Gallien Krueger MicroBass...Mystery (or WTF? if that's your style)

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A friend asked for a fix on his Gallien Krueger MicroBass, a metal boxed practice combo.
He mentioned a "cardboardy, buzzy kind of noise" when it was turned up, I expected a tear on the Eminence Beta 12" driver, maybe the surround or the spider, less likely some electronic distortion.
Driver looked OK, buzz was definitely mechanical but from inside the cabinet.
Luckily it is easy to open up, so we quickly found... 3 loose pieces of cardboard...?
All similar, double wall, like from a box, and about 50 mm (couple of inches) square.
That sure explained the "cardboardy" noise but why was it there?
The back panel has a strut with a rubber bumper on the end that looks like it should be a brace, except that it doesn't quite reach the speaker.
It was short by just the thickness of 3 pieces of cardboard...
But that's a bit odd, not just because it's a half-baked patch in an otherwise nicely assembled speaker, but because the strut is directly in line with the central hole in the speaker magnet.
Why block the speaker vent?
Seems kind of odd to want to mess up the speaker heat dissipation system, and then not even do it properly?

Anyone here ever pulled apart a GK MB ?
Not sure exactly what year, I was too puzzled to take the details.

David
 
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Custom shims, not cardboard.

No, cardboard, looked like it had been cut from a box with scissors, not particularly carefully.
I would think this had been done by a sloppy backyard repairman but that still doesn't explain why the presumably factory installed strut looks to block the speaker vent.
Maybe an attempt to add some air flow resistance with a close spaced "plug" and someone assumed it was supposed to be a support and added the cardboard.
That sounds implausible but I can't think of a plausible explanation, hence my inquiry.

Best wishes
David
 
...with Eminence drivers when did that start?

Ah! a plausible scenario finally dawns on me -
The cabinet had another driver with no centre vent, so the factory strut in the middle of the speaker makes sense.
The driver was replaced with an Eminence with a centre vent and a little shallower.
A shoddy quick fix done to pack the gap with cardboard, which was just shoved in and not secured, so it eventually fell out and rattled.
My friend said he had owned it from new and never mentioned any previous repair but it makes sense, thank you.
What manufacturer did they usually use?
Any idea what driver?

Best wishes
David
 
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They typically used OEM "Pyle drivers" but also (again custom OEM) unmarked Eminence ones.
But the cheaper stamped frame ones, not a cast frame "Greek" type (Alpha/Beta/Gamma/etc.).
It also used a foam edge.
Since they typically rot in 10 years or so, one plausible scenario is to replace it with another, perceived "better" Eminence ... pity speaker/frame depth changed.

Don´t worry too much about the vent, plugging it is no big deal, besides slightly helping dissipation its main purpose is another.
This is the original one:
MB150/112, 112MBX - Paragon 12", 100W, 8 Ohm, 37Hz, Bass Driver - gallien-krueger.com
An externally hosted image should be here but it was not working when we last tested it.


Rather than stiff cardboard, try to cut a couple "coins", slightly oversized is better, out of spongy-but-stiff "EVA rubber" foam, contact cement glue them to existing rubber plug , so they stay there, and of enough thickness so they are compressed by, say, 6mm when pushing speaker back in its hole.
You must feel the speaker "sitting on a cushion" when mounting.
The idea is not "holding the speaker" by itself, but bracing the thin aluminum back so it does not vibrate too much.
 
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Since they typically rot in 10 years or so, one plausible scenario is to replace it with another, perceived "better" Eminence ...

Yes, it all makes sense, and your ideas are in accord with my own.
I didn't worry too much about the vent and yes, the strut is intended to brace the back panel not the speaker itself.
I just replaced the cardboard but used double sided adhesive tape to make sure it stayed in place, your EVA foam idea is better.
Thank you.

Best wishes
David
 
yup i thought so Paragon speakers Speakers - gallien-krueger.com
it's even in the description that JM Fahey provided so not sure why he's saying they're Pyle or Eminence.
http://store.gallien-krueger.com/mm5/merchant.mvc?Screen=CTGY&Category_Code=S8
These particular ones are made by Eminence.
I have repaired them and they even have the typical barcode label attached to the magnet side , which does not say "Eminence" in words but have an EIA manufacturing code which starts with "67" .

