Ampeg SB-12 Finished Build - New Problems

[dawsonaudio]

Here are a few images of my new bass amp. Still trying to figure out which new speaker to get for the cabinet. The speaker I had in mind is buzzing...it's a really old Jensen field coil. Anyway, it sounds great at low volumes and thank you to the members who helped me out along the way.

Nate

[img1141]http://www.serviceairsocal.com/amp/combo.jpg[/img]
[img1141]http://www.serviceairsocal.com/amp/cabinet.jpg[/img]
[img1141]http://www.serviceairsocal.com/amp/ampguts.jpg[/img]

[Stevem]

Nice job, cool little bugger!
Take note that even the highest rated wattage Jensen field coil driver of the day ( big 15") was rated at a small 25 watts, and this was peak, not rms power!
Old speakers of that era had the voice coil winding glued to the voice coil former with hide glue ( animal glue!) and over time this glue just drys up, not to mention that taking place with the added help of wattage pumping thru the winding.
The end result is the winding coming loose, rubbing in the narrow voice coil gap, and you end up with the issue you have or a totally open winding with enough movement.
If you want the low E string to carry right even for home use you need a 12" driver, and I guess if you added a big enough tuned port slot to that cab it would work for home use and or recording.
The Eminence Alpha 12A driver goes down to 51 hz at least, has a good SPL rating and will not break the Bank at 60 bucks.
The Bata 12A is a very good pick also, ćost a bit more, but both drivers basically call for the same size box and the Bata has a better SPl level too.

[Leo_Gnardo]

On that speaker cab, do I see braces across the speaker cutout? There's sort of an X pattern behind the cloth. If that's what I'm seeing and it's not just an artifact of the photo, you'll want to make sure the cone on your new speaker can't slap into them. Either mount the speaker on an extra gasket or saw the braces out.

[xtian]

Great looking build, inside and out! Love the big knobs.

[JMFahey]

Old speakers of that era had the voice coil winding glued to the voice coil former with hide glue ( animal glue!) and over time this glue just drys up, not to mention that taking place with the added help of wattage pumping thru the winding.
The end result is the winding coming loose, rubbing in the narrow voice coil gap, and you end up with the issue you have or a totally open winding with enough movement.

Oh, come on!!! Where did you pull that from?

Animal glue may have been used to join cabinet *wood* or *Tolex/Tweed* but speaker voice coil wire (as well as cones, gaskets and in fact everything else) were glued with ... guess what .... "speaker cement" ..... (who would have imagined?) which actually is nitrocellulose adhesive (Duco/Dope family) .
Yes, it softens above 90 C , bubbles above 100 C (boiling water temperature) and catches fire above 130 C , after all it's smokeless powder dissolved in acetone, but that's a different problem.

[dawsonaudio]

Actually, there isn't any x bracing. Just an open hole. The material was rolled up in some boxes from my organ parts stash. It must be the imprint from the previous organ. The speaker is actually centered in the cabinet and the vent hole is underneath running horizontal.

The knobs came from my Ampex parts stash as well. They came off of a 440B tape machine.

[telentubes]

I see only 4 screws holding on the back panel. Any chance that is vibrating, sounding like a buzz?

[Stevem]

I have been re-coning my own vintage speakers ( Jensen, Utah , Oxford, Altec ) that I have collected over 35 years and on the very early perminat mag speakers and the field coil ones that's what the adhesive seemed to be to me by the way it failed!
If you know better that's fine, my main point was that the adhesive will just fail on its own due to age and vibration, and then you can add wattage heat into the mix!
The paper voice coil formers turn to near dust all on there own many times, have you not seen that?

[dawsonaudio]

The four screws on the back aren't an issue. Inside the amp on the back wall is some duct liner insulation which dampens things a bit. The screw are fairly snug as well.

I did take the speaker out and the rattle was still apparent. There was just a little dust on the voice coil. I decided to take out the field coil speaker and start with a new Jensen C12N. I'll put in in over the weekend along with changing some of my wiring layout and report back.

[dawsonaudio]

So I just installed my new Jensen C12N and the speaker is distorting again on the low notes. Since the speaker is new, the distortion must be originating from the amp? I also notice that there is a heck of a lot more hiss than before when the volume knob is turned up. I changed the 12ax7's with new ones and still have the hiss. My prior setup with the field coil didn't have the hiss and not nearly as much distortion. Any thoughts on what could be happening here? I'm totally stumped.

