Jess Oliver's legacy - AMPEG PORTAFLEX CLUB part 9

[jnewmark]

Originally Posted by Ric Vice

J Newmark,
Wow, that one's showroom fresh. I don't think the Ampeg covers ever had logos. The Blue Checked ones just matched the cabinet covering. My recollection is that the newer ones just had a black vinyl cover. I had a friend who owned one of these amps, and I think his cover just slipped over the cabinet, but it's been awhile, so I could be wrong. That amps in beautiful condition. I had forgotten that the newer ones, had their speaker connections, at the rear of the cabinet, that's interesting

Ric

Yeah, it really is clean, inside and out. I have'nt been able to find a cover that matches it online anywhere, so I think it was a custom job, or maybe a small run of covers that Ampeg had made. Really dig the speaker connects in the back, never really understood why the early ones had them on the side, or why, for that matter, they used that 4 pin plug, instead of the usual 1/4 jack. I think I read an interview with Jess where he mentioned that they had a bunch of them laying around, so they went with them. The extension speaker jack is obviously a 1/4 female. If you wanted to run another Ampeg cab, what were you supposed to do, make your own custom cable? Also, I see on the back of the chassis, in the top left corner, is the number 105699. Maybe it's a '69 , with a '71 replacement driver ?

[Ric Vice]

Originally Posted by jnewmark

.......... never really understood why the early ones had them on the side, or why, for that matter, they used that 4 pin plug, instead of the usual 1/4 jack. I think I read an interview with Jess where he mentioned that they had a bunch of them laying around, so they went with them. The extension speaker jack is obviously a 1/4 female. If you wanted to run another Ampeg cab, what were you supposed to do, make your own custom cable? Also, I see on the back of the chassis, in the top left corner, is the number 105699. Maybe it's a '69 , with a '71 replacement driver ?

J Newmark,

Just so you know they had a very good reason for the 4 pin connector. If you left the speaker chord unplugged, it threw the amp into standby, to protect the transformer. On the Blue Check amps there was an 1/4" auxiliary speaker jack and an auxiliary amp jack. This was a pretty forward thinking design, you can connect two B-15's together and slave one of the amps off the pre amp of the other. Your amp has both these jacks as well. Since it's an 8 ohm amp, the extension speaker jack connects to a 16 ohm tap on the transformer, so the amp maintains an 8 ohm load running two cabinets. Others, can explain how this works better than I.

Ric

[beans-on-toast]

That's one nice amp. It screams to have some R&B played through it. I've never seen a cover as nice as that one on an amp. It sure did protect it over the years.

Remember back then when people used to get covers made for their sofa's and even their car seats? Well at least in my neighborhood...

[jnewmark]

Originally Posted by beans-on-toast

That's one nice amp. It screams to have some R&B played through it. I've never seen a cover as nice as that one on an amp. It sure did protect it over the years.

Remember back then when people used to get covers made for their sofa's and even their car seats? Well at least in my neighborhood...

Funny you should mention sofa covers - my wife and I are trying to find an upholsterer who can make a cover for our 30 year old sofa ! Some things never die. RnB you say ? The first riff I played on that amp was the bass line to the Four Tops, " I Can't Help Myself ". For a few seconds there, I was James Jamerson. What do you think of that number I found on the left top corner of the chassis ?

[beans-on-toast]

Keep channeling Jamerson. I bet it sounds great. I've just been playing along with Darlene Love's Christmas (Baby Please Come Home). She's on Letterman tonight.

Your serial number doesn't fit into the 68-69 numbers that started with zero. Ampeg changed their serial numbering scheme in 1970. The amps produced from 1968-1972 were identical. They had the blue control panel with the black switches like yours. After that, the graphics were changed to black. So my best guess would be 1968-1972.

Here are the R&B number one singles from that era. That amp is in good company whatever year it was made.

1972 1971 1970 1969 1968

[jnewmark]

Wow, I did'nt know " Groove Me " was that big of a hit. Played it for years - a big club favorite for sure.

