Speaker configuration advice?

[gjesse]

I've got an old Ampeg 15/10 speaker cab, and I'm trying to decide on the best speaker configuration for it... Right now, the speakers I have to choose from are:
15" 4 ohm Peavey Black Widow
10" 16 ohm Eminence
10" 8 ohm Peavey Blue Marvel

My head is a Fender Studio Bass tube amp with a non-switchable 8 ohm output.

I'm leaning toward putting the 8 ohm 10" in series with the 4 ohm 15", making for 12 ohm total.

But there's the option of putting the 16 ohm 10" in parallel with the 15 to get 3.2 ohms.

Or should I just sell the 15 and the 8 ohm 10" and get a 16 ohm 15" to run parallel with the 16 ohm 10" to get 8 ohms, which my amp wants?

Or am I just overthinking it? I've read a 100% mismatch is ok, which would really only allow for the 12 ohm configuration, using what I've already got....The main consideration is protecting my amp, but I also want it to sound good.

Is running a 4 ohm and 8 ohm of different sizes in series just going to sound like crap?

I figured I'd throw it out to the experts before i do anything too rash

[runmikeyrun]

you need two 4 ohm speakers wired in series, or two 16 ohm speakers in parallel to get 8 ohms. Don't mismatch.

[will33]

The 15 and the 10 should be crossed over and when properly done the impedance won't work the same as when connecting fullrange speakers. If you crossed the peavey speakers at 400hz for example, you'd essentialy have a 4ohm cab that was 8ohms above 400hz.

Tube amps are completely fine to run on lower impedances than what they're set for, it's higher they don't like....the opposite of SS amps.

Fenders and some others work fine on up to a 100% mismatch going higher too, like running an 8ohm speaker on a 4ohm tap. It may not be the best idea but they can handle it. To be safe, it's best to run tube amps with impedances at or lower than what they're set for.

[WingKL]

Check out the FAQ especially the speaker size section. Quick answer is chances are the results of putting in any of those speakers in would be bad. It's like throwing on some random wheels from different cars together on one vehicle. They may or may not fit and if they're different sized, it won't work very well unless they were designed as such.

[JMac4strngr]

I'd try to stick with a combined 8 ohms, no matter what speakers You decide to use that will fit the parameters of Your cab. Whether You go with two 16ohm speakers in parallel or two 4ohm speakers in series, You'd have the 8ohms that You need.
Running that tube head with anything besides the unswitchable 8ohm tap, could be bad for the amp.

[madmachinist]

tube amps don't respond the same way to load mismatch as SS amps.
the output transformer is wired for a specific impedance , and if you have to -
you can get away with lower impedance---- too high an impedance load can damage the amp.

(the opposite of SS,which can be easily run at higher impedance -too low can damage the amp)

if you like your 16 ohm 10" , that 4 ohm BW-15" can be changed to 16 ohms .
last i heard Peavey's cast frame drivers (BW, Scorpion,etc) have field replaceable
baskets (you keep your original magnet/motor assembly and replace the basket/cone/
voicecoil assembly) i recall they were about $60-80 , and can be changed out in about
10 minutes with a screwdriver .

another word ......

perhaps the original Ampeg drivers were -balanced- ......
the output SPL of a BW 15" and a TBA 10" speaker will likely be very different, and at
different frequencies . you could have huge overlaps(peaks) and/or holes(dips) ,
which may or may not sound good to your ears.
i run different sized drivers/cabs all the time- but at different crossover frequencies
and bi-amped .

another option would be to refit the BW-15 with an 8 ohm basket , and use the 10"
8 ohm driver together with a PASSIVE crossover somewhere around +/- 200hz .
the amp would only "see" the 15" from 200hz down , and the 10" from 200hz up-
since the frequencies don't overlap, you'd have an average impedance of 8 ohms.

good luck.

[will33]

Originally Posted by madmachinist

tube amps don't respond the same way to load mismatch as SS amps.
the output transformer is wired for a specific impedance , and if you have to -
you can get away with lower impedance---- too high an impedance load can damage the amp.

(the opposite of SS,which can be easily run at higher impedance -too low can damage the amp)

if you like your 16 ohm 10" , that 4 ohm BW-15" can be changed to 16 ohms .
last i heard Peavey's cast frame drivers (BW, Scorpion,etc) have field replaceable
baskets (you keep your original magnet/motor assembily and replace the basket/cone/
voicecoil assembly) i recall they were about $60-80 , and can be changed out in about
10 minutes with a screwdriver .

another word ......

kperhaps the original Ampeg drivers were -balanced- ......
the output SPL of a BW 15" and a TBA 10" speaker will likely be very different, and at
different frequencies . you could have huge overlaps(peaks) and/or holes(dips) ,
which may or may not sound good to your ears.
i run different sized drivers/cabs all the time- but at different crossover frequencies
and bi-amped .

another option would be to refit the BW-15 with an 8 ohm basket , and use the 10"
8 ohm driver together with a PASSIVE crossover somewhere around +/- 200hz .
the amp would only "see" the 15" from 200hz down , and the 10" from 200hz up-
since the frequencies don't overlap, you'd have an average impedance of 8 ohms.

good luck.

