Ampeg B15N 1965 "8 Ohm Ext Speaker" ??

Nov 20, 2016
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Hey all,
So i was considering converting the proprietary 4 pin (protection circuit) speaker cable into a standard 1/4" speaker cable situation and simply shorting the protection pins at all times, so that i can quickly and easily use this amp standalone with other 8 ohm cabs of various sorts.

But then i saw the rear panel 1/4" jack labeled "8 Ohm Ext Speaker". But i can't find the original user manual for the life of me, and the schematic somehow isn't making things clear to me here...

What is the deal with this jack.. I don't want to start plugging things in until i know what's going on.. If i leave the proprietary 4 pin connector pig tail unconnected totally, and hook up a speaker cable to that Ext jack and hook that to an external 8 ohm cab, what will be happening at that point? Will the amp simply run as normal and power the speaker as if it were it's own speaker?

2 questions..

1, does the protection circuit still need to be artificially shorted for this Ext jack to work if the real cab is not connected?
2, is this Ext jack 8 ohms or 16ohms?

Also, does anybody know where to find the original user manual? All i can find is the dang Heritage version just released.

thanks!
 
When you connect a speaker to the EXT jack on the B-15, you are making a series connection with the speaker connection in the attached speaker cable, and connecting to the 16 Ohm tap on the output transformer, and so you need the safety circuit shunt to take the amp out of standby as well as an 8 Ohm speaker connected to the speaker wires in the cable too.
 
Also, does anybody know where to find the original user manual? All i can find is the dang Heritage version just released.

Top of the page to to the "wiki" section, check out the ampeg portaflex wiki pages. @beans-on-toast has done a very nice job collecting all of this information and linking to as many older documents as people have been able to find available. I believe there are at least a few versions of the old manuals up there, as well as schematics, product catalogs, etc.
 
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1) yes. It isn’t just about the safety circuit being closed to use the amp. The main speaker has to be connected when using the ext spkr jack as explained above.

2) the external speaker should be 8 ohms

The B-15 operating manual is in the TB Portaflex wiki. See the user manuals section.

You can buy a cable adapter from fliptops.net or build one, see the wiki, that will do what you need. The adapter cable has a 1/4” plug (or a speakon) at one end and a 4-pin connector at the other end. The shorting jumper should be included in the 4-pin connector. Plug the amps cable into the adapter cable, then the other end into a speaker cabinet.
 
When you connect a speaker to the EXT jack on the B-15, you are making a series connection with the speaker connection in the attached speaker cable, and connecting to the 16 Ohm tap on the output transformer, and so you need the safety circuit shunt to take the amp out of standby as well as an 8 Ohm speaker connected to the speaker wires in the cable too.

Thanks for the replies everyone.
So, on the tap note, just to be clear for overall understanding of how it works... With nothing connected to the Ext Speaker jack, but with the amp connected as normal to the 4pin cab, the amp is seeing only 8ohms. But with both the normal cab hooked up and the Ext Speaker also hooked up, the amp is now seeing 16ohms, with each speaker providing half of it?

I thought about repurposing that 1/4" Ext Speaker jack to be my only speaker output. Just wire the protection circuit together internally. But I'm probably just going to convert the 4pin output pin tail connector into a self wired protection point along side a female 1/4" jack.

Either way, I'm not planning on being sloppy and turning the amp on accidentally with nothing connected, and nobody else uses my rig. I just want to be easily using the amp for other application. I'll also convert the B-15N speaker itself to a 1/4" input.

Any thoughts?
 
Thanks for the replies everyone.
So, on the tap note, just to be clear for overall understanding of how it works... With nothing connected to the Ext Speaker jack, but with the amp connected as normal to the 4pin cab, the amp is seeing only 8ohms. But with both the normal cab hooked up and the Ext Speaker also hooked up, the amp is now seeing 16ohms, with each speaker providing half of it?

