

Also this issue is happening with headphones, which turns the speakers off and I've just been playing without this cable plugged in and the issue is still present. If anything it sounds like a capacitor dying maybe? It got worse as I played. I'll try and share a sound file but can't just attach and m4a unfortunately.

 

[beanondabass]

Here's a sound clip! It's A jazz bass into the passive output, with a small bluetooth speaker auxed from the phones out jack, the cab and scuffed cable aren't plugged in.

It got worse over time and with more intense playing, but I couldn't have it loud enough to actually make the amp overcurrent and cut out as it was distorting the speaker (more than it already is haha)

 

[agedhorse]

It has nothing to do with the cable, the speaker jack you installed is the wrong type and shorting the speaker minus to the chassis. This is a quick way to destroy an amp.

This is what you need (notice the plastic bushing, do not over-tighten :

 

[beanondabass]

OOOOOOOOH I see what you are saying, thank you very much, you're an amp saver Cheers horse!