With the new speaker, I wired the amp as per the original schematic below. My plate voltage is a little higher than before around 475vdc. The output tubes are biased at 22mA/s. With the higher plate voltage, I changed my electrolytic capacitors to 500vdc instead of the 450vdc's that were installed previously. Not sure if the increased voltage is causing problems. Maybe the hiss is because of my carbon comp resistors.

Here are the voltages at each electrolytic capacitor + side:
468vdc - 30uf
463vdc - 20uf
398vdc - 20uf
318vdc - 20uf

[img:800:600]http://www.drtube.com/schematics/ampeg/sb127868-jp.gif[/img]

[Leo_Gnardo]

Different speakers have different frequency responses, also different sensitivities. If your new Jensen is tonally brighter, it will expose hiss as well as the let the higher frequencies from your instrument come through. And if the speaker's more sensitive, it will sound louder than the last one overall.

New speakers need to be "broken in." After a couple weeks of playing you'll notice more bass. Some people drive the voice coil with the secondary of a filament transformer, 6 or 12 volts, let it go for hours, even days or a week or 2. Hums like mad but at the end the spider & compliance will be broken in and should remain stable for many years of playing. They are springs and like any spring, tight when new, then works as you expect for a long time, and finally gets floppy and out-of-control, let's hope that takes a couple decades. If you choose the filament transformer route I'd be careful and go with 6V, leave the 12 for higher power speakers.

Good move on going to 500V caps. Higher voltage also tends to brighten the tone.

22 mA seems a bit low bias current too. If you could 'scope the output I bet you'd see some obvious crossover distortion, that will make low notes sound crappy. Dial that up a good chunk to 30-35 mA, I'll bet that will improve the low end tone a bit.

If you think a swatch of sound absorbent material is going to stop a poorly supported panel from rattling along with low frequencies, it ain't gonna work. You have to sock down that back panel tight or it's going to sing along with low notes, no two ways about it. Get an assistant to lean on that back panel with their hand while you play low notes, that should prove in principle what I'm suggesting. Some of the low frequency energy the speaker's putting out is going into making that panel shake. When you put an end to that, you'll find more bass coming out the front. Plus your ears won't be distracted by the rattling racket. Once the cab isn't rattling there's no harm in putting in sound absorbent material to damp resonances in the space behind the speaker.

It's one of the PIA aspects of bass - getting speaker cabs to not rattle. Then you find everything else in the room rattling, windows, furniture, teeth. One rig I put together some years back could deliver 2600 watts, play a low G not even using an eighth of that power, and my vision went blurry, it was shaking the retinas in my eyes. KOOL!

[dawsonaudio]

So I did another bias to 30mA @455vdc which would put me at the 70% realm of max plate dissipation. The formula I'm using is 0.7x19W/455=0.029A. Thanks for suggesting that. I was using one of the online calculators instead of doing the math myself.

I took the speaker out of the cabinet to try and identify the distortion and eliminate the cabinet. It sounds more like a buzzing-rattle now..almost like the wires are vibrating against the back of the speaker. The wires look fine on the new speaker. Not sure if the speaker is faulty out of the box.

[SilverFox]

If you have another amp, perhaps even a stereo amp, swap the speakers to that amp and see if the buzzing sound continues.

silverfox.

[Stevem]

I do not understand why you picked that speaker for bass use as it rolls off the neeed low end that should go down to 40hz for 4 string Bass use very much way up at 100hz!
This model Jensen is one of many that does not have a Laytex treated surround , see number (2)
That being said that C12n model carrys the high end strong even up at 7Khz so the amp likly had the hiss issue all the time but you never heard it with the other speaker.
In short you are hearing distortion now from having to drive the amps output stage harder to make up for that drivers lack of bottom end.

(2) Those surrounds on that model are very stiff ( no air movement ) and if you break the speaker in for 12 to 18 hours by applying 11.5 vac @ 60hz to it you will have more low end and need less watts to get that bottom !
To me this model when new out of the box does not even have enough bottom for guitar use, no less Bass!

[dawsonaudio]

Putting the speaker in another tube amp resulted in the same distorted/buzzing coming from the speaker. It's almost like something is stuck inside the dustcap or maybe the coil was damaged in shipping.