Troubleshooting cable woes with Acme Low B2 and Rumble 500

micahwc

micahwc

May 14, 2012
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#1
So I have an odd issue with my amp and cab. I have a rumble 500 head and a 4 ohm Acme Low B2 generation 2 cab.

I recently purchased several inexpensive speaker cables (marketed as speaker cables, not instrument cables). When I use any of them to connect my amp to my cab I get a rhythmic loud popping noise. It's like some kind of failsafe on the amp is kicking in and turning it off, but the switch is on, so it turns on again, ad nauseum.

I attempted to trouble shoot by plugging my amp into different speakers and didn't have any problems with the amp, so I assume it's the cab or the cable.

Thinking it was perhaps from the cheap cables I bought a much more expensive speaker cable from my local music store. I have the same issue with it. It too is marketed as a speaker cable and not as an instrument cable.

Finally I tried a cable I made myself using two stranded speaker wire that I soldiered 1/4 plugs onto. It works with this cable fine.

Anyone have an idea for why it works with the cable I made myself and not with the cheap or expensive speaker cables I bought?
 
#2
This is a symptom of an illegal load with this power amp.

Common causes are a blown or disconnected tweeter that has an associated Zobel impedance compensating network attached. Other possibilities include a damaged or defective crossover, tweeter L pad or POSSIBLY a shorted cable (though the symptoms are usually different.
 
#4
Yes, the term "illegal load" is one that falls outside the safe operation limits of the amplifier.

This is not just a load that is nominally below 4 ohms, but there are reactive (specifically leading power factor and resonant tank circuit issues) as well as high frequency content limits that the protection can activate on as well.

I have run into this before, and in every case it was due to either a defective crossover design, damaged crossover, or a crossover with a Zobel along with an open high frequency driver (or L-Pad).
 
#7
I know less than the other two posters, but I've briefly owned an Acme B2 in the past, and my understanding is that the load is slightly below 4ohms, and some amps have trouble with this, e.g., Markbass F1 not playing well with Acme Low B2

That explains why your amp & cables worked with other cabs, but doesn't explain why it works for your homemade cable though.

Again, I'm no expert...just trying to be helpful based on what I've heard about Acme B2 cabs in the past.
 
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#9
smperry said:
I know less than the other two posters, but I've briefly owned an Acme B2 in the past, and my understanding is that the load is slightly below 4ohms, and some amps have trouble with this, e.g., Markbass F1 not playing well with Acme Low B2

That explains why your amp & cables worked with other cabs, but doesn't explain why it works for your homemade cable though.

Again, I'm no expert...just trying to be helpful based on what I've heard about Acme B2 cabs in the past.
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Is it possible the homebrew cable might be adding resistance somehow to get the impedance above the minimum the amp can handle?
 
#10
Could somebody with a B2 Series II measure the port diamters and lengths. I'm reconditioning one that had rather severe damage that included the port tubes disintergrating.

An regarding bobcruz's comment above, increasing the resistance/impedance will only make things 'easier' for the amp, but as the resistance in any cable should be very low anyway, unlikely to have been the cause with the cheapo bought cables.
 
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