DIY Airbrake...

[benoit]

Cool. I hope my question came across as more tentative and probing than contrary or argumentative. Though I don't have enough know-how to really expand on my questions I did have an intuitive sense that it probably didn't affect what you were saying too much about the 16 ohm way being best suited.

[rooster]

No benoit, this was a good concern.

Too, I would add that even as the amp sees the speaker as less than the value we call it, '8 ohms' equals 6-7 ohms dc resistance typically for example, it follows that the amp must see the static resistance as something a little different, too. Eh, so I suppose this means that the 25 ohm resistor could look like 28-30 ohms to the amp when it is coupled to it, just following the speaker dc readings vs. 'actual' speaker rated impedance pattern.

I mention this because there is some mojo here that is more science than most of us are able to decipher - myself included. As an example of this, I do not have the tools here to measure the primary value of the OT. I am solely dependant on what the manufacturer tells me it is. Eh, not that his makes a lot of difference in anything I am doing over here.....

Oh, one last comment about speaker impedance. Remember that it never goes lower than the rated load, but only higher. So as you play gtr, the impedance will fluctuate between its rating (8 ohms let's say) on the low side and a number dependant on frequency, that can go as high as maybe 50 ohms - something like that - to make an audible sound. I assume it can go higher but basically the speaker is not generating a lot of audible volume at this point because it cannot reproduce the sound. And this, on a sidebar, is where people go on about parasitic oscillations and such, burning up an amp - no audio but the amp is working hard just the same... Funny, too, how when people talk about this problem, no one ever states what the frequency was, just the it was high and beyond the range of the speaker. Maybe this just speaks to what any of us are capable of measuring and what concerns us. Eh, probably. Any scientist amp builders out there?

So keep it in mind that the AB is built of all static parts save one, the speaker. And as it gets further out into the higher clicks it becomes more unresponsive to the energy of the changing impedance that comes with frequency. ...Which is surely why it doesn't sound very good to our ear.

Rambling over.

[rawnster]

With 8 ohm speaker load connected, clicking from bypass to 'max' resistance:

7 - 8.5 - 10.2 - 11.7 - 13.7 ohms

With 16 ohm speaker load connected, clicking from etc.... :

13.7 - 10.9 - 12.2 - 13.3 - 14.8 ohms

To measure the load, just hook your AB up, pull out the plug that is coming from the amp and measure the resistance from tip to ground, pretty simple. The load will vary a bit as you play but nothing too radical.

Any, from what I see, its pretty clear to me that the original AB was made to favor a 16 ohm load. Which actually makes sense because of the typical 4/12 connection KF favored for his Expresses. But then running 8 ohms isn't completely stinky.

I am not clear what the loads are to the amp in the ABIII. Rawnster, are you game to measure yours?

Ok, I hooked my AB III with an 8ohm load and took the readings:

(6 position rotary switch)
1.1, 1.1, 4.6, 9.8, 19.5, 26.2

[rooster]

rawnster - If you have done the measuring correctly then I got a comment: Houston, we have a problem.

But I am pretty sure you measured the wrong cable end - that or you measured the right end (from cable end going to amp) but then this means you have the speaker and amp connections reversed.

?? You want to check this out and get back here?

[rawnster]

Yeah, it was early. Need coffee. Here's the readings again with a 16ohm load speaker cab:

click 1 - 9.1
click 2 - 13.8
click 3 - 9.5
click 4 - 8.5
click 5 - 9.5
click 6 - 12.4

[rooster]

OK, better. Still a little odd relative to the typical AB. I see too that your readings are going backwards from mine. In other words your 'direct to speaker' is click 6, mine is click 1.

So, if I had to point out something I would say that with the exception of click 2, your rig seems to favor an 8 ohm load. But here I have to ask: Are all the clicks reducing power gradually? Sometimes it is hard to tell but that click 2 kinda stands out.

[rawnster]

OK, better. Still a little odd relative to the typical AB. I see too that your readings are going backwards from mine. In other words your 'direct to speaker' is click 6, mine is click 1.

So, if I had to point out something I would say that with the exception of click 2, your rig seems to favor an 8 ohm load. But here I have to ask: Are all the clicks reducing power gradually? Sometimes it is hard to tell but that click 2 kinda stands out.

Well I'd be interested to see another set of readings from someone else's AB III. With the 8ohm resistor in the mix, this build is suppose to favor an 8 ohm load. Click two is what I tried to explain to you before, in that, it's louder than click one. So, to answer your question, starting with click two, the clicks do reduce power gradually. I can hear it.

[rooster]

OK, then I get it. The 13.8 is your cab straight thru the device. See? So you need to reverse the connections of pin 1 and pin 2 somehow. The first click should be just the cab without any resistance.

Let us know if this is a wiring mistake on your part or if the instructions are incorrect.

