Newbie question- Ampeg B15N

[slavax]

Hi there!

I'm working on plans for building a bass amp along the lines of the Ampeg B15N but not a clone and probably not a flip top, either. I'm interested in getting opinions from members of this forum on what they feel is the "best" B15N circuit.
I'm planning on doing SS rectification. I'm mainly wondering about the tone/feel differences between the B15NF (1968) circuit w/o the ultra hi/lo boost and the later versions of the B15 circuit.

I would be using the amp for electric and upright bass.

Thanks

 

[JimmyM]

I like them all and don't really have a preference. I never use those switches anyway. However, the later models have a fixed bias design that squeezes 30w out of them as opposed to the 25 they used to have.

And not to get all nitpicky, but the B-15NF is the design from 65-68. The ones made after that with the switches are just known as B-15N's.

 

[jimbilly]

I've got a '66 and a '72 (or is it a '73?...). The cabinets are different too, but they both sound great.
I had my tech build me a B15 standalone preamp, for the prototype he built a preamp with the early circuit on one side and the later circuit (w/ switches) on the other side. I ended up going with the early circuit, I'd end up switching the later circuit to try to make it sound like the early circuit anyway. I also like the simplicity of the early one, and less wires and switches = less noise (as a general principle).
If an amp sounds great with just a volume knob, that would be ideal, so the less knobs and switches required for great tone is a good thing.
You can use 12AX7s, but if you want to do a B15 "right" it needs 6SL7s.

 

[slavax]

Thanks for the quick replies. The other question I have is this:

Is their any DIY'er info for building the Portaflex double-baffle cabinet? I realize that Ampeg has reissued the B15 and that the design is patented but this project is solely for my own personal use.
While we're on the subject what are your opinions of the different types of cabinet designs for the B15?
The whole bass amp idea came about because I found a JBL D130 (late 50's-60's) at a garage sale which I'm going to recone (I hope.)
I figured it would be cool to put in a bass amp.

 

[beans-on-toast]

I would go with a 64 NC schematic and modify the tone circuit. They all sound good, this is just the one that I like.

In terms of solid state vs tube rectifier, if you include the circuitry for the tube rectifier, you can plug a solid state rectifier module or a tube into the tube socket. More versatility.

Weber has a proposed schematic for a B15 clone (Link Removed).

Here is another clone that you might find interesting ([Invalid or Expired Link Removed]).

 

[JimmyM]

Thanks for the quick replies. The other question I have is this:

Is their any DIY'er info for building the Portaflex double-baffle cabinet? I realize that Ampeg has reissued the B15 and that the design is patented but this project is solely for my own personal use.
While we're on the subject what are your opinions of the different types of cabinet designs for the B15?
The whole bass amp idea came about because I found a JBL D130 (late 50's-60's) at a garage sale which I'm going to recone (I hope.)
I figured it would be cool to put in a bass amp.

Don't know of any DIY info for the double baffle. Most people who can build them have bought one at one point and reverse engineered it. Then they start building and selling them themselves, so naturally they don't let too much info out about them.

My experiences are with the double baffle and the Thiele cabs. Love them both...the double baffle is a little more full range and even across the spectrum, but the Thiele has a big warm sound and a low mid that is to die for.

 

[slavax]

Another question:

In the B15 double- baffle design there appears to be a speaker baffle and a grille cloth baffle. Are the 8 oval-slot ports in both baffles or only in the speaker baffle? Thanks

 

[JimmyM]

Only in the speaker baffle. They let out the sound from the rear wave, which is then focused to the center hole by the outer grille baffle. It's kind of primitive compared to today's scientifically tuned porting methods, but it works and works great.

 

[slavax]

Interesting- I've been trying to figure it out by looking at pics of the amps in the Portaflex fan club. So the fact that the rear wave goes through the speaker hole of the grille baffle doesn't cause any phase cancellation?
For my purposes, which is not to build a clone, do you think it would be better to try to copy the double-baffle cab or to design a Thiele cab using the parameters for my speaker?

 

[JimmyM]

Interesting- I've been trying to figure it out by looking at pics of the amps in the Portaflex fan club. So the fact that the rear wave goes through the speaker hole of the grille baffle doesn't cause any phase cancellation?
For my purposes, which is not to build a clone, do you think it would be better to try to copy the double-baffle cab or to design a Thiele cab using the parameters for my speaker?

No phase cancellation...nope, not a lick. It's a design made up mostly by conjecture (albeit correct conjecture) on Jess' part, but it's the same as any other ported cab...the only difference is location of the port, and the number one rule of ported cabs is the location of the port doesn't matter.

So if you're not interested in a clone, you might as well not restrict yourself to the original designs. As for DB vs Thiele...I love them both for completely different reasons, but I don't think it has as much to do with the port as the internal volume of the cabs. The DB has a lower internal volume so it doesn't go as deep, but it has a little more even response all the way up where the Thiele has a bangin' low mid and suppresses high mids a tad. But that's exactly why people like the Thiele so much. You really can't go wrong with either. Both are superb designs that I like way better than most typical 115's, and frankly, I'm surprised more people don't use them as regular cabs these days.

 

[slavax]

Do you happen to know the size of the gap between the two baffles?