As an example, this is Ted Weber´s list:
Credence 117
Pyle 1098
Rola 285
Eminence 67
CTS 137
EV 649
Oxford 465
Altec/Lansing 391
Utah 328
Quam 270
Jensen 220
Weber 1279
Cleveland 433

OEM speaker buyers (who are amp and cabinet manufacturers themselves) obviously don´t want general public knowing that, specially that in many cases "they use exact same as competitors" which Marketing Dept will never allow anyway, so standard practice is to register some house brand and apply it to own speakers, even from different Manufacturers.

So GK uses "Paragon" as a blanket brand, Peavey applies "Blue Marvel" to Eminence and Asian speakers alike, Fender uses blue or brown (Modern or Vintage, go figure) "Fender Special Design" stickers, Randall uses "Jaguar", Crate uses them either unbranded or "V" something if made by Celestion, Mesa Boogie calls them "Black Shadow", both on Eminence/Celestion/EV made ones and so on.

As a side note, Eminence picked the most popular OEM types and sell them to the public, labelled as "Legend" , go figure.

I repair all of them and find the same frames and much alike guts in tons of them, under any imaginable "Brand".

There are not that many real Speaker factories in the World, at least in the Musical Instrument field, and amp manufacturers don´t want bthe hassle of also manufacturing own speakers, so .........

Just to break the rule, I make guitar amps and manufacture my own speakers , from scratch.

As in starting with cold rolled steel sheet which I stamp into frames (also cast a few), 1008 or 12L14 round bar, 1018 flat plate, spare unmagnetized magnets, winding own voice coils, and all the Industrial operations needed to turn that raw material into actual usable speakers.

These are a few of them, I make from 15" Bass woofers to 4" high power cone tweeters and everything in between:

12" Guitar speakers, Fahey on the right,compared to equivalent Celestion on the left, then front and back of 8" Guitar speakers, again side by side with the Celestion they replace (Musicians pull those to install mine):
 

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No special tweaks, happy they hold magnet, cone and spider where they belong :)

Went through all steps of the ladder: there are no local frame suppliers per se, each factory (there´s about half a dozen, from poor quality to people manufacturing very good speakers under licence from Altec and EV, go figure) has their own and do NOT sell them "alone", period. Which is a wise commercial decision, of course.

So I started sandcasting them in aluminum, which then requires lathe turning, at least the mating surfaces, because surface is rough and to boot aluminum contracts unevenly on cooling, meaning they work but are slow and expensive.

Then I started making them by sheet bending turning, same process used to make pans, parabolic lamp reflectors, etc, with punched round holes all around.
UGLY as hell but serviceable, and since I sold them inside my cabinets, nobody complained.

Since I am a small maker, I rent machinery and shop use, often from people making aftermarket and original car parts as subcontractors, who have the heavy hydraulic (and mechanical) presses, lots of dies, etc., and are used to charging little per finished part (or big car factories won´t buy from them).

Once I visited a factory and was marvelled, seeing what for me was a 12" speaker frame blind "cone" being stamped in the center of a 50cm x 50cm square piece of very thick sheet metal:
-"what´s THAT ? ... a speaker FRAME?"

-" huh ???????? ... what do you mean? .... that´s part of a car *wheel*"

I searched their cut and punch die storage area and found the one used to punch the "window rising machine" pocket in a car door frame, which was trapezoidal shape, about what a speaker frame "window" is.

-"can you stamp some "cones" for me, but in 16ga sheet, and punch holes around with this other die?"

-"sure, why not? ... how many do you want? 1000? ... 500? ...."

We settled on 100 and went on since then.

Later I found other conical dies which match other speaker sizes, and had a couple extra made (such as the 4" tweeter which to boot is blind) , anything which can be lathe turned out of "machinable" cast iron is relatively inexpensive, and finally, with Industry retraction because of the Oriental onslaught, bought dies from failed speaker Factories, often paying them "by the pound", as scrap metal, because there were no other bidders.
Who invests in a doomed industry?

FWIW I learnt about it 2 weeks too late, or I would have also travelled to USA to buy the BIG Peavey Magnetizer, which they used for the Black Widow speakers and their monster 220mm magnets.
Largest I currently have reaches up to 155 mm ones, what Vintage 30 and G12H use, the Peavey machine would have allowed me to make PA type 15" and 18" woofers.

Almost forgot:in the 80´s and with a couple partners made a few 30" speakers.

Cast frames of course, and since no magnet was big enough for them, had to design and use electromagnets, fed from a separate auxiliary supply.
Lost all my money invested on that :(

No buyers, so we rented them to Discos ... who were sloooowwww payers or straight nopayers. :mad: :cuss:
 
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