Ampeg B-25B: My full question list.

[michael_atw]

Alright, so I just recently bought an Ampeg B-25B stack. Or...at least I think it is. I know the cab is because it says so...but the Amp head itself just says B-25. Will a B-25B say "B-25B"?

That's my first question. I was also trying to date the thing and find out if it is all-original. The serial on it is 005751. I've learned many times that's not enough, and I don't wanna open it up and light myself on fire so I put pics up to show the chassis and back. It's got two 12AX7 tubes, a 7719, 2 7027A's, and 5AR4 - if that helps any. It looks all original to me (minus tubes of course), but the Polarity and Power switches on front are plastic so I was wondering.

Also, I posted a thread a few days ago asking anyone if they knew the impedance of B-25 heads. I learned it was probably 16ohm/8ohm a while ago but cannot find any visible label to tell me that. I was saying before that I began using it when I got it with an SVT15E I regularly use (rated at 8 ohms) because it's a much lighter cab than the B-25B that came with the head. I have read you can mod it to run at 8 ohms but it normally runs at 16. However, I also read you can plug a dummy jack into the extension speaker and imitate a cab there and bring it down to 8 ohms. However, it only has the Ext. Speaker to plug in at all, due to the fact that (as I pictured) there is an plug coming out of the amp in place of the hum-balance speaker plug. I have seen pics of these heads with an open plug in place of this, but I don't know if that's a mod. These are the only speaker jacks I know of. Does that plug automatically act as a dummy jack? I wondered this because I am forced to plug into the extension right now with my SVT15E.

Needless to say, my Bassman runs far hotter than this head, and I thought that bad impedance matches would run the head hotter. So, I have a laundry list of things to know. I figured I'd try to get them all in one sitting.

[michael_atw]

Scratch that...pics are too big and my editing program was lost in a comp. crash. If it helps I'll try to get some up...I think the info I put up might do it though.

[johnk_10]

the head is called a B25, not a B25-B, and they are 16 ohm heads. the cabs had two 8 ohm 15's wired in series for a total of 16 ohms.
and it does have an extension speaker jack that uses the 8 ohm tap on the output transformer so when you plug in two 16 ohm cabs it runs at 8 ohms.

here's a link to the schematic for it:
http://www.kbapps.com/audio/schemati...ampeg/b25.html

[michael_atw]

alright. Yeah I know the 16 ohms part.

Anyone else have any info please?

[Aquila0507]

Your fine with the B-25 for bass. There is not a difference in the amps just the speakers are different and if your cab says B-25-B your golden. I have the B-22X Jupiter and its the same as the B-25 but the B-22X has an extra pre-amp section that drives reverb, vibrato, and tremolo and it says it runs at 8 ohms. The polarity and power switches are in the front and are square, plastic, and black. The lines on the front faceplate are blue so I know its 69'-72'. I believe you could date yours the same way. Black lines are 73'-76' I believe and the power switch may still be in the front but metal. So basically date them the same as an SVT and their faceplates. I also have an ext. speaker jack and have no idea what it does as the main speaker out is labeled 8 ohms on the schematic. The hum balance has a pot you can turn with a small screwdriver and not any plug. Sorry I can't help with explaining your mod. How do you like the amp? I think the bass tone out of it kills and have even recorded with it through pedals. The headroom was amazing for such a "low" powered amp.

[Aquila0507]

Don't be afraid to open the chassis. Thats the only way I knew the amp was 8 ohms because the schematic is pasted on the chassis under the detachable metal sheet that covers it.

[michael_atw]

Thanks for the info. Yeah I wasn't worried about what type it was - I'll play bass through a Fender Twin Reverb if it sounds good enough. I was just wondering if there was a chassis that explicitly stated "B-25B" as opposed to "B-25". The lines on this faceplate are blue and the serial = 1965. Is it legit to declare it a 1965? There is no main speaker jack besides the Ext. Speaker jack...so I'm wondering where it went. It must be the plug coming out that fits in the Ampeg speaker - circle, 3 prong...like a direct box plug that would go into a soundboard. Dunno the technical name for the plug. That's what I was wondering about: is it original to the amp? I don't believe anything is modded - the two prong plug still is present. If they didn't change that out then nobody touched anything.

What I was asking is can I use the "Ampeg" plug as a dummy jack (i.e. pretending to be a speaker) and then use just the Ext. Speaker with an 8 ohm cab and have an impedance match? That was the thrust of my questions.

Can anyone help me further?

[michael_atw]

Oh I forgot - yeah I love the sound. Covers so many different genres no problem. It was either this one, a V4B, or an SVT. I hope to try them all out at gigs at some point.

[johnk_10]

the cabled 3 prong female output jack is called a 'cannon' and it is original to the amp (most likely made by switchcraft). i think that you can just use the extension jack and you'll be set to run it at 8 ohms.

[Aquila0507]

I also think that you can just use the ext. jack and be fine. You don't need a dummy load. I used to run an 8 ohm cab through the ext. speaker of a bassman 135 and the amp was fine and sound came out of it with just the ext. jack engaged. Knowing the ohm rating is important though. if its 16 ohms don't use an 8 ohm cab. But this is an old ampeg we're talking about and they usually under rated the transformers so you could run an 8 ohm cab if you dare. These weren't in production until 69'.