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Wednesday, June 20, 2012

Fender Rumble 60 Upgrade

I just inherited this Rumble 60, and I note it seems to have an 8 ohm Fender Special musical instrument speaker.  However, this speaker is actually undersize compared to a regular Fender 12", and seems to be only about a 40 watt speaker.

Note the small magnet.

The cabinet could use reinforcing,
and it looks like I could reinforce corners/edges inside,
seal the box and port too:

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).

On the other hand,
The chassis for the Rumble 60 (at least this one) isn't made of Aluminium! It seems to be soft steel.

Its solid, rectangular and just about perfect for a small 2-tube push-pull power section.

I'd have to build a separate wood cabinet for it (Marshall-style).

Click the image to open in full size.

Somebody maybe already got that idea, and sold a chassis on Ebay.

I'm thinking, - now that the Rumble 60 has been discontinued,
and it has gone down from $500 new to about $200 or less used,
it makes a good start-pack for a 'champ' project complete with speaker cabinet!.

I don't know if this speaker would satisfy a guitarist,
but the chassis should satisfy a DIYer!

 Okay here's a closer look:

Click the image to open in full size.

16.5 " wide x 9.75 " deep x 2.75 " (at back. 2.25" front) chassis


Recommendations for Amp:

Chuck integrated chip-amp,
and build tube amp on chassis, mount it in separate Marshall-style box.

Ok so the chassis is 16.5" wide (without any sides),
and 10" deep (ignoring transformer bulge and knobs).

I'm guessing a separate 'head' box will be:

(inside: 16.75" x 12" deep (allow for knobs/fuses) x 8" (3" + at least 5"))

Add 3/4" ply (x2) box:

18.25" x 12 (or 13)" deep by 10" high.
You can use the metal speaker-grill (cut) as a front/back grid above the chassis / knobs, to keep fingers out of front and back, and have plywood top, bottom and sides.
Carpet (or woodfinish) and corners as desired.

Makes for a sweet looking 60w push-pull head.


Recommendations for Speaker Cabinet:
As it is, the cabinet has a horrible "honk/peak" on the low E string (G,G#),
and another one about an octave higher (Gb, G, G# smeared).

Its a very uneven bass response, almost unplayable in certain areas of the fretboard!

The four-knob EQ doesn't fix this,- not even in the ballpark.

I'm thinking: put a real 200 watt speaker in it,
seal the port with a plywood block,
maybe add a midrange speaker (in separate compartment) + crossover.

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

 To update:

I've added wheels (a necessity with any amp larger than a 10watt).


I reinforced the inside with a cross-piece horizontal pine board about 1" thick and 4" wide, edgewise toward speaker-back, cutting back-panel in half. Screws in sides and back and glue to hold.

I added a 16" piece of 2x4 in bottom, after removing fruity lightshow panel, right on bottom and right against port-hole inside blocking it: glued and screwed from bottom/sides (1.5" screws).
Long-screws were used on ends (from side).

I used a spray-can of insulating-foam, filling port, corners and edges all around inside box. Worked well.

I replaced speaker and tested:

The two main resonances were still there, (49 Hz and 98 Hz), but a little less bad. Much of the cabinent distortion on other notes however were cleaned up, so this cabinet reinforcement is really worth doing.

I went back to my buddy at The Speaker Store, asked about a notch-filter. This was not good news, as the cap/inductor combo would require 150 mH (HUGE if aircore) and 20,000 - 40,000 uF caps. The cost was unjustifiable, and the coil was made of unobtainium.

But the good news is my speaker expert recommended
stuffing the cabinet full of quilt-stuffing material to kill all standing waves.
So I gutted a 'comforter/pillow' and stuffed cabinet and replaced speaker:

The two main resonances were about 80% killed off (still a slight and noticable peak/resonance but the bass guitar was actually playable and the notes near the 'resonance' weren't all blended together and indistinguishable.

Also, the whole bass fretboard was not only playable but sounded musically awesome (especially two-note stuff and octaves)!

Bass Guitar amp went from pure honky crap to sonic bliss.