The last half of your post is probably the best bet here and that's only if the cab is suitable for the BW after you build the 10 in a subchamber if it's not already. 15's can play up to 800, maybe 1k before they beam too badly so I'd play with an active xover and biamping to find the point that sounds best, then build you're passive xover at that frequency. The end result would be a match for your amp assuming you can find impedance curves for all the drivers used.

[gjesse]

great responses! Thanks for the advice!
That's a lot to chew on...
Seems like the best way to go here is with a crossover.
Until I can figure that part out, I think I'll just run the 4 ohm 15" by itself and leave the 10" disconnected....it's in a separate compartment.

[billfitzmaurice]

Run the fifteen full range, use the 8 ohm ten, wired parallel but with this between the jack and the driver:
Dayton Audio DMPC-60 60uF 250V Polypropylene Capacitor

Impedance is not an issue with your amp. It doesn't need an 8 ohm load, that's just the highest load that should be used with it.

[gjesse]

Why, does that create a high pass filter for the 10", or something?
So I understand, I put the capacitor in series with the 10" speaker? Do I put it in-line with the wire going between the + or - terminal of the speaker/jack, or does it matter?

[madmachinist]

yes. it's a hi-pass filter .

personally, i'd go with a less expensive $1.50 electrolytic cap...

68uF 100V Non-Polarized Capacitor

those poly caps are wonderful for hifi tweeters and such,
in my HIFI speakers.... Snell , Mission , Paradigm, JBL L112,
Mirage ... for low/ midbass -almost always use electrolytics,

values above about 22-30mfd are rare and extravagent.

in a bass guitar setup , i don't think anyone could tell the difference.

edit-
the cap is bipolar, and the circuit is AC, so itshouldn't matter which side you apply it to.

that extra $14 could be put to better use toward better drivers(cast frame , neo , alnico, etc..)

[BassmanPaul]

I'm with Bill on this one. Bi polar electrolytic caps are poor at best. A good plastic cap will always better them.

[madmachinist]

not trying to start a capacitor-conundrum here.....

but please explain why , in a circuit that tops out @ 5-6khz
a premium 60mfd polypropylene cap will outperform
a practical electrolytic of similar value.

e-lytes might have a shorter predicted lifespan and larger
tolerance, but have been used successfully in speaker designs
for many decades without fuss.

if it matters... a polyester cap is still about 1/5 the cost of
PP and has a similar ESR .

at 60mfd , most any cap's esr is neg'ble .

[billfitzmaurice]

Originally Posted by madmachinist

yes. it's a hi-pass filter .

personally, i'd go with a less expensive $1.50 electrolytic cap...

The voltage capacity of an electrolytic is the problem. While it may be rated 100v I've seen them blow at 1/4 that. And no manufacturer would dare use an electrolytic for a pass-through on a quality speaker. For a shunt they're OK, but good hi-fi speakers won't even use them there due to their high ESR. Polyester is OK, so long as rated for at least 200v.

[gjesse]

Wow, I'm learning way more than I expected from my question...Thanks again for all the input!

[will33]

Originally Posted by gjesse

Wow, I'm learning way more than I expected from my question...Thanks again for all the input!

Yeah, it's highpassing the 10. Without also lowpassing the 15 there will a dip in impedance where the 2 overlap which with a tube amp is a nonissue.....just know it's there should you ever use an ss amp with this cab, it'd have to be safe at 2ohms......or add a low pass to the 15. That only applies to an ss amp, with your tube amp, just using the capacitor will be fine.

[gjesse]

Thanks again all for your great advice and explanations...I got the capacitor Bill suggested, installed it with the 8 ohm 10", and WOW...my rig sounds perfect now! What a difference it makes with the highpassed 10" in the mix. Really rounds out my sound, even seems to add some sort of compression or something...I can really pound the strings, and it still sounds great---no BWOW on the low notes, just a nice heavy THOOM. Above the 12th fret is just butta now, so round and warm...the 15 alone just couldn't express very well up there. Everyone in the band noticed the difference, and one of the guitar players had to play bass for a few songs to take it for a drive. Seriously, this forum is awesome! I'll be spending a lot more time here I think....

[CormacBass]

I cant buy this cap Dayton Audio DMPC-60 60uF 250V Polypropylene Capacitor where i live (ireland).
Anyone know where I can get it?

[CormacBass]

Dayton Audio DMPC-60 60uF 250V Polypropylene Capacitor cant buy this in Ireland, anywhere else i can get it?