Fender Rumble 60 - what mods to clean this up?

[nazaroo]

Originally Posted by nikolozj

Hi nazaroo, could you please share with more pictures?

I'm building copy of Rumble 60 and desperately looking for any material.....

From the ones you uploaded, I see that fender didn't use crossbraces? Is that very important?

Yes, I'd say thats VERY important!

I found that playing the Rumble 60 was awful,
as in not playable usefully past a very low volume.
This is because on at least two spots on the bass fretboard,
the notes for a 3 or 4 fret span were all blurred together and way too loud.

This was because of built-in resonances,
and made the little 12" speaker/cabinet useless.

I added heavy bracing, and stuffed the cabinet FULL of
blanket/comforter stuffing to stop the resonances,
and make the amp/speaker playable.

I also sealed up the reflex-slot and removed the light-show,
to make the cabinet have a more flat response over the whole fretboard.

This was the only thing that saved the amp/speaker combo.

Note that some volume was lost with those mods,
but this was necessary to restore musicality and balance.

One could additionally add a Karlson-type interface,
increasing the size of the cabinet,
but also doubling the volume for a given watts or loudness setting.

12" underpowered "Fender Special" speaker, small magnet (prob.50w)

Bass Reflex Cabinet w. slot along bottom:

9" deep X 15 3/8" wide x 15 1/4" high (16 3/4" at back) interior box,

= 2140 cubic inches (1.24 ft³, .035 m³ )

with port/slot = 3/4" high x 16" wide, (12 Sq. In.)
and about 7" deep. (84 cubic in. in volume).



As it is, the cabinet has a horrible "honk/peak" with the low E string,
and another one about an octave higher. Very uneven bass response,
the four-knob EQ doesn't fix this,- not even in the ballpark.


Recommendations:

Put real 200 watt speaker in it,
seal port with plywood block,
add midrange speaker (in separate compartment) and crossover.



Reinforce cabinet walls with crossbraces,
and foam 3 sides inside to stop standing reflections.

[nikolozj]

Thanks for your response nazaroo!

I also sealed up the reflex-slot and removed the light-show,

you mean, you sealed that .75" gap on the bottom of front?
isn't that needed to let the pressure out, so speaker won't blow up?

[nazaroo]

Originally Posted by nikolozj

Thanks for your response nazaroo!


you mean, you sealed that .75" gap on the bottom of front?
isn't that needed to let the pressure out, so speaker won't blow up?

Yes I blocked that, to get a smoother frequency response.


No, unless you are pumping ridiculous levels of power into it.

The sealed cabinet should limit the excursion somewhat,
protecting the speaker against physical overload.

I think there is some problems with paper-accordion surrounds (i.e., guitar speakers) handling extreme excursions of bass,
and supposedly the surrounds can tear.
It may be that this speaker (the original equipment) is susceptable to this, if pushed:
It certainly looks rather underrated (small magnet etc.) for a bass speaker.
But I think Fender would have put a speaker in that can handle bass,
since they contract speaker-makers to customize their speaker requirements.

I haven't overdriven it, but no one should with a small cab/speaker like this. Its strictly for practice and coffee-shops.

I was tempted to drop in a guitar-speaker (higher power) but resisted the temptation, since guitar-speakers are not made for bass-excursion.

I am still tempted to further modify the cab:

That is, put a Karlson-skirt on it to greatly amplify the loudness for a given wattage. If this was done properly,
the amp/speaker combo could probably power a rock-band / bar scene.

[nikolozj]

I've heard that paper ones are the better (more "natural") sounding speakers generally.
I guess I will have to experiment on sound practically...

I was tempted to drop in a guitar-speaker (higher power) but resisted the temptation, since guitar-speakers are not made for bass-excursion.

Sure, bass guitar may tear guitar speaker, but it also depends on what power you push into it. But also, speaker can't produce the frequencies that is not in its range, right? So scientifically speaking if speaker range is 80-5000hz, then let's say 5 string bass's low string B can not be heard at all there, speaker will just not produce it, although it may be wasted in thermal energy in coil, but I don't think it'll tear it.

[nikolozj]

Oh, I just forgot, how thick are the walls for box of Rumble 60?

I chose 0.7 inch (18mm) thick for sides, top and bottom.
And I want to find (still looking for) 0.5 inch (12mm) for front.

is that okay?

[nazaroo]

Originally Posted by nikolozj

Oh, I just forgot, how thick are the walls for box of Rumble 60?

I chose 0.7 inch (18mm) thick for sides, top and bottom.
And I want to find (still looking for) 0.5 inch (12mm) for front.

is that okay?

You don't want to skimp on the frontplate.

It will be flexing, and if the speaker is significantly smaller, then the whole front-plate will warble.

If anything, I'd use at least 3/4" ply, and even reinforce it
by running a strip or support from each corner to the speaker-edge. Especially with a bass cab, where real motion is involved.

[nikolozj]

Originally Posted by nazaroo

You don't want to skimp on the frontplate.

It will be flexing, and if the speaker is significantly smaller, then the whole front-plate will warble.

If anything, I'd use at least 3/4" ply, and even reinforce it
by running a strip or support from each corner to the speaker-edge. Especially with a bass cab, where real motion is involved.

Hm... Weird. Speaker is not so heavy - 5.3lbs (2.4kg)
I've been suggested to get thinner, as long as, if the speaker has some sort of unbalanced frequency responses, thin wood would sort of balance it, in a way that it would eat up some frequencies (of course we're not talking about some certain measures - just experiment)....

I'll go for thick, like you suggest. But why exactly ply and not MDF? Because of weight?

MDF is pretty heavy.

[nazaroo]

Originally Posted by nikolozj

Hm... Weird. Speaker is not so heavy - 5.3lbs (2.4kg)
I've been suggested to get thinner, as long as, if the speaker has some sort of unbalanced frequency responses, thin wood would sort of balance it, in a way that it would eat up some frequencies (of course we're not talking about some certain measures - just experiment)....

I'll go for thick, like you suggest. But why exactly ply and not MDF? Because of weight?

MDF is pretty heavy.

I guess you could use MDF, but it may be more prone to crumbling, and its heavier. Also, MDF dulls sawblades real fast. Hardly worth it.
Its supposed benefits over plywood appear marginal.

[Nigel Goodwin]

Originally Posted by nazaroo

I guess you could use MDF, but it may be more prone to crumbling, and its heavier. Also, MDF dulls sawblades real fast. Hardly worth it.
Its supposed benefits over plywood appear marginal.

As it's only a fraction of the cost, I wouldn't say that was marginal?.

Personally I've always used chipboard, which is even cheaper - and many (if not most?) professional speakers used to do the same.

Only real problem with chipboard and MDF is the weight, otherwise it's fine - and acoustically 'deader' than plywood.

[nazaroo]

Originally Posted by Nigel Goodwin

As it's only a fraction of the cost, I wouldn't say that was marginal?.

Well, two qualifiers:

(1) I get plywood free usually, in the garbage,
or bins at nearby construction sites.
Canada is a rich country in regard to wood.

(2) If you're a manufacturer this becomes more significant.
But if you are building a single pair of speakers,
who cares, if its a one-time cost, and
when plywood is only $20-$60 a 4' x 8' sheet?

Personally I've always used chipboard, which is even cheaper - and many (if not most?) professional speakers used to do the same.

Only real problem with chipboard and MDF is the weight, otherwise it's fine - and acoustically 'deader' than plywood.

If your speakers are going to sit in your living room for years at a time, I guess weight really isn't a big deal.
But if you're lugging the speaker cabinet to and from gigs,
in cars or up stairs, then it probably becomes significant.