What is the "Ext Amp" jack on the back of the Ampeg B-15? Yeah, I know it's some sort of low-level output, but is it preamp output, or some other type? What was it originally meant to be used for?
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"Ext Amp" jack on Ampeg B-15
- Thread starter Soulfinger
- Start date
Essentially a line out. You can connect it to the input of another amp, extension amp jack of another amp, or as a line out to a mixer/PA. The old ampeg manuals aren't very specific, but they say you can connect it ext amp to ext amp and everything will run off of the controls on the first amp so I would say it's at least post-eq.
- Feb 13, 2008
- 1,250
- 1,921
- Disclosures
- Owner, Vintage Blue (repro cabinets)
To add to what Corey already said, here is a quote from the mid-60's B15N owner's manual:
"If you feel you need more power than your equipment is designed to produce, you might consider obtaining an additional PORTAFLEX. All PORTAFLEX models are equipped with an external amplifier jack. This enables you to hook two complete PORTAFLEX amps in tandem, which, depending on the model, can double your output wattage. This tandem set-up is accomplished by using a regular mike cable with a phone plug on each end, connected to the external amp jacks of any two PORTAFLEX units; or, you may plug the other end of the cable into the input of any amp. If used in this manner, adjust the volume on the PORTAFLEX first, then adjust the volume on the secondary amp, which in most cases will be a very low setting. What happens is this: The signal from the pre-amp on the PORTAFLEX is also sent into the second amp, so you get the output from both units. If the second amp has vibrato or tremolo, it will sound as if both units have it. This is another way to get more power if you ever feel you need it."
I keep meaning to try it sometime but I never have.
"If you feel you need more power than your equipment is designed to produce, you might consider obtaining an additional PORTAFLEX. All PORTAFLEX models are equipped with an external amplifier jack. This enables you to hook two complete PORTAFLEX amps in tandem, which, depending on the model, can double your output wattage. This tandem set-up is accomplished by using a regular mike cable with a phone plug on each end, connected to the external amp jacks of any two PORTAFLEX units; or, you may plug the other end of the cable into the input of any amp. If used in this manner, adjust the volume on the PORTAFLEX first, then adjust the volume on the secondary amp, which in most cases will be a very low setting. What happens is this: The signal from the pre-amp on the PORTAFLEX is also sent into the second amp, so you get the output from both units. If the second amp has vibrato or tremolo, it will sound as if both units have it. This is another way to get more power if you ever feel you need it."
I keep meaning to try it sometime but I never have.
I haven't looked at the schematic yet - but are the 1/4 jacks acting like inserts on a mixing board then? With the ability to both send "preamp out" and receive "power amp in"? I always just thought it was a basic preamp/line out...
- Feb 13, 2008
- 1,250
- 1,921
- Disclosures
- Owner, Vintage Blue (repro cabinets)
Clint - I'm not smart enough to know much about mixing board inputs but I'm pretty sure it behaves just the way you described. Here is a schematic for a '65 B15NF. It shows the jack sitting right between the preamp and the phase inverter. I'm pretty sure it was like this in all the B15N variations.
I just got home and so I hooked up my SVT 3-Pro to my Portaflex. I can confirm that on my '63 B15N, the "Ext Amp" jack works both as a "preamp out", and also as a "power amp in" jack - all in one. Not that I would ever need or want to send another amps preamp signal into the Portaflex - but it can be done.
I played with this a little more--sending the signal out of the B-15 (mine's a '66 I think) into the input of the my "big rig" set neutral---the tone is AMAZING! It's tubey, but surprisingly hi-fi. It doesn't sound like the B-15 itself (I guess the power amp and the speaker are a big part of the lo-fi tone). Why can't someone make a preamp that sounds like this?
Ok, so if this is the case with a B-15, would it be the same for a B-25? I don't know where my schematic is right off hand. If that is the case, I wish I would have known this years ago when I got my B-25, and later my Carvin R1000. 1000 watt B-25?
Will it send the signal if the amp is in standby, or does it need to be full on for this to work? If it needs to be full on, I need to get a dummy load as soon as possible. I need to try this out!
Will it send the signal if the amp is in standby, or does it need to be full on for this to work? If it needs to be full on, I need to get a dummy load as soon as possible. I need to try this out!
Yes, the B-25 and any other Ampeg with an ext. amp jack will work this way.
The amp needs to be operating, standby off.
Having owned my 70s Ampeg V4 for about two years now, I only just today stumbled upon the above info regarding the ext.amp-jacks. Very interesting stuff indeed, but now I feel I need to run something by the tech-savvy ones 
My V4 has two of these jacks - both simply marked extension amp. Seeing as they apparantly double as preamp out and power in, seems to me they could work as an effects loop. Am I right in this, or have I got it backwards somehow?
My V4 has two of these jacks - both simply marked extension amp. Seeing as they apparantly double as preamp out and power in, seems to me they could work as an effects loop. Am I right in this, or have I got it backwards somehow?
Hi, im reviving this old thread to see if i can upgrade my live and recording setup. I have an Ampeg B15N 64´hooked to the original cab and from the 8ohm external speaker from B15N head to an Ampeg SVT 410HE to push it a little bit with an external amp too, and i would like to incorporate an Ampeg SVT DI (12AX7 Vacum tube DI) sent to the mixer and finally to the PA or Avid mbox mini 3. I would like to know if it´s possible to use de ext amp out jack from B15N into The Ampeg SVT DI box>mixer>PA, Mbox mini 3 or would this harm my gear. Or what would be the best setup trying to keep the signal from the 6SL7 pre tube from the B15N togheter with the 12AX7 tube of the SVTDI box.
On the SVT DI box spec sheet it says mawimum input signal 2.5V RMS, Maximum Bal. out signal 0.5V RMS. I would be greatfull if anyone could help me out as i don´t want to mess up my gear. Thanxs
ps: here in South America we use 220V
On the SVT DI box spec sheet it says mawimum input signal 2.5V RMS, Maximum Bal. out signal 0.5V RMS. I would be greatfull if anyone could help me out as i don´t want to mess up my gear. Thanxs
ps: here in South America we use 220V
One thing I noticed with my 67 B15NF is that if I plug a cable into the Ext amp jack into a DI box for recording, the output of my B15 drops considerably...is something wrong??
Nothing wrong, there’s an impedance mismatch. Some B-15’s and other Ampeg amps have a cathode follower circuit before the ext amp output that deals with this issue, other B-15 revisions don’t. Your B-15NF amp doesn’t have the cathode follower.
Just discovered this today. Question - if I have a B15 head running 'ext head' out to say the effects return on a micro SVT, assuming I will need the B15 cab plugged in as well? It would be cool to just be able to take the B15 flip top head off and use it independently but don't want to blow it out w/o a load. Just wasn't sure if the 'ext head' out would count as a load.
Yes the speaker cabinet normally needs to be connected otherwise you need an 8 ohm dummy load connected. The ext amp is not a power am load, it’s a preamp output or power amp input.
One way around this, you can remove the two 6L6GC power tubes. Then you can use the amp without a speaker load connected.
