15 inch 4 ohm Speaker for Polytone Mini-brute IV

[harpdevice]

Hello,
I recently bought a used Polytone Mini-Brute IV (made mid 90's)
I really like it except for a strange mid-range muddiness - enough to fatigue my ears after a few minutes.
I've tried different EQ settings and the muddiness remains.
I'm playing a Tele with Texas Special PU's - recently professionally rewired.

So - I'd like to try another speaker.

Does anyone have a speaker recommendation?
Or one they'd like to sale?

Thanks,
Kevin

[marcwhy]

Are you sure it's wired for 4 Ohms and not 8?

Check with Warehouse Guitar Speakers.

[docbop]

We're bringing one in for testing....

[harpdevice]

Are you sure it's wired for 4 Ohms and not 8?

Check with Warehouse Guitar Speakers.

It's a 4 ohm speaker.
Here's a pic.

[SuperFour00]

I put an 8 ohm Eminence speaker in my Polytone, which, like most Polytone amps with a single speaker, was designed for a 4 ohm load. I couldn't tell much difference in the volume. The amp was useful in the same volume ranges as before, with no noticeable ill effects or issues. I've had that amp for over ten years. In general, a higher impedance load on a solid state amp will merely reduce the wattage output by a certain percentage, based on the specific design of the amp. As long as you go higher, not lower, you won't damage the circuit. For me, there was no practical difference in volume.

[Franz 1997]

+1; the eminence legend 15" speaker is so efficient ( 102dB) that it will more than compensate for any mismatch loss. It also sounds less muddy in the midrange.

[harpdevice]

+1; the eminence legend 15" speaker is so efficient ( 102dB) that it will more than compensate for any mismatch loss. It also sounds less muddy in the midrange.

Would a 4 ohm version be better than the 8 ohm?

Eminence Legend CA154 15" Bass Guitar Driver 300W 4 Ohm

[SuperFour00]

Would a 4 ohm version be better than the 8 ohm?

Eminence Legend CA154 15" Bass Guitar Driver 300W 4 Ohm

Just make absolutely sure your amp wants a 4 ohm load. Putting a 4 ohm speaker in a solid state amp that is wired for 8ohms would be a bad thing. You should do some research; I'm not absolutely certain that every Polytone MiniBrute 4 in every production year was designed with a 4 ohm load. Since Polytone isn't so great at customer service, I'd look around at finding a tech that's familiar with Polytones and get his/her input. A tech could determine what impedance your amp needs. There may be schematics circulating on the web, too.

Is the speaker currently in the amp marked 4 ohms? If not, does it register around 3 ohms resistance on a multimeter? If the amp is designed for 4 ohms, that's a better match.

[Greentone]

Given that the OEM speaker in the Polytone was also an Eminence, it could be that the 4-ohm Legend is pretty close to what was in there. It is more modern and will likely sound cleaner. It is the driver I would pop into a MB III/IV if I were replacing a driver in one of my Polytones.

[Greentone]

I feel confident that you need a 4 ohm speaker. Every Mini Brute I - IV I have seen is 4 ohms. 8 will work though.

[BigDaddyLoveHandles]

Would a 4 ohm version be better than the 8 ohm?

Eminence Legend CA154 15" Bass Guitar Driver 300W 4 Ohm

That's a bass speaker. I see it's 96.9 dB sensitivity and useable freq of 3 kHz. The guitar one (Legend 1518) is 103.4 dB and goes up to 4 kHz. I would choose the guitar one, for sure.

[Greentone]

You guys realize, don't you, that Polytone put the same speaker in the III and the IV. The III was their combo bass amp. The IV was the same amp, but had reverb. It's a guitar/accordion amp, but it has the same speaker as the III. Works great. Turn the reverb off and it is a great bass combo amp. Many times, I have used the II and the IV as small bass amps. The I and the II have the same 12" driver; the III and the IV have the same 15" driver.

[Greentone]

Reducing the impedance of the speaker load below that intended in the design of a solid-state amplifier causes the amp to try to deliver more power to the speaker(s). This results in additional heat that must be dissipated by the transistors, power transformer, and other devices in the amplifier. The chief worry is the set of power transistors. They can destroy themselves in short order due to thermal runaway in an amp that has no thermal protection circuit if you present it with too low an impedance load.

