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Official Ampeg Portaflex Club

#881
beans-on-toast said:
The output transformer on the B15N has 8 and 16 ohm taps. The 4-pin connector is connected to the 8 ohm tap. The speaker in the ampeg cab is rated at 8 ohms. So when you plug in the cab through the 4-pin connector, you are matching the 8 ohm tap with the 8 ohm speaker.

The external jack is intended to be connected to a second 8 ohm speaker cabinet while the 4-pin cable is connected to the main cabinet. In this configuration, you are using the 16 ohm transformer tap connected to the two 8 ohm speakers connected in series whose impedance totals 16 ohms.

The connections are wired (at least on my amp) so that you can not use just the external speaker jack and leave the 4-pin plug unconnected.

If you had a dummy 4-pin socket with pin-2 and pin-3 shorted, and pin-1 and pin-4 shorted, you could plug this into the 4-pin plug coming out of your amp and connect a 16 ohm cabinet to the external speaker connector on the amp.

This should all be as clear as mud now. :crying:

David
Click to expand...

So, with two cabs, you are running a 16ohm load with about 25 watts?
 
#882
jnewmark said:
So, with two cabs, you are running a 16ohm load with about 25 watts?
Click to expand...

That's correct. The wattage varies depending on the model of B-15N that you have. The early ones were stated as 25 Watts. If you use one cabinet or two, in this case, the output wattage will be the same.

Now, in general, there are those that claim that the sound will vary depending on which output tap is used. I don't buy it but that's another story.


David
 
#886
kittyboy said:
Are they sharing the watts but there's still a net gain, or do they each have 25watts going through them?
Click to expand...
They share the watts. I wish I could explain scientifically why when you split the watts between two cabs the overall volume is much louder, but all I know is there's more air movement with two cabs.
 
#887
kittyboy said:
Are they sharing the watts but there's still a net gain, or do they each have 25watts going through them?
Click to expand...

What Jimmy said is correct. If you have two identical speakers, half the power (watts) will be going through each.

more speakers = more air movement = more volume BUT you can't just keep adding speakers. This can only be done correctly if the impedances are matched.

In the case of the B-15N, the amp sees the same speaker load (it can't tell the difference) if you have one 8 ohm, 15" speaker connected to the 8 ohm tap or if you have two 8 ohm, 15" speakers in series connected to the 16 ohm tap. If you have two speakers connected, more air will be moved and it is going to be louder.

Since the speakers are sharing the power, they are being pushed less, which means less distortion. This means that you can turn up the amp and get even more volume out of the rig. So you win two ways.


David
 
#890
kittyboy said:
Thanks for the info fellas!

So just to be totally clear:

I run one cab off the main tap, one cab off the "external speaker" tap, and I'm good? Both are running at 8ohms and are happy?
Click to expand...

Yes. Just be sure that the external cab is rated 8 ohms.

I don't want to confuse you even more, but if the speakers in each cab are the same and the cabinets are the same, the sound will be balanced. If you are using different types of speakers and/or cabinet designs you might find that one cab will be louder or sound different than the other. Some people find that this is a good thing.

It's all very complicated and confusing.


David
 
#891
beans-on-toast said:
Yes. Just be sure that the external cab is rated 8 ohms.

I don't want to confuse you even more, but if the speakers in each cab are the same and the cabinets are the same, the sound will be balanced. If you are using different types of speakers and/or cabinet designs you might find that one cab will be louder or sound different than the other. Some people find that this is a good thing.

It's all very complicated and confusing.


David
Click to expand...

+1 I tried this with a bigger cab, way dominated the other.
 
#893
JimmyM said:
That's why my mind is boggled when people run 410/115 rigs. The 410 clearly dominates the volume so the 15" might as well not even be there IMHO.
Click to expand...

I've wondered about that as well. If the 115 had a higher sensitivity spec, that would help balance the volume. According to Ampeg, their 410HE enclosure has a sensitivity of 95dB; their 115E has a sensitivity of 97dB. It will take less power to drive the 115 to the same level as the 410HE. Maybe this is sufficient to balance the volumes between these two cabs.

On the other hand, the 410HLF has a sensitivity of 99dB. It is going to blow the 115E away.


David
 
#894
beans-on-toast said:
I've wondered about that as well. If the 115 had a higher sensitivity spec, that would help balance the volume. According to Ampeg, their 410HE enclosure has a sensitivity of 95dB; their 115E has a sensitivity of 97dB. It will take less power to drive the 115 to the same level as the 410HE. Maybe this is sufficient to balance the volumes between these two cabs.

