OHMs ? Ampeg B25B

[zoomduck]

Just purchased a 1974 Ampeg B25B in very good condition . It has two speaker outputs rated at 16 ohms . The seller said to connect it only to 16 ohm speakers . I have ordered a jensen MOD 15" 16 ohm . What would happen if I connect my Avatar cabs at 8 ohms each ?

[zoomduck]

Forgot to mention....I need to replace the volume pot on channel 2........can I just unsolder it from the circuit board and replace it ?

[soundmasterg]

If you connect your Avatar cabinets at 8 ohms each in series, then you will get a 16 ohm load and you will be ok. If you connect them in parallel then you get 4 ohms and that will be ok at very low volumes, but at higher volumes, your amp will be working much harder than normal and be 'that' much closer to a dead short, and it may not like it every much and blow out the output transformer. There may be other taps off the transformer that just aren't hooked up to the outisde of the amp, in which case you can just change which wire is soldered to the jack and get a different tap on the back of the amp. I did this on my Sunn 200S.

For the pot, assuming you have the correct replacement part, then yes, just unsolder from the circuit board and replace. Be careful about too much heat on the board or you can lift the traces on those old boards.

[zoomduck]

Greg . By in series do you mean daisy chain style ....or each output to each cab ( sounds like parallel to me ) Thanks .

[thx1955]

Barry,
If you can get your B-25 chassis out of the cabinet and look at the external speaker jack.

There should be two taps coming from the output transformer, one Yellow which is 8 Ohms, and one Green, which is 16 Ohms.

Take a look and see how these are connected to the jack(s) and from there you can see if the jacks are linked in series or parallel.

For the record, my B-25 has only one external speaker jack, and it's wired to the 8 Ohm tap.

Hope this helps.

[soundmasterg]

Yes Barry, series would be daisy chain style and parallel would be each output to one cab. The best approach would be to get the correct tap wired to the back of the amp for whatever load you want to run as Jim suggests.

[zoomduck]

OK . Here is what I see . 2 jacks one above the other . The top one is " ext speaker " the bottom is " speaker " . The top jack has both yellow and green wires from the transformer . The schematic says " 8 ohms ext speaker " thats the top jack . On the outside both jacks say 16 ohms . How should I run two 8 ohm cabs ? Daisey chain or build a series patch box ? Thanks .

[thx1955]

Hi Barry,
Could you send / post a pic of the wiring of the two jacks ?

[zoomduck]

Jim . Had to send the photo to your email . Thanks .

[thx1955]

Hi Barry,
Received and replied, give me a call.

[zoomduck]

Hi Jim . Too much for my non-tech head . I think you are right about a 16 ohm in the bottom and 8 ohm in the top jacks . I am just going to make a series patch box and play it safe . Thanks very much for all the info . Barry .

[ben_brown]

I looked at my B25B and there is one speaker out. This is closer to the bottom. Above that there is a toggle switch that says 8ohms one way and the other way says 16ohms. Eveidently someone installed this themselves. It is labeled with the old style plastic stick on labeling tape.

[beatlefreak]

Run the speakers daisy chained together - Positive out put of the amp to the positive of one speaker. Negative of that same speaker to the positive of the second speaker. Negative of the second speaker to the negative of the amp. 16 ohms, and the amp will be happy.