This is less of a problem with all-tube amplifiers, although a gross mismatch is to be avoided. Fender amps, for example, can run all day on four ohms even though they may be designed for an eight-ohm load. (Case in point: a Fender Deluxe Reverb has an 8-ohm speaker; if you add an extension cabinet with a single 12" speaker, the amp will see a four-ohm load. This mismatch is no big deal.)

The Polytone MiniBrute IV left the factory in Hollywood, CA, with a circuit that was designed to see a four ohm load. With that amp, adding an extension speaker actually presents a series load of eight ohms, rather than two ohms. Otherwise, the MBIV would generate lots of heat inside that smallish cabinet. You can see this most clearly with the MiniBrute PA. Its two cabinets snake together in series to keep the amp from seeing an ultra-low impedance load. The PA head is merely a standard MiniBrute amp with three preamp channels.

[SuperFour00]

To make a long story shorter, modern Solid State amps will run cooler and output less power if you present them with a higher speaker impedance than their design calls for. Run them with a lower speaker load (than specified) and they "run away" with producing more power than the components can stand. You can literally burn an amp up that way.

(Tube amps want a specific load that matches the output transformer and if it's too high or too low, you stress either the transformers or the tubes, and assorted other bits.)

[Franz 1997]

The Polytone MiniBrute IV left the factory in Hollywood, CA, with a circuit that was designed to see a four ohm load. With that amp, adding an extension speaker actually presents a series load of eight ohms, rather than two ohms. Otherwise, the MBIV would generate lots of heat inside that smallish cabinet. You can see this most clearly with the MiniBrute PA. Its two cabinets snake together in series to keep the amp from seeing an ultra-low impedance load. The PA head is merely a standard MiniBrute amp with three preamp channels.

Might be worth noting that the service tech notes for the late 70s/ early 80s 378A model Poly power amp ( for all Brutes) specifies testing with a 2 ohm, NOT 4 ohm, dummy load. I suspect all poly power amps were designed with the same nominal output impedance.

All Poly amps over the years that I have owned allow an 8 ohm speaker to be connected in parallel, not series, thus presenting an approx 2.8 ohm load to the power amp when in parallel with a 4 ohm internal speaker. Maybe the MB IV is an exception, but it seems unlikely.

All that said, the 8 ohm high efficiency 15" Legend guitar speaker would be a good replacement, if the 4 ohm version is hard to find.

[SuperFour00]

Whether testing the amp with a 2 ohm load implies it's designed to run that way at full power over an extended period isn't clear to me. That the original speaker is 4 ohms suggests that a replacement speaker should be 4 ohms at least.

Greentone wrote:
"The Polytone MiniBrute IV left the factory in Hollywood, CA, with a circuit that was designed to see a four ohm load. With that amp, adding an extension speaker actually presents a series load of eight ohms, rather than two ohms."

One of y'all is wrong on the workings of the extension speaker jack.

4 ohms sounds like the safest speaker impedance for that amp.

[BigDaddyLoveHandles]

It's perfectly safe to go with a higher impedance speaker in a SS amp. The amp will produce fewer watts, but with a more efficient speaker that may not mean the rig is any quieter.

[Franz 1997]

Whether testing the amp with a 2 ohm load implies it's designed to run that way at full power over an extended period isn't clear to me. That the original speaker is 4 ohms suggests that a replacement speaker should be 4 ohms at least.

One of y'all is wrong on the workings of the extension speaker jack.

4 ohms sounds like the safest speaker impedance for that amp.

I don't disagree - I'm not suggesting a 2 ohm speaker. 4 ohms upwards indeed sounds safest. I'm not dogmatically insisting that 2 ohms is right for the minimum speaker load, I'm just adding some service manual info which suggests that the amps are tested into 2 ohms. Various sources over the years have suggested that they come stock with a 3 ohm (dcr) speaker, and I've certainly had one like that, but I can't generalise from that info.

So, it's not fully clear to me either, and as the Polytone factory seems to have gone out of business with the death of the founder, it may remain unclear. I hope all we're trying to do here is add to the pool of knowledge, as well as help the OP. I'd say that service manuals should be of some help in cases like this, so that's why I'm quoting the info….

