Power amp for Ampeg B-25...maybe?

Bruiser Stone

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Dec 7, 2017
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So I acquired a 1969 Ampeg B-25 tube head yesterday because the tone was just too sweet. My needs remain with finding a louder setup. Apparently I am at war with myself on this issue because I went and traded for 50 tube watts, which ain’t so quiet through two 15”s. I guess for small club-blues-jazz stuff it will work fine, but not a Sabbath tribute band (but the tone, man, the tone!)

But it does have an “Ext. Amp”
out jack. From what little bit I gleaned from old TB posts, the opinion seems to be it can be connected to another amp.

In an effort to get the best of both of tone and power without spending too much dough, what if I:

1. Acquire something, for example, like a Peavey Firebass, a big, burly, reliable chunk of American heat sink (I’m fond of Meridian MS products), something clean and loud primarily for the purpose of power amp, and to also to serve double-duty as an alternate/backup amp? Would it detrimentally affect the B-25’s tone?
I’m not against Class-D stuff with a power amp jack, but my first thought was old school Peavey for loud, cheap, and durable.

2. Would I be better getting just a dedicated power amp? If so, suggestions?

3. Or be patient and score something like a Peavey 120 or 120/120 Classic all tube power amp? Is this a novel but unnecessary idea?

3. Or, will I do great and costly harm to the Ampeg if I attempt to mesh 50-year old technology with equipment it was never intended to meet?

I’m aware that on its own, the Ampeg is 16-ohms out of one jack, 8-ohms if both are plugged in. I have a Palmer cab merger to run both of my 8-ohm Black Widows in series for 16, and a dummy jack plug when I only want to run one at 8. It is my understanding the Ampeg’s speaker jack impedence quirks are moot if and when a power amp feeds the Widows (unless of course I’m wrong). Thx in advance everyone.
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If you are not wanting to use the B25 as a pre-amp, I would find another solution.

Yes you can run the pre amp output (EXT AMP) into the unbalanced input of a ppwer amp. The B-25 would still have to be connected to a speaker cab and the output would be split between the external power amp input and the B25’s power amp input.

It might work, you will have to try it and see. Worst case, with an external power amp connected, the level going to the B25’s power amp will be reduced —less volume out of the B25’s power amp and the B-25’s pre might not drive the external power amp well.

Now what follows may be confusing.....

How well it will work depends on the external amp. Unfortunately, the B25 topology is instrument input into two preamp stages, then into a passive tone stage, then into the power amp and the ext amp split off in a Y output. There could be an impedance mismatch when driving an external power amp.

If they were smarter they would have used another tube to create a post EQ gain makeup stage followed by a cathode follower, then split to the EXT AMP and into the power amp.
 
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What is the "Ext Amp" jack on the back of the Ampeg B-15? Yeah, I know it's some sort of low-level output, but is it preamp output, or some other type? What was it originally meant to be used for?

"Ext Amp" jack on Ampeg B-15

beans-on-toast: Thanks for the information. Part of my research included your contributions to the 2009 post quoted above. I intend to keep this little thing either way, but I wondered of it had a role in a louder context. My initial research led me to think it was possible.
 
You never know, if you have another power amp input, give it a try.

Connect an instrument cable from the B-25’s EXT AMP into another amp’s power amp in. Plug into the B-25, this will use the B-25’s pre to drive both amps.

You know, I think the sound is great, but 72 hour return periods don’t come around very often, so I’ll leave the experiment to someone else. Thanks for a fast reply.
 
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A B25 was the basis for my rig of doom. B25 to TL606 cab. B25 to 3 way crossover. 3 way crossover to a DC150 driving some EV horns, a rack of DC300's driving ProCo 2x15 cabs, another rack of DC300's driving EV 18" TL cabs (forget the number). Give it a try, just make sure you have a cab connected to the B25.
 
If you have a real PA, micing the cab is the best way to get that tone louder. Even my relatively cheap Audix D4 mic does a great job live or in the studio. I don't know why people get up tight about micing bass amps. My amp is as much a part of my sound as the bass, so direct isn't an option.

Alternatively, if you really need to get that sort of sound louder from an amp, I suggest you look into the SWR Interstellar Overdrive preamp. I have one, and it is sitting next to my V4B, which is on a B25B cab. I have posted on this---it sounds almost exactly the same as the V4B. The V4B looks cooler, but the tone is almost indistinguishable. I can scale that tone up with whatever size power amp I want.
 
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More great suggestions for future reference, especially the Interstellar. The ‘lil Ampeg (probably wisely) went back home this afternoon, and I’m on the hunt again before my next jam session in a couple of weeks.
 
Looking at these pictures, I see that the B25B used two 12AX7's, a 12DW7, and 12AU7
as their pre amp tubes as did the B-15S. I guess I'd missed that somewhere along the way. That's a departure from the 6SL7's the always used in the B-15N's. Apparently the B-18's were the last 50 watt amps to use octal tubes. Probably cheaper to do this. When the Helmet Head's were introduced that would have made the B-15N the only Ampeg bass amp using 6SL7's. You have to think that the bassists and engineers who were at Ampeg, still preferred the sound of those tubes in the preamp of the B-15 as it was the only bass amp in the entire line that used them.
 
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This is great information! I aquired a B-25 from a fellow TB'er not to long ago and I noticed that going through a single 12 cab (rated 350w), I was getting some distortion at louder volume. I thought about running it through a tube power amp but, wasn't sure how it would go.
 
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You need one of these. 100 Watt B-15 clone I had built. It was cost effective compared to all the solutions everyone comes up with trying to get decent volume out of a low watt amp.View attachment 3068905 View attachment 3068906

This makes perfect senes to me for those who want a B-15 only louder. and it has varriable ohm taps on the transformer to accomidate different cabinets. Using a single B-15N only works for me in the studio or on gigs where

there's an acoustic piano and a tasteful drummer with a small kit.
 
Sorry for the late chime... Beans is absolutely correct on all points. I run mine as he suggests out of the B25 EXT AMP into an ampeg "slave" A-120 through an 8 Ohm B40 410 cab and it is brilliant.

Good to see the A120 being used for bass applications. I always thought they were better suited for keyboards and some guitars.