Ampeg B-15N Schematics

[Humber]

I built a B-15nc from Joe Piazza's schematic on the Amp Guide site (it's also at Schematic Heaven.) The voltages are all dead on, but the tone controls do very little, and it distorts at low volume. I'd love to double check the schematic. Or maybe someone out there has built this same amp?

[loudthud]

I just noticed that the B-15NC schematic calls for 4 meg treble pots. In the later schematics that specify 1 meg pots, the treble caps are 470pF and 4700pF instead of 47pF and 470pF. There is another schematic that uses the 1 meg pot with the lower value caps. Confusing! What value pot did you use?

Looking across many Ampeg schematics you see they eventually ended up with the volume pot between the two gain stages and the tone controls right before the phase inverter. This may have been a move to address the distortion you are experiencing. The Ampeg Basses may not have had enough output to distort the gain>tone>gain>volume confiruration but other basses in the marketplace had more output and ran out of headroom.

[Humber]

Thanks for the reply. I couldn't find a 4 meg pot, so first I used a 2 meg, then a 5 meg, then a 1 meg (I saw the same thing you mentioned)--no real difference, but the 1 meg was worse. We tried a couple different cap values, but I don't know if we tried the one you mentioned. We messed with the feedback, and a few other things to get the gain down, but afraid that in attempting to make it a workable amp we're also losing whatever qualities made these cool--you know? I have other 6L6 heads that would work, but I was hoping this would be something special.

The tone section is odd in this one--pre-volume, and with that weird circular thing. It wasn't easy for a non-genius like me to work a layout for it.

That's an interesting point about bass output levels. We have been testing it with a guitar and 2 12 cab--not a great way to test, but I've played guitar through these, and there's no way it should sound like it does.

[loudthud]

It's been my experience that many times there are early vintage examples of amps or guitars that were made before 'they got it right'. Ever played one of those 52 or so Les Pauls with the trapeze tail piece? Not a bad guitar but the bridge gets in the way. I think the B15N is the model with the magic.

Over at the Talk Bass Forum (you have to register) there are quite a few portaflex fans that may have some insite if the B15NC design has headroom limitations. It's more of a players forum but still has valuable info.

[GregS]

I recently built a Portaflex B15N from the project in Kevin O'Connor's TUT3 book. I do not have any problem with distortion at low or moderate volumes. I get some distortion at high volumes as is expected as the amp doesn't have that much headroom (25W). The tone controls are also effective. Initially the treble control wasn't effective because of a mistake in the schematic but once wired correctly it works fine.
I don't know about the B-15NC. Maybe this version of the amp has problems.

Greg

[Fletcher Munson]

There are some pretty major differences between the Piazza schematic and the “Ampeg” schematic found here:

The Free Information Society - Ampeg B15N Electronic Circuit Schematic

The tone stack and the volume control swap places, and then there are the aforementioned value changes. If Piazza’s schematic really was taken from a 1962 amp, then that’s the one I’d try to stick with. As near as I can tell, the B15N and the B15NC have only minor differences.

I’m thinking of building a B15N also, so I’d be very interested to see if/how you’re able to work this out. Would you be willing to post some pictures and share your layout? Maybe we can help you troubleshoot it, as I suspect something is just wired wrong (no shame, we’ve all done it).

If you haven’t seen it, there’s an interesting interview with Jess Oliver (the amp’s designer) here:

Keith Rosier Online

[Fletcher Munson]

I recently built a Portaflex B15N from the project in Kevin O'Connor's TUT3 book. I do not have any problem with distortion at low or moderate volumes. I get some distortion at high volumes as is expected as the amp doesn't have that much headroom (25W). The tone controls are also effective. Initially the treble control wasn't effective because of a mistake in the schematic but once wired correctly it works fine.
I don't know about the B-15NC. Maybe this version of the amp has problems.

Interesting to hear that KOC has a B15 project. Is O’Connor’s schematic pretty close to Piazza’s? I know KOC always adds something to the design or he wouldn’t include it his book. Just wondering how close it is to the original. Also, does he include a layout? Looks like I’ve got to buy TUT3…

[txstrat]

I copied this: http://www.schematicheaven.com/ampeg..._portaflex.pdf (the link to the B-15N 1968 Portaflex) with a Eminence Kappa 15 speaker and four instead of two 6L6 (and a matching OT).
It sounds absolutely great and has enough headroom for every club. Only thing I discovered was, the Ultra Bright switch doesn't seem to make a big difference. Everything else was "Tone of glory".

[Fletcher Munson]

I copied this: http://www.schematicheaven.com/ampeg..._portaflex.pdf (the link to the B-15N 1968 Portaflex) with a Eminence Kappa 15 speaker and four instead of two 6L6 (and a matching OT).
It sounds absolutely great and has enough headroom for every club. Only thing I discovered was, the Ultra Bright switch doesn't seem to make a big difference. Everything else was "Tone of glory".

