The main out of a B15N is 16 ohms; the extension is a separate 8 ohm tap. Neither would be suited to 4 ohm load. You'll work the output tubes WAY too hard and may even burn out the rectifier tube trying to supply the additional current. It won't hurt the cabinet, but you'll probably damage the amp. There is NO such thing as "underpowering," in and of itself and even at continuous full square wave clip (which the B15N can't even do, let alone into a 4 ohm load) you'd be "hammering" your NV610 with a whopping 70 watts of horrible sounding distortion. It's not enough to hurt even a single speaker in that cabinet.
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Ampeg B-15 thread... first ever?
- Thread starter Ben Clarke
- Start date
The main out of a B15N is 16 ohms; the extension is a separate 8 ohm tap. Neither would be suited to 4 ohm load. You'll work the output tubes WAY too hard and may even burn out the rectifier tube trying to supply the additional current. It won't hurt the cabinet, but you'll probably damage the amp. There is NO such thing as "underpowering," in and of itself and even at continuous full square wave clip (which the B15N can't even do, let alone into a 4 ohm load) you'd be "hammering" your NV610 with a whopping 70 watts of horrible sounding distortion. It's not enough to hurt even a single speaker in that cabinet.
I'm in that pathetic area of tube amp existence where a little knowledge can be dangerous...
however, isn't running a amp with no speaker essentially a high impedence load?
I think this is where I got my idea of damaging the transformer from...
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch
here's a link to what I'm assuming is the later thiele cab b-15n (the hi and lo switches):
Link Removed
No speaker loads shows as an inifinite impedance, and you're on the right track and that FAQ is generally correct. However, output transformers in bass amps are usually made for higher current than those in guitar amps and the output tubes and their associated circuitry usually arc before the output transformer suffers damage. Tube arcing is almost certain in big bass amps like the SVT where output tube plate voltage is already skirting the limits of the tubes under normal conditions. In the B15N's fairly conservative design with average plate voltage, no load is more likely to damage the output transformer than the tubes, but IME fixing them, the output transformer is of high enough quality that damage occurs elsewhere
Hey, PBG, this reminds me of something else I wanted to ask you. Have you read this quote from Jess Oliver in his interview with Keith Rosier?
"What are the benefits of re-coning the 8-ohm speaker in Portaflex amps with a 4-ohm kit, as you did with my B-18?
After being in the amplifier business for many years, I found out you don't always have to follow the book rules. When it comes to speaker load on a tube amp, I've found you can load a tube amp down to saturation without damaging the amp. You just get more effective power out of the amp. Just as long as the secondary of the output transformer "sees" a path, some resistance, it doesn't matter how low that resistance is, as long as it sounds good, and four ohms is not below the saturation point."
What do you think of this idea? Apparently Jess has been doing mods on a few people's B15 series amps with this 4 ohm recone on the speaker.
If anyone's interested in the full interview (although I suspect most of you have read it) here it is:
Link Removed
Matt
I wouldn't do it. I have seen this recommendation before. Even though Jess Oliver designed the amp and I respect his thoughts, what he proposes is more for those who want a different sound out of a B15N and not simply to use a 4 ohm load. There's a lot of things that are touched on in that quote, such as output tube saturation, others that are ignored, such as damping and others that are assumed such as a better power supply with s/s rectifiers and upgraded capacitors that HAVE to be addressed before you can do what he suggests.
Given his comments about Everett Hull not liking loud music, I assume that he does not care for the sound of the B15N in stock form because it is "too weak" due to restrictions on the original design that Hull placed on him. The modifications he suggests won't leave the amp sounding very much like a B15N at all.
Given his comments about Everett Hull not liking loud music, I assume that he does not care for the sound of the B15N in stock form because it is "too weak" due to restrictions on the original design that Hull placed on him. The modifications he suggests won't leave the amp sounding very much like a B15N at all.
Wow, thanks for all the info. The search continues to find a louder B-15. From what I've heard, the B25-B has a tone very similiar to the B-15, or B-18. I think 50 watts would be enough for certain gigs that I have.
Yeah, this thread has been great. Really informative.