[jnewmark]

Originally Posted by Ric Vice

J Newmark,

Just so you know they had a very good reason for the 4 pin connector. If you left the speaker chord unplugged, it threw the amp into standby, to protect the transformer. On the Blue Check amps there was an 1/4" auxiliary speaker jack and an auxiliary amp jack. This was a pretty forward thinking design, you can connect two B-15's together and slave one of the amps off the pre amp of the other. Your amp has both these jacks as well. Since it's an 8 ohm amp, the extension speaker jack connects to a 16 ohm tap on the transformer, so the amp maintains an 8 ohm load running two cabinets. Others, can explain how this works better than I.

Ric

Ok, I get the aux amp jack function, but my ext speaker jack question still stands. It clearly calls for a 1/4 end from the B15. But what if you wanted to use another Ampeg cab from that era as an ext cab, ( along with the internal speaker connected ), that has the 4 pin jack on it ? You would have to use a 1/4 male jack/4 pin female cable to do it. Did they exist back then ? I know they do now, I just ordered one from Fliptops.

[beans-on-toast]

Originally Posted by jnewmark

Ok, I get the aux amp jack function, but my ext speaker jack question still stands. It clearly calls for a 1/4 end from the B15. But what if you wanted to use another Ampeg cab from that era as an ext cab, ( along with the internal speaker connected ), that has the 4 pin jack on it ? You would have to use a 1/4 male jack/4 pin female cable to do it. Did they exist back then ? I know they do now, I just ordered one from Fliptops.

I've never seen extension cabs or cables offered in the Ampeg catalogs or parts lists. Matching extension cabs were mentioned in the user's manuals so they sold them.

A music store or neighborhood radio/TV repair shop would build whatever cables you needed. One brochure says that Ampeg had four factory service centers in the US, located in Chicago, Hollywood, Nashville, and Linden. This was in addition to over 50 authorized service centers.

[jnewmark]

Originally Posted by beans-on-toast

I've never seen extension cabs or cables offered in the Ampeg catalogs or parts lists. Matching extension cabs were mentioned in the user's manuals so they sold them.

A music store or neighborhood radio/TV repair shop would build whatever cables you needed. One brochure says that Ampeg had four factory service centers in the US, located in Chicago, Hollywood, Nashville, and Linden. This was in addition to over 50 authorized service centers.

Oh well, no big deal, I've got cables on the way if I ever want to mix n match cabs. Just wondered why, in one instance, Ampeg went the 4 pin route, and then in another instance, on the same amp, went the 1/4 route. Why not make it 4 pin in both ? One of the mysteries of life, I guess, like, why do we park on a driveway, and drive on a parkway ?

[Ric Vice]

Originally Posted by jnewmark

Oh well, no big deal, I've got cables on the way if I ever want to mix n match cabs. Just wondered why, in one instance, Ampeg went the 4 pin route, and then in another instance, on the same amp, went the 1/4 route. Why not make it 4 pin in both ? One of the mysteries of life, I guess, like, why do we park on a driveway, and drive on a parkway ?

+1
J Newmark,
My educated guess is that someone at Ampeg (probably Jess Oliver) considered the possibility that players might want to use a cabinet built by another manufacture. If they had gone with a 4 pin on the rear of the head, the available cabinets you could have used with that head, would have been limited to Ampeg Cabinets. I've never seen the dual cabinet B-15ND that was sold with a matching extension, but my guess is that they just sold a cord with a 1/4" plug on one end and a 4 pin on the other. I've made a couple of cords like that, it's pretty simple.

Ric

[jnewmark]

As far as the vinyl cover goes, I read a 1971 Ampeg catalog online, and it mentions an " optional " cover for the B15N, but, of course, does'nt show one. The one offered at Fliptops, says it's " an authentic '60's B15N cover", but it doesn't look like mine, the way it is made.

[beans-on-toast]

The four-pin vs 1/4" is explainable.

