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Old 5th May 2012, 07:25 PM   #1
nazaroo is offline nazaroo  Canada
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Arrow Fender Rumble 60 - what mods to clean this up?

I know that the Rumble 25 had some underrated cap problems, which when changed (25v --> 50v) solved distortion/clip/pwr supply problems.

I just inherited this Rumble 60, and I note it seems to have an 8 ohm 100 watt Fender Special musical instrument speaker. Is this the standard guitar-speaker, or is it a different one made for bass? Doesn't sound like a bass-speaker.

The important thing is, can I close off the slot at the bottom of this cabinet for better (flatter) bass response? Max volume isn't as important as the two apparent peaks in the response, one at the low E on a bass-guitar, and one about an octave higher. I'm thinking these peaks are due to the port at the bottom, and I'm wondering if plugging it with 3/4" plywood would improve the sound.

I'm noticing the peaks using a fender precision bass with a simple set of stock pickups.
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Old 6th May 2012, 08:40 AM   #2
nazaroo is offline nazaroo  Canada
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Well, I pulled the speaker in the cabinet,
and it looks underpowered. claim 100w but looks more like 50.
It could be distorting. There doesn't seem to be any rubbing,
but I'm not a speaker expert.

The cabinet could use reinforcing,
and it looks like I could reinforce corners/edges inside,
seal the box and port too, and drop in a higher-power smoother speaker.

What should I use for cabinet reinforcement, epoxy?


the speakers on my Twin dwarf the speaker here,
even though its a bass! Looks like Fender cut some corners here.

Has anyone got better bass sound by switching out speaker or cabinet?

Looking at the chassis, the actual amp is a little integrated chip on a heatsink. Doesn't look like much!

I'm wondering if its worth it to just use the chassis to build a 'champ' or Deluxe 60 w tube amp on, and build a separate box for the head!

As they say, the amp is just not loud enough for rock bands,
and the distortion / lack of flatness in the first two octaves is annoying.

What would a circuit for a good 60w tube Bass amp look like?
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Old 6th May 2012, 04:02 PM   #3
nazaroo is offline nazaroo  Canada
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The basic speaker looks like this ($45 on Ebay used)

Click the image to open in full size.

The magnet is quite a lot smaller and lighter than those on the Twin:

Click the image to open in full size.

I'm thinking of swapping them!

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 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!
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Old 8th May 2012, 03:06 PM   #4
nazaroo is offline nazaroo  Canada
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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:

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

------------------------------



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 ft, .035 m )

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

Click the image to open in full size.

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.
Attached Images
File Type: jpg chassis1.jpg (31.8 KB, 158 views)
File Type: jpg chassis2.jpg (44.0 KB, 24 views)
File Type: jpg rumblebox.jpg (32.1 KB, 125 views)
File Type: jpg Rumble60Cabinet.jpg (21.6 KB, 219 views)
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Old 9th May 2012, 04:17 AM   #5
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Interested to see what you do here. I've got a Fender Rumble 100 sitting around (100W amp, 15" speaker) - looking for ideas of what to do with it. That is assuming I don't just sell it (naturally, I already have a half dozen half finished projects)

If I remember correctly, the chassis is quite substantial (or at least enough to hold a couple of transformers without issue. Could be fun to turn it into say a 50W tube amp. For pure laziness, tempting to get a 5F6-A Bassman kit (minus chassis) and put that in there. About to make an order from Mable Audio, and I think I could get the kit minus chassis for around $250. Otherwise, maybe beef it up with a couple of bridged chip amps?
Think I would write off the speaker to be honest, IIRC it's a low end Eminence in the Rumble 100, but it's far from the prettiest sounding speaker.
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Old 9th May 2012, 10:38 AM   #6
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by MellowTone View Post
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Interested to see what you do here. I've got a Fender Rumble 100 sitting around (100W amp, 15" speaker) - looking for ideas of what to do with it. That is assuming I don't just sell it (naturally, I already have a half dozen half finished projects)



If I remember correctly, the chassis is quite substantial (or at least enough to hold a couple of transformers without issue. Could be fun to turn it into say a 50W tube amp. For pure laziness, tempting to get a 5F6-A Bassman kit (minus chassis) and put that in there. About to make an order from Mable Audio, and I think I could get the kit minus chassis for around $250.
This sounds like a great idea!
The chassis seem to be steel, which is why this may be a great choice.
If Fender had put some quality in the cabinet rather than the chassis,
it would have had a winning amp. As it is, maybe its a winning DIY project!


Quote:
Otherwise, maybe beef it up with a couple of bridged chip amps?
I can't see how putting any chip amps can improve it at all.
Its already got chip amps (unless the 100 has a couple of discrete transistors instead).