Pull all your bass woofers, reinforce sides and stuff the box full!
You will be amazed.

Now the amp actually doesn't sound so bad (for a transistor),
although the EQ is still crap.

In an ordinary room (practice, coffeeshops) it seems more than adequate.
Also seems to work good as a small guitar amp (maybe better than as a bass).

Still, speaker seems underpower for useful playing.

I'm now thinking of just adding a tube-circuit (12ax7 etc.) inside the box for an 'overdrive' channel:
You don't have to make a separate 'head' for this, to get a 'tube' sound.

I will get back again on this.

 "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.


  1. heya naz.

    new reader (tonight, actually). happy to make the (virtual) acquaintance...

    got to yer blog by way of the diyaud mu-stage -> mu-stage ultimate amp thread.

    not on about any of that -- i want to aid yer cab resonance deal at 49 & 98 cycles somewhere or other around here:

    my concern (i do "heat stuff" for a living) is that i don't know how 'stuffed' yer cab is vs. what the rig needs for free-convection and i really really really hate to see component life compromised. life is often a 4th-order function of temperature (eg twice as hot => 1/(2^4) = 1/16th life. in the best case, heating leads to power compression and parameter changes as you well know.

    also been building speaks for a good 30-some and i can say that inserting a couple strategic wedges (slightly reducing your cab's vol) is an angle to try if the stuffing (a) kills something tonewise and/or (b) insulates that which must be cooled/free to convect.

    say we start with a rectangular box, everything square/plumb/blah. you get vibration modes as functions of each dimension between parallel walls, most-offensive of which is usually the long dimension. standing waves each direction.

    doesn't take much--5 degrees a wall or 10 on one side only--and those modes go bye-bye; you're left with only the helmholtz.

    other thing is one batt of R19 or whatever across the long dim is an angle if that works with your gear heat/other/wise.

    free input in reciprocation for your own sharing: buzz a couple 10+ degree wedges almost the full size of whatever cab wall they'll occupy and listen to that in lieu of the "full-stuffing approach". rtv them in if you want them semi-permanent--personally, i use packing tape and light dabs of silicone . season with limited stuffing to taste

  2. i guess it all depends on the drivers and the cab hole sizes (more infinite baffle vs. more reflex vs. pipe modes vs. ...) easier to 'go empirical' and do ear-time. damping box modes, IMO, is often a straight trade of subjective "openness" vs. transient performance. if you work with non-parallel walls (ie use wedges), it really helps sort "pure cab volume" vs. same + standing waves.

    you only have to saw a few sizes once and a lot of experimentation is possible.

    the other thing about stuffing--even with open-back cabs--is it really changes the amount of 'cab/box-reflection-sound' coming out the front of the cones. there's a certain amount of that ordinary backwave output that, once-delayed and bounced around and back through the cone, is another Whole Tone Thing (tm)

    the way to find the happy spot, IMO, is to get two test tracks--one with some giant soundbody thing--cello, piano, whatever--and one with drums and lots of drums.

    if you've never before done the exercise, it's worth finding the extremes Once

    you wank for max openness of the hollowbodies. best-piano, etc. you then wank for best transient performance of the drum stuff. so, in most "normally-tuned" [sic?] enclosures, there are your two extremes. at that point, the learning really happens if you're able to add/subtract on the fly with the cab on its back and with one panel loose but laying on weatherstripping or a bead of RTV, etc.

    add/remove/experiment with the program material running. this takes an hour or four in a continuous effort, but when you are done balancing the two extremes, you will have Zero doubt that you left anything on the table (or whether the cab even allowed either to work let alone a compromise). voicing slightly-mistuned enclosures, in 4-words, or "optimizing" in one

    so anyway, i know that stuffing -- enough to whack standing waves down -- can also whack a driver's ability to "breathe" and...well that's basically what touching-base here is all-about. i wanted to share the wedge/angle-thing in case it's of use and in the same spirit (aka "FWIW"

    be well.
    Forward Message

  3. The above were comments from Grindstone (USA), a DIYer with some speaker cab experience!

    Thanks to Grindstone! Good points.