On the other hand, the 410HLF has a sensitivity of 99dB. It is going to blow the 115E away.
Click to expand...
How trustworthy are sensitivity specs, though? I was reading on here that they list sensitivity for a given frequency but it changes with the frequency range. I've done the 410HE/115E thing with rentals and it always seems to me like the 115 is pretty well useless. But this is just something I've started trying to learn about, so I could be all wet and just heard the 410 better because it was closer to my head.

BTW, it blows me away that the HLF has a higher sens rating than the HE. It's almost always the exact opposite with bigger cabs that have a huge low end extension.
 
#895
If you had a powerful enough stereo power amp, and ran each cab in it's own channel, could'nt you give the 115 more juice than the 410 to balance it out?
 
#896
JimmyM said:
How trustworthy are sensitivity specs, though? I was reading on here that they list sensitivity for a given frequency but it changes with the frequency range. I've done the 410HE/115E thing with rentals and it always seems to me like the 115 is pretty well useless. But this is just something I've started trying to learn about, so I could be all wet and just heard the 410 better because it was closer to my head.

BTW, it blows me away that the HLF has a higher sens rating than the HE. It's almost always the exact opposite with bigger cabs that have a huge low end extension.
Click to expand...

I'm thinking that the 115 in these rigs is there to add some of the lower end that the 410 can't produce. Maybe you're supposed to feel it more than hear it. I would trust your ears. If they're telling you that the 115 isn't making a difference, then it's probably the case.

It's true that you have to be careful with specs. They are not always what is claimed.

You are right that some specs are frequency dependent and sensitivity is one of them. When a spec is provided, it is based on standards adopted by the audio industry to help people compare different products. Specs can vary depending on how a company interprets a standard and performs the measurements. Here is one example of how to sensitivity is measured. http://www.goodsound.com/features/2008_02_01.htm
The sensitivity number that they come up with is an average over a range of frequencies measured in a consistent way.

Speaker impedance is another good example. An 8 ohm delta-15A speaker varies in impedance between 6 to over 60 ohms depending on the frequency applied. When you play a note on your bass, you are producing the fundamental frequency and it's harmonics. The speaker is seeing a range of frequencies all at once. The impedance is always changing. It is an 8 ohm speaker but when it is connected to a working amp, it doesn't measure 8 ohms. This happens with some of the other specs as well.

I think that specs get you in the ball park to help you decide what speaker or enclosure to match with your amp but the only real way to evaluate something is to use your ears.

In the case of the two Ampeg cabinets, if they derived their sensitivity specs by making the measurements in a consistent way, then you should be able to rely on their claims when comparing one cab to another. When you start comparing cab specs from different companies, all bets are off.


David
 
#897
jnewmark said:
If you had a powerful enough stereo power amp, and ran each cab in it's own channel, could'nt you give the 115 more juice than the 410 to balance it out?
Click to expand...

Yeah, that's a nice way to go. Or use an attenuator on the 410 cab.

A lot of players never seem to be happy with their sound. People spend their life buying and selling equipment in search of musical nirvana. It's a wonderful thing when you can find it ... a B-15N.


David
 
#898
Thx for the link and explanation, David. Sensitivity is something I never really noticed before, since you can always turn the amps up. But in a mixed rig, it becomes way more important. Just another reason why I don't like to use mixed rigs. They're always finicky somehow. A stereo amp with separate volumes like Jnewmark was asking about would help, but I can't hang with the phasing, either. If I get a mixed rig supplied on a gig, I always unplug one cab unless it's someone's personal rig that they're using.
 
#899
Follow up:

Gigged last night locally at a smallish venue with my 66 b15n and 65 Precision bass. Acoustic Guitar, vocals, drums, percussion and electric guitar. I was not going through the PA. My volume knob was set just around 9 o'clock.

I had plenty of head room and the sound was dark and warm. Glorious! I am loving this amp.
 
#900
Recently had to replace the original stock CTS in my '67 B15N. Man, did I have a tough time getting that bottom nut off the wood screw at the bottom of the speaker! The screw stuck out so much that I could'nt get a socket wrench deep enough to get at the nut. I finally had to use the socket by itself and hand turn it until the nut got loose enough to screw off by hand. I replaced the 15 with a JBL K140. Different sound, as the K140 is more of a full range sounding speaker, but can handle more power than the CTS. Getting a little more top end from the JBL also.
 

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