 

[Vintage-Blue]

3/8".

 

[jsbarber]

For those interested in Jesse Oliver and the B-15 here's an interesting quote about the design:

What particular electronics design or type was used for the Portaflex combos?
I believe I got the design for the Portaflex amp right out of the back pages of an RCA tube manual; that is for the power amp. For the preamp I used the circuitry common to some of the earlier models. (Tube manufacturers, in order to increase sales, would sometimes provide manuals for making circuits to be used with their tubes.)

This is from a 2002 interview, which can be found here:
Link Removed

Jim

 

[BassmanPaul]

I would go with a 64 NC schematic and modify the tone circuit. They all sound good, this is just the one that I like.

In terms of solid state vs tube rectifier, if you include the circuitry for the tube rectifier, you can plug a solid state rectifier module or a tube into the tube socket. More versatility.

Weber has a proposed schematic for a B15 clone (Link Removed).

Here is another clone that you might find interesting ([Invalid or Expired Link Removed]).

Neither circuit is a B15 schematic and as such neither will sound like a B15. I agree that the switches are unnecessary and can be eliminated. As all the B15 circuits are very similar just pick one and go for it. For a first attempt I'd stay with the 6SL7s as the tube basses are larger and easier to work on.

Paul

 

[slavax]

Right now my biggest problem w/ circuitry is figuring out the right PT w/o spending $200 for a repro. Looking at Hammond Xformers they have one that produces 425V on the secondaries and on that produces 500V. But the one schematic that has voltages shows something in the range of 450V on the 6L6 plates. The rectifier (5AR4) has a 17V voltage drop. Anyhow it seems like I need a PT in the 450-475V range.
I'm planning on using 6550A tubes which can take more power but I don't want to overload the speaker (JBL D130 F). And I might do the rectifier/plug in SS module idea too. I've come up w/ many ideas for bells and whistles but on second thought Keeping It Simple Stupid is probably a better principle for me to follow.
I have some other questions about the double-baffle design:
What is the thickness of the grille baffle?
What supports the grille baffle at its edges?
Is the speaker baffle screwed into cleats so it con be removed or is it glued/screwed into the sides of the cabinet?
Thanks

 

[BassmanPaul]

If you are going to build your first amp stay with the design you chose. Sticking 6550s in will not give you more power and adds a variable you know nothing about. When you know what you are doing, bells and whistles can be retrofit.

Paul

 

[JimmyM]

What is the thickness of the grille baffle?
What supports the grille baffle at its edges?
Is the speaker baffle screwed into cleats so it con be removed or is it glued/screwed into the sides of the cabinet?
Thanks

Grille baffle looks to me like 1/4" though I can't tell exactly and don't want to take apart my cab. Might be 3/8", though I think it's 1/4". Definitely not bigger than 3/8".

Grille baffle is supported by bolts on each corner that go under the grillecloth and poke through the speaker baffle and attach with nuts on the inside. The spacers, as Mark said, are 3/8".

Third question I'm not sure about, but I think it's glued. I'm pretty sure it's not removable.

 

[beans-on-toast]

Right now my biggest problem w/ circuitry is figuring out the right PT w/o spending $200 for a repro. Looking at Hammond Xformers they have one that produces 425V on the secondaries and on that produces 500V. But the one schematic that has voltages shows something in the range of 450V on the 6L6 plates. The rectifier (5AR4) has a 17V voltage drop. Anyhow it seems like I need a PT in the 450-475V range.

The output transformer used in the later models, OT-214, had the following specs: primary - 6.5K, DC resistance 290 ohms
secondary - 8 and 16 ohm taps, 1 and 1.4 ohm DCR.

People like the Hammond 1650F.

The power transformer on my early model B15NA (PT-108) also used in the NC, had a 750VCT (375-0-375) @ 150mA with 117VAC on the primary. It produced 430 V on the plates of the 6L6GC's with a 5AR4 rectifier. Something like a Hammond 274BX or 374BX.

 

[Vintage-Blue]

Jimmy - the grille baffle is actually thicker than what you may think, 1/2".

Slavax - as Jimmy said, the grille baffle has (4) #8-32 threaded studs that go through the speaker baffle and are held tight with a washer and a lock nut. I don't know if this will make sense or not, but I'll try to describe the basic cross-section of a double-baffle cabinet. Start with a 12.5" wide side panel. A .75" thick back panel and a .75" thick face frame get attached. That gives you an over-all cabinet depth of 14" (.75 + 12.5 + .75). Spacer strips get attached behind the face frame. The strips are .625" (3/8") thick. The speaker baffle is then glued to the back of the spacer strips. As Jimmy said, the speaker baffle in a double-baffle cabinet is not designed to be removed. The grille baffle installs from the front and is inset within the face frame. It is held away from the speaker baffle by (4) 3/8" spacer blocks. On the original cabinets these blocks were attached to the speaker baffle. I find it easier to attach them to the grille baffle. Plus, they cover the mounting stud hardware better that way.

I hope this makes some sense.

 

[JimmyM]

BTW, if you ever decide not to DIY a cab, you can't get better B-15 cabs than Vintage Blue. Mark is a real artist, and made some very nice improvements that make using it on a regular basis a lot easier.