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Official Ampeg Portaflex Club

#8,161
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.
 
#8,162
beans-on-toast said:
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.
Click to expand...

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 ?
 
#8,163
Can anyone in hear comment on the tonal differences between the PF115He/hlf and the SVT15e? I currently use a diesel 1x15 which I love the tone of but i am having a really hard time find another one to pair with it. Its ported, but has really tight lows, the mids are very aggressive, and the highs sparkle. Im not a fan of tweeters, though I guess I could just turn it off if I got a cab that has one. Also I am superficial as hell, I hate the looks of the PF cabs but if they sounded great itd be tolerable. Havent seen svt15e's or pf cabs at my local guitarcenters or i wouldve tried them out.
 
#8,164
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.
 
#8,165
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.
 
#8,168
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.
 
#8,169
jnewmark said:
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.
Click to expand...
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 ;)
 
#8,170
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.
 
#8,171
I used a 2 cab B-15 stack back in the day (66-69). Yeah, wheeled 'em in, took the dolly off the main cab and fastened the lid of the ext cab into the bottom of the main cab with the dolly's knobbed bolt and fastened them together with the lid clips. I skipped the fasten once and it fell off, so from then on every time. I cant put a number on it, but adding the 2nd B-15 cab made a huge difference, made my bass audible among 2 guitars with Twin Reverb amps, a Farfisa playing through a Gemini II and a flashy drummer (think Dino Danelli). It also helped me and the others to have some sound coming out closer to ear height. My local idol worked at Ampeg in Linden and had the same stack with a B-18 head (before the ND was marketed). He used to be known as the loudest bass player around. (He sold me my ext cab for $25! I always wondered if it was hot). Remember, this is all prior to FOH support for instruments - if there was a PA it was for vocals only, at least at the club level - and prior to the 100+ watt amps, V4, SVT, Sunn etc. You couldn't get by with the B-15 once the guitarists started going to Marshalls and heavy distortion or keys went to B3 through twin Leslies ( my next band).

Today I have B-15s, B-18 head etc. And I use them mostly in the studio or practice room, occasionally a light gig. My regular gigging amp is a PF500/PF115E and I stack a 2nd 115 when I need more umph. At 61 I like light, simple.
 
#8,172
^ Hmmm, not really sure what you did there with the two cabs. Did the ext cab have a threaded hole on top or something ? Or did you turn the cab upside down so the cabs' latches met up together ? I just can't picture this somehow. I assumed that, once you take the dolly off the main cab, it just sat on top of the bottom ext cab, no ?
 
#8,174
Getting a cab up to ear level makes a big difference when playing. I sometimes stick my single cab on top of a Leslie to get the height. I use a gramma isolation riser to prevent any vibrations. The Leslie cabs can be wobbly. Seems to work out well.
 
#8,176
Thnx Jimmy! And Happy Christmas to all.

Anyone else any experiences with putting a DI-box between the Ext Amp of an SVT or V-4B and the FOH...? Or using no DI at all (but then the signal is not balanced and isolated)?

Stiles - are you using a modern SVT head with an XLR out? Didn't have those back in the days, so the Ext Amp is all I have. But I like the idea of skipping the DI-box altogether, or just using a high-end clean 1/4" jack through XLR kinda thing, if such a thing exists...

Live, I am always miking my 810 used with my SVT or V-4B. Most of the time, I am surprisingly successful in talking the tech into using just that, thanks to the incredible performance of the Heil mic. But if he insists on a second channel for a DI (only a DI is out of the question, of course;-), I just want the cleanest possible Ampeg sound converted into an XLR-signal. Don't have those beasts just for monitoring, right? I have tried a speaker sim DI between the head and the 810 (Palmer PDI-09), but did not like the speaker sim. I can buy a REDDI, but even being the best DI in the traditional sense (tapping right from the instrument) it has little to do with the sound I am looking for. I know sound techs will kill me for not wanting to give them a clean signal straight from my (passive/vintage) bass, but hey - we're on a bass forum.

Now Styles72 set me thinking of tapping from the Ext amp-signal and using that for conversion into an XLR-signal and then to the FOH. Is that a stupid idea? What kind of signal is the Ext amp? Just Pre-amp, I thought... so very well usable for the PA imo. If my theory is correct, I just need the cleanest DI box on the market and feed it from the Ext amp. Brilliant, and the simplest and purest (and cheapest) solution for all of us vintage Ampeg users. Or am I missing something....?

David, is it correct that the signal out of the Ext Amp is pre-amp minus volume/power? I found some older threads with useful insights from, among others, you and Jimmy, but I haven't found the exact question above addressed. Old Ampeg manuals say the Ext amp can be used to feed the PA. So... the next question is how to convert the 1/4" jack to the desired XLR connection for the FOH. Just an adapter cable? Or a simple, passive DI from a trusted brand?
 
#8,177
jnewmark said:
^ Hmmm, not really sure what you did there with the two cabs. Did the ext cab have a threaded hole on top or something ? Or did you turn the cab upside down so the cabs' latches met up together ? I just can't picture this somehow. I assumed that, once you take the dolly off the main cab, it just sat on top of the bottom ext cab, no ?
Click to expand...

The ext cab had a removable lid just like the main cab, but no amp. Screw the blank lid of the ext cab in place of the dolly on the main cab. Stack and then clip the clips. Worked well, not that much work, just a bit of ritual. Had to take dolly off to stack anyway. I figured this was the way the B-15ND dual cabs connected.
 
#8,178
ddbassGA said:
The ext cab had a removable lid just like the main cab, but no amp. Screw the blank lid of the ext cab in place of the dolly on the main cab. Stack and then clip the clips. Worked well, not that much work, just a bit of ritual. Had to take dolly off to stack anyway. I figured this was the way the B-15ND dual cabs connected.
Click to expand...

How do you screw the blank lid in to the bottom of the top cab, there is no hole in the blank lid to do so, at least none I've ever seen .
 
#8,180
So I've pinpointed either a cause or symptom of why my B15 is blowing fuses. There is a small indentation on the heater winding for the power tubes that can be seen in this picture. When I fire up the amp with a fresh fuse I'm getting a spark between approximately this point and the terminal on the fuse casing. Is it possible for the casing on the wire to wear through, or is this indicative of a short somewhere in the wiring or some kind of fault in the transformer? Can it be repaired or reinsulated to prevent these shorts in the future or does the transformer need to be replaced? All of the wiring impedance checks seem to be ok based on this troubleshooting guide.

B2SxC.jpg
 

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