Bass amp with KT120's!! Also, crossover design question

pylet2000 · #1 10-22-2011, 07:02 PM

Hello all, I've just built a 150W tube amp using a pair of KT120's! It is deep and sufficiently loud for most venues. Used the Mercury Magnetics bass output transformer, sounds great with no hint of saturation. I'll post some pics and maybe even some sound clips if anybody is interested. It's generally a Blackface Fender AB763 circuit, goes great with my Fender P Bass Special.

The cabinet I've built for this amp is where I have some questions. I bought a 3015 (non LF) with the hopes that it would be full range enough to use by itself. In the Mesa cab I have it temporarily loaded it, it does a decent job of being full range, but I wouldn't mind getting more definition to the upper end. So here's my question, can I use this speaker in conjunction with a closed back mid driver? Does it need a crossover to work proper? If so, would the crossover for the venerable fEARful's be "close enough" to perform well in a non fEARful cab with speakers that it wasn't designed for? I do realize that question made every purist cringe... My apologies. I do well with amps, made dozens of guitar amps, and now one decent bass amp, but this bass cabinet / crossover / T-S parameter stuff is new to me.

A little info about the cab itself, 25"x24"x12" @ 3/4" solid pine for the sides, 3/4" ply front and back. Shelf ported, 19.5"x1"x8". Dovetailed. I was originally going to build it as a flip top, but the amp chassis is just too deep. Live and learn. I would have just built (or more likely bought from one of you fine builders) a fEARful if I hadn't hoped to make a single 15" 150W Mega Portaflex... Now I'm just hoping to end up with a workable full range cabinet instead of $40 worth of firewood and a "lesson learned."

Thanks a ton for any help or info you can share!!!

-Jeff

will33 · #2 10-22-2011, 08:06 PM

The closedback mid would be an easy enough add on but you'll have to do a crossover designed for those particular 2 drivers which the fearful's is not. Depending on the impedance plot at the crossover frequency, some off the shelf might work, might not.

will33 · #3 10-22-2011, 08:23 PM

Looking at the impedance curves the closest off the shelf thing that might work is emi's 800hz 2-way and a la6cbmr. That crossover lines up pretty well with the 6 but your woofers impedance is around 6ohms in the cab there, still better to make your own.

christw · #4 10-22-2011, 09:14 PM

Sorry I can't help with the speaker stuff. But the amp sounds like it'd be pretty awesome. Have you got pictures?

Oh, and one day I will own a true mega fliptop. Don't give up my friend!

pylet2000 · #5 10-22-2011, 10:20 PM

Thanks for the info Will, I appreciate you taking a second to spec that out.

I just got done trying the raw cabinet with the single 3015 (non LF). The cab goes low, but has several notes that don't perform properly. The attack is pretty slow, too. So much for my bass cab design skills. I'm gonna stick to amps and just build a proper fEARful. Lesson learned.

That said, looking closer at the 15/6, there may be enough room to build a fliptop behind the mid chamber... Perhaps the Mega Fliptop issue isn't dead yet.

Here's some pics of the KT120 amp. I'm still waiting on my chrome barrel knobs to show up, forgive it's raw look with them absent.

pylet2000 · #6 10-22-2011, 10:22 PM

And here's what the back looks like...

will33 · #7 10-22-2011, 10:26 PM

Ya, your cab is super shallow front to back. There might be some cancellations happening between cone and back wall making some notes sound wierd. I'd have to dig up the chart to see just what freq's that would be in but may be part of the problem.

The amp looks really nice and I think a 150 watt fliptop would be totally cool.

will33 · #8 10-22-2011, 10:36 PM

Is your cab lined on the inside with foam or insulation or something?

will33 · #9 10-22-2011, 10:52 PM

How about bracing? Look at the diagram in post #4 of this thread. Vertical 212 Design & Possible Build Shouldn't be too hard to add some crossbracing like that to stiffen it up. If you don't have enough room because of the driver to get front to back sticks out away from the edges, put a couple spine braces along the back wall something like you see in the first post of that thread and in the fearful builds.

You might be getting some panel resonance from the pine. That can work to your advantage in guitar cabs but for bass you want the panels stiff, no vibration or resonance. Your cab is at the small end of what that driver wants but it might be salvageable.

If you already have the lining and bracing there isn't much more you can do. If not, you're getting frequencies bouncing around inside the box creating peaks and nulls and making some notes sound wierd and panel resonances making it even worse.

pylet2000 · #10 10-22-2011, 11:35 PM

Thanks for your compliment and suggestions Will, much appreciated!

The cab isn't lined / filled at all, so standing wave cancellation is very likely going on. I just tossed the 3015 in to see if it was worth pursuing this particular cab any further. I didn't brace it in any way either, so the back panel is resonating on certain notes.

The 12" depth was limited by my dovetail jig. The dovetails aren't required, as evidenced by the many great designs out there that don't use them, but to me they seemed prudent for a bass cab.

I'm feeling pretty good about getting a flip panel behind the mid chamber of a 15/6. Has anybody ever tried building a fliptop fEARful? Talk about having it all! A renowned high performance cab with top notch neo speakers, and a great sounding powerful tube amp, all in an easy to carry lightweight package! Mega fliptop!! This definitely seems worth pursuing...

will33 · #11 10-23-2011, 12:26 AM

The bracing and lining can be done in an afternoon's time for no more than $20-30. I'd at least do that and see what you think. It'll go quite a ways toward fixing the sound. You can still persue the fearful but it'll probably turn this into a decent oneway cab.

will33 · #12 10-23-2011, 12:28 AM

I don't know where yours is tuned at but if you still find it a little boomy due to the small box, lowering the tuning (longer port) can remedy that too.

pylet2000 · #13 10-23-2011, 01:22 AM

You are correct, it's probably worth the effort to see what the bracing and maybe some stuffing will do. I've come this far, right?

The cab is tuned to roughly 39 hz, and has about 3.2cf internal volume before figuring in the speaker displacement and the corner bracing. Or at least that's what the calculators suggest it should be... 39 hz seems lower than most folks aim for, but the band I play bass in has a guitar player that tunes down a half step, throw in the occasional drop D song (concert C#) on top of that and you're not so far from 5 string territory.

bb5000 · #14 10-23-2011, 02:08 AM

Nice build ! What's the plate voltage ?

will33 · #15 10-23-2011, 07:32 AM

You shouldn't have to adjust tuning then, really no point in going lower. Tuning a little higher would bring the lowend up a little should you want that but the bracing and lining will clear up the wonky notes. I too am a little more interested in the amplifier.

pylet2000 · #16 10-23-2011, 10:34 AM

I'm going to start a new thread that focuses on just the amp.

Thanks for all your ideas and suggestions on how to clean up the cab. I'll probably salvage the cab as best I can, and start running down the fEARful path too. Prototypes are often a living experiment, and they don't always turn out like you hope...

Thanks again!!