[rawnster]

OK, then I get it. The 13.8 is your cab straight thru the device. See? So you need to reverse the connections of pin 1 and pin 2 somehow. The first click should be just the cab without any resistance.

Let us know if this is a wiring mistake on your part or if the instructions are incorrect.

I just measured the ohms on another AB III that a friend owns here locally. His readings were almost identical to mine, except for the second click. His second click read about 9.1 ohms. This leads me to think that something mechanically is wrong with my switch. Both AB's are wired exactly the same, and using the same components. FWIW, his AB also showed the last click of around 13 ohms....and the first click around 9 ohms.

I'm pretty confident that I didn't make a wiring mistake; I followed the instructions that were given by mhuss, as well seeing a real Dr. Z AB. Try throwing in the 8 ohm resistor off of pin 4; bridge pins 1-3; connect the other side of the 8 ohm resistor and one end of the bridged 1-3 pins to the middle tab of the non-multi-tabbed 25 ohm resistor where it's connected at 13ohms. Perhaps you'll get a similar reading to what I and others get with the AB III.

Below are more pics of my AB III. Let me know if you see something wrong. The pin numbers are difficult to read, so I superimposed them on the first pic.

[rooster]

Q: How can your friend's box read 9 ohms if this is the cab bypassed from the box? It should read 12-14 ohms typically. Is this not the loudest position? OR are you saying that your box does not go loud to quiet, that it jumps at the end or something? That first reading is what is odd.

Measure your speaker box without the device. What is it? This is what my first click is, just the speaker box. So this is either 7 ohms for the 8 ohm cab or 13.7 for the 16 ohm box.

Man, if that switch is bad all I can say is what are the odds? I would start disconnecting a few wires and check the operation of the switch first off. As you say, it could be the switch even if the build of it, the switch doesn't really allow for such things. Its just a rotary device afterall. But hey, maybe so.

[rooster]

Hmm. I just got to thinking about this and I have another idea. Do you think it is possible that you wired the wrong pins on the switch? I realize that you did some graphics work on the one pic - but do these numbers actually coincide with the actual pins on the switch?

This is where I would start, just confirm that the switch connections are what they are supposed to be. It could be that you started the hookup 1 pin out of sequence.

And again, please explain to me how on the friend's box he got a reading of 9 ohms for a 16 ohm cab? I can't get my head around this. Does the ABIII not have a bypass by design?

[rawnster]

I measured my speaker load and it's about 13.7 ohms. This is what my second click reads. So my second click is doing what you say the first click should be doing--bypassing attenuation and sending signal straight to the speakers. From click 2 to click 6, it progressively increases attenuation. Cool! The issue is with my first setting (click). Pin 12/6 is suppose to be the first setting. pin 12 is bridged to pin 11. I disconnected the two pins and left pin 12 disconnected. The first click(12) reads 25ohms, and the second(11) click reads 13.7ohms.

[rawnster]

Hmm. I just got to thinking about this and I have another idea. Do you think it is possible that you wired the wrong pins on the switch? I realize that you did some graphics work on the one pic - but do these numbers actually coincide with the actual pins on the switch?

Yep! You know it's like in school, if you actually take notes you have a better chance of remembering what you're listening to and writing down. I did that layout drawing so I could learn and remember. Per your request, with jewelers magnifying glasses on, I made sure pins 1 through 12 were connected correctly compared to my drawing, and my friend's AB. Rooster you can see by the pictures I just uploaded how it's wired. There's something weird going on around 12/6 and 11/5 that belays its hookup.

And to clear up the discrepancy with my friend's AB: when I took the readings, I did so quickly and didn't realize that I had it upside down. So, the first click was really the sixth click and so on. This means that his first click reads as you would expect...a robust 13.7ohms--bypassed signal to the speakers. So, I'm confessing my oversight/mistake/novice intelligence to you this forum. You're right.

However, if I incorrectly wired my AB, then it's a mystery and is hiding; for I don't see it at all. And to be clear, my friend's AB and mine are wired exactly the same.

[rooster]

Hmm. Reading your last post I still think you are out of sequence with your hookup somehow, and that the switch is OK.

Here, remember when you measured the opposite end of the box you got:

(6 position rotary switch)
1.1, 1.1, 4.6, 9.8, 19.5, 26.2

Well, I realize you probably had the speaker load plugged into the opposite jack at the time - but I don't know how this would skew things. You should have read something like this if nothing else was plugged in:

0, ?, ?, ?, ?, ?

Now, I don't really know what the ABIII should read on clicks 2-6 but click 1 has to be '0' - because there is nothing connected, right?

Try this then, don't plug anything in except the speaker cable and take a reading.

For example, this is mine:

0, 30.6, 35.4, 40.3, 49

[Structo]

What is the AB III?

I know this is a shot in the dark but I was looking at the photos and noticed what appeared to be some melted insulation on one of the wires to the switch.
Like the terminal below it got hot and melted it.
Could it be possibly shorting into the wire that is melted?
I circled it.