That’s where I was heading too… 4 6L6’s should give plenty of headroom without changing the tone too much. Did you stick with a tube rectifier or change it to solid state?

Any pictures?

[GregS]

Interesting to hear that KOC has a B15 project. Is O’Connor’s schematic pretty close to Piazza’s? I know KOC always adds something to the design or he wouldn’t include it his book. Just wondering how close it is to the original. Also, does he include a layout? Looks like I’ve got to buy TUT3…

I don't have Kevin O'Connor's schematic in front of me right now but right away I can see there are significant differences from Piazza's which I'm currently looking at:
- power supply uses a solid state rectifier instead of a tube rectifier. There are numerous other differences in the power supply.
- there is no standby in KOC's version.
- there is one pre-amp channel instead of two. There are only two 6SL7 pre-amp tubes: V1 comprises two gain stages with bass and treble controls in between and the volume occuring after stage 2 and before the phase splitter if I remember correctly. V2 is used as the phase splitter.

There are a number of other differences as well as a number of similarities. I believe there are a lot of different variations of the B15 circuit - some were fixed biased and some were cathode biased. The one in KOC's book is cathode biased and probably represents one of the many versions of circuit. There are some of his own additions as well but the original circuit is still largely intact.

He does include layout as well. There are many other amp projects in the book besides the Portaflex and there is also a lot of general information on amp construction as well.

Greg

[txstrat]

Did you stick with a tube rectifier or change it to solid state?

Any pictures?

Solid state recto (4x 1N4007).
Didn't copy the flip top cabinet. Put it in a combo.
Unfortunately this is the only picture I got. The speaker housing is sealed (except the two vents) and the amps chassis has a open back (tubes hanging upside down). The slot above the speaker is for ventilation reasons.

[Fletcher Munson]

I don't have Kevin O'Connor's schematic in front of me right now but right away I can see there are significant differences from Piazza's which I'm currently looking at:
- power supply uses a solid state rectifier instead of a tube rectifier. There are numerous other differences in the power supply.
- there is no standby in KOC's version.
- there is one pre-amp channel instead of two. There are only two 6SL7 pre-amp tubes: V1 comprises two gain stages with bass and treble controls in between and the volume occuring after stage 2 and before the phase splitter if I remember correctly. V2 is used as the phase splitter.

There are a number of other differences as well as a number of similarities. I believe there are a lot of different variations of the B15 circuit - some were fixed biased and some were cathode biased. The one in KOC's book is cathode biased and probably represents one of the many versions of circuit. There are some of his own additions as well but the original circuit is still largely intact.

He does include layout as well. There are many other amp projects in the book besides the Portaflex and there is also a lot of general information on amp construction as well.

Greg

Thanks for your post. Funny thing is after I asked my question, above, I went home and checked my book collection. I knew I had one of the TUT books, but couldn't remember which one. Sure enough, it was TUT3 with the chapter on the Portaflex. In my defense, I bought that book a couple years ago when I had zero interest in bass amps, so I probably glossed over that chapter.

KOC gives a nice overview of the different B15 versions. The preamp started as gain stage -> tone stack -> gain stage -> volume control. On later versions, they changed this to gain stage -> volume -> gain stage -> tone stack. The power amp started as cathode biased, then switched to fixed bias for more power. The phase splitter was consistent through all iterations.

For his version, KOC chooses the earlier preamp with the tone stack between the two gain stages. He includes a cathode bypass cap on the second stage to "add a little sparkle" and eliminates the largely unnecessary second channel. He sticks with cathode biased 6L6's for "a rounder sound." The phase inverter is unchanged. Also, if I remember correctly, he used 1MA pots for both bass and treble.

I was impressed with how closely he follows the original. He didn't seem to feel any need to "improve" on the design; he just tweaked it a bit.

[dpacek]

I copied this: http://www.schematicheaven.com/ampeg..._portaflex.pdf (the link to the B-15N 1968 Portaflex) with a Eminence Kappa 15 speaker and four instead of two 6L6 (and a matching OT).
It sounds absolutely great and has enough headroom for every club. Only thing I discovered was, the Ultra Bright switch doesn't seem to make a big difference. Everything else was "Tone of glory".

Can u share a diagram of how you added another pair of 6L6s and a new OT for more headroom ?
thanks
dan

[txstrat]

Just like in every other 4 power tubes amp. I added one 6L6 on either side in parallel and used the wires from the OT for the four plates (two on each side) and added a second wire to the girds right after the coupling caps of the PI. No big deal. Compare the schematics of a two 6L6 Fender (such as the AB763 Super Reverb) and a four 6L6 Fender (such as the AB763 Twin)

[dpacek]

Thanks, that is pretty straightforward. Did you use a version of the Fender 125A30A for an OT ?