Yes. Inserting the plug into the ext amp jack connects the speakers in series and switches the speakers onto the 16 ohm tap. The impedance is matched.


I thought about repurposing that 1/4" Ext Speaker jack to be my only speaker output. Just wire the protection circuit together internally. But I'm probably just going to convert the 4pin output pin tail connector into a self wired protection point along side a female 1/4" jack.

Either way, I'm not planning on being sloppy and turning the amp on accidentally with nothing connected, and nobody else uses my rig. I just want to be easily using the amp for other application. I'll also convert the B-15N speaker itself to a 1/4" input. Any thoughts?

This is a commonly seen mod.

In terms of quiet operation, use isolation washers on the main speaker output jack to prevent the speaker return from running through the chassis. This is done with a Switchcraft S-1028 and S-1029. The chassis hole may need to be slight enlarged just enough so that the washer’s collar fits in. This can be done with a couple of turns of a small file. This is already done for the ext spkr jack. A new set of washers there never hurts. The original cable isolated the speaker return from the chassis. It can help the amp to be quieter. The speaker return should be connected close to where the phase inverter is grounded. The black OPT secondary ground should also be connected here. This is easiest to implement on the turret based B-15’s.
 
Yes. Inserting the plug into the ext amp jack connects the speakers in series and switches the speakers onto the 16 ohm tap. The impedance is matched.




This is a commonly seen mod.

In terms of quiet operation, use isolation washers on the main speaker output jack to prevent the speaker return from running through the chassis. This is done with a Switchcraft S-1028 and S-1029. The chassis hole may need to be slight enlarged just enough so that the washer’s collar fits in. This can be done with a couple of turns of a small file. This is already done for the ext spkr jack. A new set of washers there never hurts. The original cable isolated the speaker return from the chassis. It can help the amp to be quieter. The speaker return should be connected close to where the phase inverter is grounded. The black OPT secondary ground should also be connected here. This is easiest to implement on the turret based B-15’s.

Thank you this is perfect.
I dug around the web for the s1028 and s1029 and don't quite get images that make sense to me. Then again this may be because the images don't actually represent the real items.
But on all the images i don't see any collar'ed washers. Just two different materials of flat washers.
Can you help me out by explaining which should be shaped like what?
 
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Thank you this is perfect.
I dug around the web for the s1028 and s1029 and don't quite get images that make sense to me. Then again this may be because the images don't actually represent the real items.
But on all the images i don't see any collar'ed washers. Just two different materials of flat washers.
Can you help me out by explaining which should be shaped like what?

To see an example, you should have a pair of these washers on your amp's ext spkr jack,otherwise the amp will have a ground loop hum. Where you buy them from depends on where you live. In the US mouser.com and tubesandmore.com both sell them. Data sheets from Switchcraft are attached below.

The washers are made out of an electrically non conductive material. One piece has a collar that fits in the chassis hole from inside. The jack is inserted.This centers the jack and prevents it from comming in contact with the chassis. The flat washer goes on the outside, then the jack washer, then the jack nut. The outside washer prevents the jack washer and nut from contacting the chassis.
 

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To see an example, you should have a pair of these washers on your amp's ext spkr jack,otherwise the amp will have a ground loop hum. Where you buy them from depends on where you live. In the US mouser.com and tubesandmore.com both sell them. Data sheets from Switchcraft are attached below.

The washers are made out of an electrically non conductive material. One piece has a collar that fits in the chassis hole from inside. The jack is inserted.This centers the jack and prevents it from comming in contact with the chassis. The flat washer goes on the outside, then the jack washer, then the jack nut. The outside washer prevents the jack washer and nut from contacting the chassis.

Ok great thank you now i see in the data sheet that the second has a collar. Totally makes sense and i'll check out the Ext Speaker jack just out of curiosity.
Awesome!