After talking to my speaker expert guy,
I'm convinced that 90% of the (bad) sound from these units,
is coming from the cheaply made speaker enclosure.
I'm going to reinforce mine, and block at least part of the port.
If I can flatten out the honky peaks in the lower range (30-160 Hz),
Then it will sound far better at every volume above 3.

I may not even need to replace the speaker,
as stiffening and sealing the cabinet will allow it to handle more power without distorting excursions.


Quote:
Think I would write off the speaker to be honest, IIRC it's a low end Eminence in the Rumble 100, but it's far from the prettiest sounding speaker.
I agree the speakers are cheap in these things.
But I can't help feeling the cabinet is contributing
the greater part of the horribly unflat response.

I'd rather lose a little volume and get a flat fretboard,
because I might even eliminate the need for compression,
while preserving playing dynamics when I actually want them.

If I were you, I'd pull out your 15" and take a look in there.
I'll bet you'll see cheap.
And you can tap on each side of the enclosure with it empty:
I think you'll see that every side plays a different annoying note, like a cheap African drum.

I'm thinking of putting 1" x 1" strips across each side, on a diagonal,
and screwing/gluing them in place to deaden each side.
Then I'm going to put a layer of thick and weighty mattress stuffing on three opposing sides to stop reflections and standing waves.

I may also block up corners inside with triangular blocks.

I am betting the cabinet will sould 400% better with the same speaker.

I will keep you posted.

The speakers are definitely underpowered, but this is my first strike at the problem.
Maybe it will be better if not okay.
Then I will assess the need for replacing the speaker.
Tough call, cause almost any good woofer with ridged edges (relfex type) would improve the power-handling.

Last edited by nazaroo; 9th May 2012 at 10:41 AM.
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Old 9th May 2012, 03:28 PM   #7
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Quote:
Originally Posted by nazaroo View Post
This sounds like a great idea!
The chassis seem to be steel, which is why this may be a great choice.
If Fender had put some quality in the cabinet rather than the chassis,
it would have had a winning amp. As it is, maybe its a winning DIY project!
Yeah I'm weighing off the gains from just selling vs. modding... hmm, oh the choices...
It does occur to me that I currently have a 1970's era 100w tube head to refurbish, a 410 I'm going to rebuild into 2x 210's, etc etc, but this could be an interesting diversion.
If I went for turning it into a tube head, I'd probably remove the chassis and make a new head for it (not enough space for the transformers and tubes) but it's 19" wide and that's wider than I'd like for it. Then figure out what to do with that 15. Perhaps make an ultralight 112 cube?

The question is, where is the point of diminishing returns? That is, how many mods to plan before just selling it and putting it toward new gear instead of simply modding a poorly designed unit?

Quote:
Originally Posted by nazaroo View Post
If I were you, I'd pull out your 15" and take a look in there.
I'll bet you'll see cheap.
And you can tap on each side of the enclosure with it empty:
I think you'll see that every side plays a different annoying note, like a cheap African drum.

I'm thinking of putting 1" x 1" strips across each side, on a diagonal,
and screwing/gluing them in place to deaden each side.
Then I'm going to put a layer of thick and weighty mattress stuffing on three opposing sides to stop reflections and standing waves.

I may also block up corners inside with triangular blocks.

I am betting the cabinet will sould 400% better with the same speaker.
I remember I took the speaker out a few years back and it did look rather woeful inside. They seem to have cheaped out a bit too hard on this.
Admittadly, I haven't played through it in quite a while (it's currently lying down with a couple of amps and whatnot on top) so I can't quite remember the sound - BUT I'd be willing to take a stab and say you're right - add bracing, some stuffing (don't think mine has any either) and perhaps seal it off and you might be on a winner. Obviously, just flat out sealing without changing the volume can go either way, but without specs on that speaker, only one way to find out. Once you get into the territory of replacing the speaker

EDIT: also, out of interest, how wide is the chassis itself on yours? Looks to be around 14" (~350mm)
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Last edited by MellowTone; 9th May 2012 at 03:30 PM.
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Old 11th May 2012, 07:04 PM   #8
nazaroo is offline nazaroo  Canada
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Quote:
EDIT: also, out of interest, how wide is the chassis itself on yours? Looks to be around 14" (~350mm)
Ok 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.
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Old 11th May 2012, 07:17 PM   #9
nazaroo is offline nazaroo  Canada
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To update:

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

Also:

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 ridiculous, and the coil was made of unobtainium.

But the good news is 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:

FANTASTIC!
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.

I will get back again on this.

Last edited by nazaroo; 11th May 2012 at 07:20 PM.
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Old 19th May 2012, 11:06 AM   #10
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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?
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