Initially they used the four-pin because two wires went to the speaker. The other two were used a safety interlock. They wanted to protect the amp in case it was turned on without the speaker cab connected. If you do this with a tube amp you can blow the power amp. The solution was to route the high voltage from the power supply through the four wire speaker cable to the cab and back to the amp. The four-pin connected acted like a switch. If it was plugged into the cab, the two end of the high voltage wires got connected together, the circuit was completed and the high voltage would turn on. Ampeg used this scheme on the early SVT as well as other amps.

The safety regulators were not happy with this because it was dangerous and could lead to electrocution. Ampeg's approach was to keep the four-pin connector but to not send the high voltage out on the speaker cable. Instead they used a circuit loop through the speaker cable to disable the phase inverter in the amp. This is a tube stage just prior to the power tubes. This only carried a couple of volts so it was safe. It accomplished the same thing, it shut down the power amp if the cable to the speaker cab was not plugged in.

They changed this approach when your amp came along. They eliminated the four-pin connector and went with a 1/4". They used another way of protecting the amp. They installed a 250 ohm resistor across the output transformer where the speaker connects. If the amp is turned on without the speaker connected, the resistor is seen as a load and the power amp is protected. This is the best and most elegant approach to use. It is used by other amp manufacturers as well. Now the speaker cable only has two wires in it so they went with the 1/4" connectors.

In some ways the four-pin connector was better in that, like a speakon connector that followed, it locked into place. You couldn't pull it out as easily as a 1/4" plug could be. The advantage of the 1/4" connector is that it was easy to use other cabinets with the head. With the four-pin connector, unless you had an adaptor, you had to use the Ampeg cabinet. Maybe this was for marketing purposes, they sold more Ampeg cabinets. Having said that, I didn't know anyone back then who used an extension cab with their portaflex. If the amp wasn't loud enough, it would be mic'ed.

[jnewmark]

Originally Posted by beans-on-toast

The four-pin vs 1/4" is explainable.

Initially they used the four-pin because two wires went to the speaker. The other two were used a safety interlock. They wanted to protect the amp in case it was turned on without the speaker cab connected. If you do this with a tube amp you can blow the power amp. The solution was to route the high voltage from the power supply through the four wire speaker cable to the cab and back to the amp. The four-pin connected acted like a switch. If it was plugged into the cab, the two end of the high voltage wires got connected together, the circuit was completed and the high voltage would turn on. Ampeg used this scheme on the early SVT as well as other amps.

The safety regulators were not happy with this because it was dangerous and could lead to electrocution. Ampeg's approach was to keep the four-pin connector but to not send the high voltage out on the speaker cable. Instead they used a circuit loop through the speaker cable to disable the phase inverter in the amp. This is a tube stage just prior to the power tubes. This only carried a couple of volts so it was safe. It accomplished the same thing, it shut down the power amp if the cable to the speaker cab was not plugged in.

They changed this approach when your amp came along. They eliminated the four-pin connector and went with a 1/4". They used another way of protecting the amp. They installed a 250 ohm resistor across the output transformer where the speaker connects. If the amp is turned on without the speaker connected, the resistor is seen as a load and the power amp is protected. This is the best and most elegant approach to use. It is used by other amp manufacturers as well. Now the speaker cable only has two wires in it so they went with the 1/4" connectors.

In some ways the four-pin connector was better in that, like a speakon connector that followed, it locked into place. You couldn't pull it out as easily as a 1/4" plug could be. The advantage of the 1/4" connector is that it was easy to use other cabinets with the head. With the four-pin connector, unless you had an adaptor, you had to use the Ampeg cabinet. Maybe this was for marketing purposes, they sold more Ampeg cabinets. Having said that, I didn't know anyone back then who used an extension cab with their portaflex. If the amp wasn't loud enough, it would be mic'ed.

As usual, you da man ! Thanks. So, if I do use another cab out of the extension cab with the 1/4 jacks, can the 4 pin cable just hang there ? In other words, can the extension jack be used exclusively sans 4 pin cable ? Or do you always have to have two cabs running when using the extension speaker out ?