One last concern.. I've done lots of shopping/constructing of line audio connections/jacks and such, but i've never assembled a speaker cable connector/jack. Is there any sort of "gauge" issue with jacks/connectors in this application? I'm using 12 gauge speaker wire from Redco which i already had built a custom long version of the proprietary speaker cable and used new 4 pin xlr connectors of the same brand as ampeg had used. That cable seems great. But for example, do all Switchcraft 1/4" jacks/connectors do a good enough job conducting so that i'm not adding any significant impedance to the signal?
 
One last concern.. I've done lots of shopping/constructing of line audio connections/jacks and such, but i've never assembled a speaker cable connector/jack. Is there any sort of "gauge" issue with jacks/connectors in this application? I'm using 12 gauge speaker wire from Redco which i already had built a custom long version of the proprietary speaker cable and used new 4 pin xlr connectors of the same brand as ampeg had used. That cable seems great. But for example, do all Switchcraft 1/4" jacks/connectors do a good enough job conducting so that i'm not adding any significant impedance to the signal?

For this application, a low wattage amp, a 1/4” jack works well. Over 300W, a speakon is the right choice. The only concern about a 12 gauge cable is the weight. Some cables are very heavy and can apply too much downward pressure on the jack and bend the internal jack contact.

If you look at a standard instrument plug, that size plug is good for a lighter gauge. By lighter I’m talking about 18 gauge. Normally, with a larger 12 gauge cable, larger sized speaker cable plugs are used. This helps ensure that no shorts occur inside the the plug case. A heavy gauge wire with a standard sized plug has the wires too close. A little bending and it shorts.
 
For this application, a low wattage amp, a 1/4” jack works well. Over 300W, a speakon is the right choice. The only concern about a 12 gauge cable is the weight. Some cables are very heavy and can apply too much downward pressure on the jack and bend the internal jack contact.

If you look at a standard instrument plug, that size plug is good for a lighter gauge. By lighter I’m talking about 18 gauge. Normally, with a larger 12 gauge cable, larger sized speaker cable plugs are used. This helps ensure that no shorts occur inside the the plug case. A heavy gauge wire with a standard sized plug has the wires too close. A little bending and it shorts.

Roger that. Understood.
Could you perhaps point me to a known and proper 1/4" speaker connector on Mouser/Allied/Digikey that will work well with 12 gauge? (I'm in the US) I feel like "speaker connector" doesn't give me at all the direction i need for searching. Does Switchcraft make one? Also, know of a similarly robust 1/4" female jack for panel mount?
 
Here are a couple, you can see if the model number is available from the other suppliers. Check the specs to ensure that the end will slide over the cable.

Anything Neutrik is good.

"Neutrik Rean NYS225L 1/4" Mono Plug Nickel For Speaker Cable" from www.parts-express.com!

"Neutrik Rean NYS225 1/4" Mono Plug Nickel For Speaker Cable" from www.parts-express.com!

"Neutrik Rean NYS225B 1/4" Mono Plug Black For Speaker Cable" from www.parts-express.com!

Switchcraft makes one, 184L as I recall.

Thank you for all of this!
 
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I just acquired a B15N, 1967, myself. The amp is nearly perfectly original. The person who I received the amp from showed me the receipts of the most recent work he had done and it was done by the man who designed the circuit for these amps. Ver cool indeed.

The only thing different is at some point in time, someone removed the 4-pin-XLR and reattached a 1/4 inch cable. I do not know why anyone would do this, other than like you describe, to use as a standalone head. I know this thread is old, but I would strongly suggest anyone thinking about using a B15N as a standalone head and desiring a 1/4 inch connector to consider a converter from the stock 4-pin-XLR. If you have the means to acquire such an amp, and another enclosure to pair it with, you can probably afford a converter.

I am strongly thinking about switching it back to the XLR for the original appeal, as well as added safety. I know why companies prefer 1/4 inch over 4-pin-XLR (cost, simplicity, etc), but I do not know why engineers allowed this change to happen in the industry. I think I would sooner insert a 4-pin-XLR in a different cab before changing a b15N.

Just my thoughts.