It's perfectly possible that some amps have come with series ext spkr jacks; I'm simply saying that the ones I have seen have been parallel.

[Greentone]

The MI store owner who trained me back in the 70s was a Polytone dealer (and player). He did tell me that Polytones were tested at two ohms--I do remember that. He attributed the great tone of the amps to the fact that they were working derned hard--a notional 4-ohm speaker will drop below that impedance over some parts of its frequency response range.

I don't know if the tone is, in fact, due to the low-impedance load of the speaker (4-ohm) or not. I always suspected that the early Polytones got their sound due to the trashiness of the preamp ICs that were spec'd. Those Polytone amps on a nominally clean setting were never all that clean, which is what I think most players were drawn to. Other solid-state designers approximated this with 'scruff' knobs, etc.

I have popped old JBL 8-ohm speakers into Polytone amps and the results were pleasing, but I think that getting an Eminence driver as close to the original specification would be desirable.

Remember, part of the fun of owning guitars and amps is messing around with them. ;-)

[Franz 1997]

.

I don't know if the tone is, in fact, due to the low-impedance load of the speaker (4-ohm) or not. I always suspected that the early Polytones got their sound due to the trashiness of the preamp ICs that were spec'd. Those Polytone amps on a nominally clean setting were never all that clean, which is what I think most players were drawn to. Other solid-state designers approximated this with 'scruff' knobs, etc.

)

I'm with you on that point. I have a rebuilt 80s baby brute Poly with a new class D power amp section, but original preamp and ICs ; that Poly 'trashy' warm tone comes right through to the speaker, whereas plugging a very similar, but new, preamp with modern ICs through the same power amp/speaker/ cabinet is as clean as a whistle.

But whether this was engineered by design, or simply a by-product of the ( very common at the time ) ICs Poly used, we may never know. Polytone did recognise that the amps had an unusual tonal signature; they called it ''cool blue sound" - or maybe it was ''warm blue sound"…in any event, they claimed the sound as an asset.

A lot of players say that the later Polys, with the Sonic Circuit features, didn't have that familiar sound. That was my experience; I sold my 2001 Poly pretty fast.

[Greentone]

+1 on Mojo is in the preamp. The power amp of the Polytone is a pretty straightforward design. I don't see where it could contribute much to the party, other than volume. The speaker in the amps seems, to me, to be pretty neutral. I put a JBL 8-ohm PA speaker in a Baby Brute once...didn't really change anything, which surprised me--pleasantly.

I think that the funk in the front-end of the older Polytones, i.e., anything before the sonic circuits, really gives the warmth that folks liked about those amps.

If you don't want an amp to color the sound, don't use a Fender, a Polytone, a Gibson, a Marshall, a Vox, etc. AI can sound pretty neutral. Some old Ampegs can sound pretty neutral. I think, though, that most guitarists are quite used to signal coloration via amplification. Go direct to the board and it can be shocking to the average guitarist what things sound like.

[Engine Swap]

I have popped old JBL 8-ohm speakers into Polytone amps and the results were pleasing, but I think that getting an Eminence driver as close to the original specification would be desirable.

I too have had good results with various 12", 8 ohm speakers into Polytones. For a 15", agree with getting something as close as possible to the 4-ohm Eminence. I tried a 4-ohm 15" CTS and it totally killed the Polytone sound.

[jads57]

I believe Eminence makes a 4 ohm speaker modeled on a JBL 15" for the Quilter Steel Air model amps.

[harpdevice]

Thanks everyone for the discussion and advice.

I agree with sticking to a 4 ohm speaker.
I don't want to loose that Polytone sound - I'd like to just clear it up!

I do have a 15 inch 8 ohm speaker in a Community PA cabinet that I may try out - sounds like there's no risk to the electronics.

I think parts-express will allow me to "audition" a speaker - just gotta pay return shipping.


Question is: Which one to audition 1st? At what $$$ will really make a quality difference?

[BigDaddyLoveHandles]

I believe Eminence makes a 4 ohm speaker modeled on a JBL 15" for the Quilter Steel Air model amps.

I can see that steel players would like JBL-clone speakers but would an aluminum-dustcap speaker be too bright for jazz, especially if your are going for the polytone sound? I see that speaker is neodymium, for weight reduction.