NickyBass, I had a B25B and found it to be plenty loud for most situations and styles I played it in. Ran it through an Acoustic 406 2x15 cab. Sounded fantastic. One of the few rigs I actually regret selling, and I've bought and sold many over the years. Frequently pushed it to overdriven levels. Not necessarily because I had to, but because wanted to. Loved that sound! I did find myself getting drowned out in one three-piece I jammed with--screaming Marshall half-stack played by a completely tasteless guitarist and a loud, heavy hitting drummer. (Although I had played with the drummer before, and I know that he was only playing that loudly that night because he had to to compete with Mr. Tasteless Marshall Dude). Ultimately it didn't matter, though. That guitar player sucked, and I had no interest in pursuing any kind of musical relationship with him. All other situations I played the B25B in it worked just fine for me.
Matt
I see. Personally, I'd rather have the option of running the amp through another fifteen now and then anyway, so I definitely won't be considering this mod. Now that you mention it, though, Jess does recommend the s/s rectifier in that interview. Don't recall if he said anything about the caps. Will have to go back and look again. But if all that's going to equate to a radically changed sound, I'm not particularly interested.
Matt
I think I read somewhere that the B-25B circuit is very similar to the V4/v4B. I know you have the V4 because I emailed you about buying it, but money's a little tight right now. I also saw your ad for the SVT and the custom B-15 cab. I would think the V4 with that cab would do exactly what you are seeking...
I have no idea if this is true or not. I do know that I have owned both amps and never thought they sounded much alike. Never owned both at the same time, mind you, so I never got to do an honest A/B-ing of them. I just know that the B25B always made me go, "Yeah, that's the sound!" The V4B makes me go, "Yeah, that's a very good sound, but it's not the sound..."
Sorry for the totally unquantifiable subjective opinion. I just know that I'd be surprised to learn that the circuits are very similar, as my ears suggest differently.
Matt
From what I've read the V-4B cab is what really changes the sound. I have no first hand experience. I assume the only way to truly tell is to have both right in front of you and have them go thru the same speaker...
Is the B25 similiar to the V4 or the B15? I've heard that it was a higher powered B15. The V4 is a great amp, but it doesn't have the nuance that I love in the B15. I tried them both with the same cab, and came to the conclusion that the B15 has a more open growly sound. This may not make any sense at all, but the B15 seemed to add more to each note, whereas the V4 had more focus on the whole phrase.
Psycho, how powerful is the B15-S? Can that be run with an 8 ohm cab?
The B25 is closer to being a mix between the B15N and B15S. It has the tube rectifer of the B15N, but higher power like the B15S. An actual B15S is more powerful since it has s/s rectifiers. It's not very close to the V4.
It's 60 watts and can be run into 2, 4, or 8 ohms.
Sold it because I wasn't using it enough. I also think that after some 29 years a change was due. I maybe shoulda bought a new strap or something.
I ended up keeping the original dolly, if someone needs it.
My current "if I win the lottery amp" is a V4B head and two B15S cabs (EV B's or some Emminance's) from Fliptop, Anvil flight cases.
Quick question. How much of the B15 tone is from the speaker? Many used CTS's (15G54B I think), not all obviousely, some had Emminance, some had JBL'S and maybe even Altecs. Some other Portaflexs came with Altecs.
Best of Luck,
Wesley R.
I ended up keeping the original dolly, if someone needs it.
My current "if I win the lottery amp" is a V4B head and two B15S cabs (EV B's or some Emminance's) from Fliptop, Anvil flight cases.
Quick question. How much of the B15 tone is from the speaker? Many used CTS's (15G54B I think), not all obviousely, some had Emminance, some had JBL'S and maybe even Altecs. Some other Portaflexs came with Altecs.
Best of Luck,
Wesley R.
X
X Wolf
Guest
I had an Ampeg B-15S Portaflex back in the early Seventies which I had bought new and kept for 7 or 8 years, I wish I still had it! It came with an Altec 421A 15" and had 60 watts. What a great amp! I did a lot of recording and performing in small venues with that amp and the only reason I sold it was the weight of it finally got to me, LoL.
George
By the way, I consider my B-15S, SVT with 2-V4B bottoms and my Sunn 200S with 2-215 bottoms the three best sounding amps I ever played through.
George
By the way, I consider my B-15S, SVT with 2-V4B bottoms and my Sunn 200S with 2-215 bottoms the three best sounding amps I ever played through.
Okay, how about this?
What settings do you use on your B-15n?
e.g. where do you have your volume, bass and treble set? if it's a later b-15n, do you engage the hi and/or lo switch?
What settings do you use on your B-15n?
e.g. where do you have your volume, bass and treble set? if it's a later b-15n, do you engage the hi and/or lo switch?