[beans-on-toast]

The short answer is you have to use the main cab if you are plugging in an 8 ohm extension cab. If you just plugged in to the ext spkr jack the amp would be expecting to see a 16 ohm load.

How this works varies depending on the B-15 revision. On your particular amp, if you just plugged in to the ext spkr jack, your speaker load would need to be 16 ohms for it to not have any impedance matching issues. So if you had two 8 ohm cabs and a cable that serial daisy chained them to give you 16 ohms, you could plug that into the ext spkr jack and not use the main cabinet.

Some people rewire the amp a little. They remove the speaker cable and install a 1/4" jack in its place. Then they can plug in any 8 ohm cab to the new jack and add a second 8 ohm cab via the ext spur jack. It makes life a little less complicated at the expense of a reversible amp modification.

[beans-on-toast]

To add to what I was saying above, here is a copy of the output transformer secondary and speaker wiring for the 1968 and later B-15N revisions. The earlier B-15 models are wired a little differently, they don't have the 250 ohm resistor, but otherwise are the same as far as speaker wiring/routing is concerned.

There are a couple of things to note. The green wire is the 16 ohm transformer output or tap. The yellow wire is the 8 ohm tap. Built into the ext. spkr. jack (J6), there is a switch depicted by the arrow that I have circled in green. The switch is normally closed as shown in this image. When you insert a plug, the switch opens.






The image below shows how the main cabinet is connected to the output transformer when no external speaker is connected. The 8 ohm tap is connected to the speaker through the switch in the ext spkr jack. The 16 ohm tap is not connected. The path of the circuit is shown in yellow.






When you plug an extension cab into the ext. spkr. jack, the switch opens. The arrow isn't connected to anything now and this disconnects the 8 ohm tap. At the same time, the external speaker inserts itself, in series, to the main cab's speaker and both speakers are connected to the 16 ohm tap. The path of the circuit is shown in green.






Note the 250 ohm, 10W resistor. This is the safety resistor that I mentioned earlier. It is connected to the 16 ohm tap. When an external speaker is connected, the 250 ohm resistor is in parallel with the speakers. Some wonder, doesn't this affect the impedance? The answer is not really. When you connect a small resistor in parallel to a large resistor, the parallel resistance is about the same size as the small resistor. In this case, 250 ohms in parallel with 16 ohms works out to 15 ohms. Close enough. In practice, the true impedance of the speakers is lower than 16 ohms so the 250 ohm resistor matters even less. So it doesn't cause an impedance mismatch.

[nouroog]

Amazing contribution as always, David.

Thanks a lot

[jnewmark]

Originally Posted by nouroog

Amazing contribution as always, David.

Thanks a lot

+10000 !

[jnewmark]

I know Jimmy M does this, but does anybody else gig with two cabs and the B15N ? Just wondering how much more volume, loudness I will get with another 15 cab ( loaded with a JBL K140 ), and if it would be worth it to start lugging around two heavy pieces of gear, not to mention having to unscrew that dolly everytime.

[JimmyM]

Originally Posted by jnewmark

I know Jimmy M does this, but does anybody else gig with two cabs and the B15N ? Just wondering how much more volume, loudness I will get with another 15 cab ( loaded with a JBL K140 ), and if it would be worth it to start lugging around two heavy pieces of gear, not to mention having to unscrew that dolly everytime.

Nice thing about two cabs is not having to unscrew the dolly because a cab is up by your ear. And while I wouldn't say it doubles your available volume, it adds a good amount. That said, I'm taking either my Micro VR or BA110 to my gig today

[coreyfyfe]

I've been doing my cover gigs with my B15 and PF210 as an extension cab. Volumes use its been pretty good. I've had the B15 pushing pretty hard to keep up but it sounds great. Unfortunately it blew a fuse last night, so I'm working out the cause at the moment. Hopefully nothing serious.