70’s Ampeg B25B

[thelastinline]

Hey!
Was lucky enough to pick this up at what I thought was a great price. It smells like an old library - spent 15 years in a basement, untouched. According to the family, the original owner took pride in his gear.

The dilemma… this is a 16 ohm amp and I only have 8 ohm cabinets.
What are my options here? Would really like to find something that suits the head, but any vintage gear is seldom sold in my area.
Can my Avatar 2x10 be rewired for 16 ohm? Is there a modern 16 ohm cab that would be a good fit?

 

[eniac]

Plug your 8 ohm cab into the main speaker jack. Then insert a dummy 1/4" plug (tip and sleeve NOT connected inside the shell) into the external speaker jack. That will switch the output secondary tap from the 16 ohm tap to the 8 ohm one.

At least that's the way I'm reading the switching jacks shown on the schematic. However, please wait for verification from someone with more knowledge about these amps before proceeding. I might have missed something.

 

[coreyfyfe]

You can do the dummy plug thing for running an 8 ohm load, or wire the two taps to separate jacks.

When I ran mine with two 8 ohm cabs I used a Palmer speaker merger to run them in series for a total 16 ohm load.

Palmer CAB M | Signal Splitter & Switcher | Palmer

The concept behind Palmer's Cab Merger may be very simple, but the unit fills an important gap! Have you ever wanted to connect two guitar cabinets to just one amplifier output? It's quite simple with the PCABM! In addition, you can...

www.palmer-germany.com

 

[thelastinline]

The amp came with a microphone cord with a 1/4" plug on one end, wound in a loop and taped. Could the original owner have been using that to run an 8ohm cab?
Just want to make sure there's no risk of damaging the amp. If I need to gut a 1/4" to run the tip and shell, or if just a 1/4" cable left unplugged is fine.
Unfortunately electronics aren't my strong suit.

 

[eniac]

Look at the cord's XLR connector. It has 4 pin sockets, not 3. That's Ampeg's speaker cord, not a microphone cord. It was designed to mate with the 4-pin XLR connector on the back of earlier B25B cabs. You can set it aside; you don't need it. But save it for when you sell the amp.

For the external speaker jack, you could get by with a short speaker cable with the far end not plugged into anything and above all not touching anything metal. But I never recommend such non-serious choices and I am totally against that one because it's the "lazy man's way out" and, frankly, the mere idea of it stinks. Buy a right angle plug and it won't stick out asking to be damaged during moves. If you buy the Switchcraft below, you won't even need to check under the shell for shorted tip-to-sleeve contacts because there aren't any. Just make sure 2 the screws are snug. (Not socked down, just lightly snugged up.)

https://www.parts-express.com/Switchcraft-228-1-4-Mono-Cable-Mount-Male-Shielded-Flat-Plug-with-Solder-Terminals-093-1035?quantity=1

 

[eniac]

I would like to add: If that amp's been sitting around unused for that long, I wouldn't power it up just yet. I would bring it to a tech first, and tell him how long it's been sitting idle. The tech will plug it into a variac and bring the voltage up slowly, checking for excessive current draw during the process.

 

[thelastinline]

You guys are a wealth of knowledge. Thanks for the help, it's appreciated!

 

[Ric Vice]

I was under the impression that the two Eminence 15's in the B25B cabinet were wired in parallel thus dropping the cabinet's load to 8 ohms. At least that's what I remember. But the speaker out says 16 ohms weird. The 4 pin speaker connectors were designed to throw the amp into Standby is you neglected to plug in the cabinet. As I recall protecting the transformer. You should post this to the B-15 forum here on TB David (Beans on Toast) is a wealth of knowledge. There is also an Ampeg B-15 Portaflex Wiki that will have the specs on the B-15S which I believe was essentially the combo version of the B-25B.

 

[coreyfyfe]

I was under the impression that the two Eminence 15's in the B25B cabinet were wired in parallel thus dropping the cabinet's load to 8 ohms. At least that's what I remember. But the speaker out says 16 ohms weird. The 4 pin speaker connectors were designed to throw the amp into Standby is you neglected to plug in the cabinet. As I recall protecting the transformer. You should post this to the B-15 forum here on TB David (Beans on Toast) is a wealth of knowledge. There is also an Ampeg B-15 Portaflex Wiki that will have the specs on the B-15S which I believe was essentially the combo version of the B-25B.

There were two versions of the B25B cab. One with no switch, these were 16 ohm only, and one with a series/parallel switch for 4 or 16 ohms. The drivers should be 8 ohms each.

There were no production B25B cabs at 8 ohms.

The output on the B25B, as mentioned above, expects 16 ohms on the main with the extension cab jack switching the two parallel connections to the 8 ohm tap when running two cabs.

The available schematics show both cab options, one has the series/parallel switch and one is wired with the speakers in series only for 16 ohms.

https://ampeg.com/data/6/0a0005091563662004548f485/application/pdf/

https://ampeg.com/data/6/0a000509157f6620041ee6e4b/application/pdf/