Ampeg B-25 schematic confusion

[nsubulysses]

I am a little confused by this schematic and don't want to pass along wrong info. It seems that the regular speaker jack is 16 ohms, and when you plug into the extension speaker jack it drops to an 8 ohm tap, meaning it is best to use two 16 ohm cabs. Is this correct? This seems right, but also strange for a bass amp.

The amp is a head not a combo. By the schematic it also makes it seem you can plug into either for either impedance, but I know from other Ampegs that the ext speaker jack doesn't work unless you're plugged into the regular speaker jack also.

Ampeg B-25 schematic.psd

[Tom Phillips]

... It seems that the regular speaker jack is 16 ohms, and when you plug into the extension speaker jack it drops to an 8 ohm tap, meaning it is best to use two 16 ohm cabs. Is this correct?

Yes that is correct.

...This seems right, but also strange for a bass amp...

I don't understand why you feel that is strange. It should work fine and makes sense to me.

...The amp is a head not a combo. By the schematic it also makes it seem you can plug into either for either impedance, but I know from other Ampegs that the ext speaker jack doesn't work unless you're plugged into the regular speaker jack also....

That is correct and it is easier to understand if you have the original version of the schematic published by Ampeg. Attached is a copy for your reference.

Cheers,
Tom

[nsubulysses]

Real schematic is much better

Guess it just couldn't think of another bass amp that is 8 ohm load minimum. They usually go lower. And I worked on a B-15 recently and kinda expected it to be the same. oh well. Thanks!

[Leo_Gnardo]

Guess it just couldn't think of another bass amp that is 8 ohm load minimum. They usually go lower. And I worked on a B-15 recently and kinda expected it to be the same. oh well. Thanks!

One of my customers didn't know better, and slammed his B25 thru a 4 ohm cab, eventually wrecking the OT. Subbed in a generic Bassman/SuperReverb OT from MagicParts/Ruby and all was good. Similar OT's available from Antique/CE and I'm sure other places, if you want to make B25's more user friendly. Not terribly expensive, under $100. Unless you go Mercury of course.

Early Ampeg heads had "flying lead" outputs, connector on a wire hanging out the back, instead of a jack. Many had a 4-pin XLR (mic) type connector with an interlock feature that defeated signal going thru the amp unless it was plugged into a mating Ampeg speaker cab. A bid to sell Ampeg cabs or a safety feature to keep people from wrecking their amps? That explains their output scheme. By now just about all surviving amps have been converted to output jack.

[AL30]

One of my customers didn't know better, and slammed his B25 thru a 4 ohm cab, eventually wrecking the OT. Subbed in a generic Bassman/SuperReverb OT from MagicParts/Ruby and all was good. Similar OT's available from Antique/CE and I'm sure other places, if you want to make B25's more user friendly. Not terribly expensive, under $100. Unless you go Mercury of course.

Early Ampeg heads had "flying lead" outputs, connector on a wire hanging out the back, instead of a jack. Many had a 4-pin XLR (mic) type connector with an interlock feature that defeated signal going thru the amp unless it was plugged into a mating Ampeg speaker cab. A bid to sell Ampeg cabs or a safety feature to keep people from wrecking their amps? That explains their output scheme. By now just about all surviving amps have been converted to output jack.

Sorry for the late response. What did the Bassman OT sub do to the sound? Just curious. I've got a few old Ampegs and I love the sound of them. I've toyed with the idea of building one. The transformers in the V-Series amps (and the B25's) are pretty beefy. I always assumed that a different OT would drastically change the feel of the amp.

I just picked up an early 70's V4 full stack. It still has the original cables and jacks on the cabs. Interesting for sure.

Thanks,

AL

[nsubulysses]

The person believes the SVT is 300W at 2 ohms, and something less at 4 ohms, because the original SVT 8x10 cab was only rated at 240W total, so the speaker jack scenario creates an impedance mismatch purposefully at 4 ohm (1 cab) setting to protect the cab.

I guess what I was meaning is that the mismatch would come from plugging a 4 ohm cab into the 4 ohm regular speaker jack, with the head claiming 4 ohms, but actually the OT provides 2 ohms at all times, which can only reached when both speaker jacks are used. Basically protecting their own single 240W 8x10 from possibly being overloaded.

Thank you for commenting and clearing up this untrue info.

[Tom Phillips]

Comments based on the SVT circuit version shown in the previous post.

…The person believes the SVT is 300W at 2 ohms, and something less at 4 ohms, because the original SVT 8x10 cab was only rated at 240W total…

I have not found any substantial difference in the 2Ω vs. 4Ω output power of a properly operating vintage SVT. I cannot remember seeing an official speaker cab rating but I have heard this story before.

…so the speaker jack scenario creates an impedance mismatch purposefully at 4 ohm (1 cab) setting to protect the cab...

There is no mismatch because each cab is eight 32Ω speakers in parallel = 4Ω. Because of the power interlock jumper setup in the main 4 pin speaker connector the amp wont operate if you plug into the extension speaker jack without first connecting the main speaker. Therefore, if the stock vintage SVT speaker cabs were being used there would be no impedance mismatch with either one or two speaker cabinets connected.

…To me it seems to be an OT with a 4 ohm that drops to 2 ohms when the 2nd cab is used, both delivering full power because of impedance match. Does anyone know if this is true?

I agree. That’s true.

[nsubulysses]

I guess what I was meaning is that the mismatch would come from plugging a 4 ohm cab into the 4 ohm regular speaker jack, with the head claiming 4 ohms, but actually the OT provides 2 ohms at all times, which can only reached when both speaker jacks are used. Basically protecting their own single 240W 8x10 from possibly being overloaded.

Thank you for commenting and clearing up this untrue info.

Whooops I was trying to reply with a quote and i edited my whole question by accident so now the thread doesn't make sense.

For those who may read this in the future: The truth of the old SVTs with no impedance selector switch is that they they provide a 4 ohm and 2 ohm tap from the output transformer secondary, both providing 300W (full power) as long as the impedance of the cab is matched. When the regular speaker jack is used it is 4 ohms, and when the regular AND extension speaker jack are used the amp uses a switching jack that goes to a 2 ohm tap.