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B25-B Cab Modification

G

gimp

Jan 13, 2011
363
7
Catonsville, MD
facebook.com
#1
So I'm putting together a strange rig.

I currently have a B-15NF for home use, and for playing out/practice I use a BT-15 (solid state) into a Mosvalve MV-925 into a 8ohms 1x15. It's not ideal, as the Mosvalve is a 4ohms output, and I need to move a little more air.

I've got the BT-15 and Mosvalve sounding really good after A/B switching it with my B-15NF, so that part is good to go.

I've got a line on a well loved B25-B cab. I'd like to rewire this cab so that the two 15s were independent from each other (two jack plates). This would allow me to load it with two 4ohms 15" speakers and use the full output of the Mosvalve.

That seems pretty straight forward. But, would there be any way to wire this up so I could either have two independent 4 ohms inputs, or a single 8 ohms input? Pretty sure the answer is no, but you guys would know best.

Thanks!
 
#2
The answer is yes, and the way to do it is to play a little game with 4-way Speakon connectors.

For the dual 4-ohm connection, you can use anything you want: a pair of phone plugs, a pair of Speakons, one 4-way Speakon, banana plugs, whatever. Just wire directly to the speakers.

For the (series/mono) 8-ohm connection, we will use a separate 4-way Speakon connector. From the amp side, you will use pins 1 and 2 of the plug for amp output (+) and amp output (-). You will connect a piece of wire between pins 3 and 4 to short them. At the cab connector, connect pin 1 to speaker 1's (+) side. Connect pin 2 to speaker 2's (-) side. Connect pin 3 to speaker 1's (-) side. Connect pin 4 to speaker 2's (+) side. Now when you plug in your "special" series-connection Speakon plug, the plug's short between pins 3 and 4 will make a connection between speaker 1's (-) and speaker 2's (+), thus connecting them in series, while the plug's pin 1 will connect to speaker 1's (+) and pin 2 will connect to speaker 2's negative. The beauty of this scheme is that when you remove the plug, the series connection between the two speakers goes away, and you can use the speakers independently again via the other cab connector(s) you've chosen.

Someone else PLEASE check my work. I have about ten things going on right now and I am not 100% confident in this scheme. Thanks in advance.
 
#3
I just whipped up a quick diagram, if someone wants to check my math.

speakers.jpg


This was my idea, assuming two 4 ohms speakers. Just have to throw the switch to "on" when using the 8 ohms input.
 
#4
craig.p said:
The answer is yes, and the way to do it is to play a little game with 4-way Speakon connectors.

For the dual 4-ohm connection, you can use anything you want: a pair of phone plugs, a pair of Speakons, one 4-way Speakon, banana plugs, whatever. Just wire directly to the speakers.

For the (series/mono) 8-ohm connection, we will use a separate 4-way Speakon connector. From the amp side, you will use pins 1 and 2 of the plug for amp output (+) and amp output (-). You will connect a piece of wire between pins 3 and 4 to short them. At the cab connector, connect pin 1 to speaker 1's (+) side. Connect pin 2 to speaker 2's (-) side. Connect pin 3 to speaker 1's (-) side. Connect pin 4 to speaker 2's (+) side. Now when you plug in your "special" series-connection Speakon plug, the plug's short between pins 3 and 4 will make a connection between speaker 1's (-) and speaker 2's (+), thus connecting them in series, while the plug's pin 1 will connect to speaker 1's (+) and pin 2 will connect to speaker 2's negative. The beauty of this scheme is that when you remove the plug, the series connection between the two speakers goes away, and you can use the speakers independently again via the other cab connector(s) you've chosen.

Someone else PLEASE check my work. I have about ten things going on right now and I am not 100% confident in this scheme. Thanks in advance.
Click to expand...

Wow! That is much cooler than my idea in using a switch.
 
#9
WhoDidTheElf said:
Wouldn't having two 8 ohm speakers be better? That way you can drive both speakers of the cab, each with one channel from the mosvalve?
Click to expand...

The Mosvalve has an A and B channel. Each channel is rated at 80 watts @ 4 ohms, and there is no mono bridging. Each one of those channels has two outputs.

So, I would run A to one input on the cab, and B to the other, giving me 80 watts @ 4 ohms to each speaker, for a total of 160 watts.


With my previous setup (two 8 ohms cabs) I was just running both outputs for channel A into the cabs (so their combined load would be 4 ohms). I'm trying to get the most out of this power amp.
 
#12
Bring this up to the top. Speakers are ordered, and cab (while not a B25-B, is another older Ampeg cab) is in hand.

My real question comes down to the switch. I could spend 17 bucks on a reproduction switch from Fliptops, but I feel like there is a more local, cheaper solution. Can anyone tell me what I should be looking for (spec-wise)? Max to each speaker will be about 300 watts.
 

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