Why can't I plug a base guitar into a guitar amp?

[akis]

Will something blow? Is the PSU of the guitar amp (purposely) weak and cannot handle the low frequencies?

Thanks

[ghoshsubha444@gmail.com]

I am not sure but I heard something that Bass notes draw more current than higher pitch notes. Besides We will not get good response from normal guitar speaker. The cabinet construction is some what different. And you may cause permanent damage inside the speaker. But you can play in comparably less volume. i think that will do no harm. 

[QReuCk]

To be heard, bass notes have to be at a considerably higher power. That's why the average bass player uses an amp 5times more powerfull than the guitarists he plays with.
They also generate a higher signal amplitude, which might push the speaker into its excursion limits. That's the main concern when plugging a bass guitar into a guitar amp. If it's just for training at home, you can get away with cutting a lot the bass bands of the amp's tone stack to prevent damage and avoid playing too loud.
Another one is that depending on the circuitry, your amp itself can enter into occillation if driven hard with frequencies lower than what it is designed for.

On another hand, a fairly high number of guitarists do plug a looper stompbox with integrated rythm box in it into their guitar amps without any troubles. These rythms sometimes contains very low frequencies (but they are recorded and compressed, which is a different matter than an uncontrolled live instrument.

[akis]

Would that mean that the average bass amp requires a speaker with multiple times the wattage of the guitar amp's? Would you be able to take any old cabinet and plug a bass amp into it or would you require a specially made cabinet?

[Roly]

Well you can, but...

This is a simple question with a fairly complex answer.

I'll take your questions in reverse order.

Is the PSU of the guitar amp (purposely) weak and cannot handle the low frequencies?

No.  It makes little difference to an amp power supply if it is asked to deliver, say, 50 watts at DC, 50Hz, 500Hz, or 5kHz - a watt is a watt is a watt.

The amplifier portion of a guitar amp, particularly the preamp, is often constructed with deliberately poor bass response simply because tenor guitarists usually don't want a lot of bottom end.  This is not universally true and some guitarist like old valve (tube) bass amps because they like a bit of "body" and thump available in their playing.  Typical guitar amps however are simply built as if the bass control were turned down a lot, the "bass" control going from nothing much to not a lot.

Will something blow?

The primary problem with putting bass guitar into many guitar amps, combos with open backs in particular, has to do with the speaker(s) and air loading of the driver(s).

Open back cabs are constructed to provide air loading of the driver only down to the lowest frequency of a tenor guitar, and in small combos not even that.  You will already know that some smaller amps will "frap" on lower guitar notes, blow a bit of a raspberry rather than produce the bottom open E on a tenor guitar if hit hard.  Plug a bass guitar into such an amp and the "frapping" problem is much worse.

The basic reason for this is the nature of air which is what we call "compliant".  At low frequencies when the cone moves the air tends to simply flow out of the way instead of being compressed and producing a sound wave as happens at higher frequencies where air is "stiffer", so the air loading on the speaker cone is reduced and the cone is freer to move more and starts to produce nasty noises as it gets to the limits of its available travel.  Carried to extremes the speaker will be damaged - a bit like over revving an unloaded car engine.

There is another factor that conspires against the bass guitarist and that is the nature of human hearing.  The human ear is quite sensitive to mid band frequencies, but it rapidly gets less sensitive as the frequency goes down.  This means that to sound about as loud as a tenor guitarists a bass guitarist has to push quite a bit more power, but this is also where speaker systems start to run into the problem of air compliance, so bass players (and synth/keyboardists) get hit with a double whammy of a deaf audience and poor speaker performance.

If a bass player were using an open back combo, say like a classic Fender Twin, and simply turned up to try and compete the most likely outcome would be damaged speakers as they try to travel a long distance to pump the required air, but can't get a "grip" because it slips out of the way (and simply flows around the back).

So the first thing you notice about bass rigs is that they are almost invariable closed or sealed cabs, possibly with a tuned port of vent, but never open back.

The second thing is that they often have either very large diameter speakers (15-18 inch), or lots of smaller ones (e.g. 8x 10-inch).  These give a large effective cone area which can move a lot of air, and smaller drivers in particular will often have quite long cone travel available compared to normal guitar speakers.

Bass cabs are almost always quite large, and sometimes resort to fancy tricks such as horns like W-bins and J-bins.  The classic Acoustic 360 bass amp had a cab about the size of a refrigerator and if you look inside you discover that it was a W-bin constructed as a split and folded horn off the front of the internal rear-facing drivers, and the nature of a horn is to increase the air loading on the driver.  This alone can produce as much as ten times the Sound Pressure Level of a normal direct radiator cab.

Reproduction of bass frequencies is an interesting and complex business which is a particular interest of mine (as a synth/keyboard player) and I have only skated over the surface of the problems and solutions here, but some general rules are;

- bass drivers need to be large so they can move a lot of air with little cone movement, and drivers must have a low self-resonant frequency (because their ability to reproduce drops very rapidly below their self-resonant frequency, f₀).

- "Small" and "bass" are a basic contradiction in terms; chose one.

- need to be in a sealed or properly tuned cab so they don't unload at low frequencies

- need to be handle fairly large amounts of power because of ear insensitivity


At low levels you can get away with a lot, but as you need to get louder you are more constrained to large sealed cabs with large drivers and amps with lots of watts.


If you are a bit handy with woodwork there are plans for J and W horns that are quite effective for bass guitar, and also quite efficient in their need for drive power to get reasonable levels in club gigs and the like.  When it comes to bass a good cab can save a lot of amp watts.

This is one that I built that will serve most small/medium gig situations that uses a 15-inch driver and only requires a 50 watt amp ('tho it will handle up to 200 watts);



There are other designs such as J-bins that are quite home buildable.

General cabinet construction techniques;
http://www.ozvalveamps.org/cabinets/fane-loudspeaker-book-pages-01-11.pdf

1x15" rear loaded horn J-bin can be scaled up or down for 18" or 12" driver;
http://www.ozvalveamps.org/cabinets/fane-loudspeaker-book-pages-34-41.pdf

HTH

[Steve Dalllman]

The info given is good. It isn't the amplifier but the cabinet that is important. An open back has no internal air cushion to limit the excursion of the speaker. The speaker matters as well. Guitar speakers do not have the excursion needed for the lower bass notes. The higher the excursion, (Xmass) the better for bass. The cabinet should be designed with the intended speaker in mind. Most speaker manufacturers give the specs on their speakers so the box size, and the port size (if ports are desired) can be calculated.

As for the amplifiers...bass amps generally require more power and headroom than guitar amplifiers. Guitar amps often have the low end restricted for tonal reasons, and that may not sound great for bass. A guitar amp will not be hurt by using bass.

The tone controls of a bass may be tuned lower. Individual amplifier stages will have larger coupling caps. The filter capacitance may be much higher.

In bands today, amplifiers from 15 to 100 watts are generally used, with the trend being toward smaller amps, which are often mic'd. 300 watts is considered the lowest power for a gigging bassist, with 500-1000 watts being common.

Trends change with all instruments. 100 watt stacks were common decades ago, but today, only metal and hard rock bands use amps that big. PA's have improved over the decades.

With bass, large cabinets were the norm. Today, with better speakers and good design, smaller cabinets are common. 10's (in multiples) are the most popular bass speakers. 12's are becoming very popular, giving 10-like highs and 15-like lows. 15's are still popular, but often in single 15 cabinets.

Foster/Fostex horn tweeters were and are still popular, although a trend has begun of using midrange cone drivers instead of tweeters, or with tweeters. Popular speakers for gigging bass players are often a 15 or 12 with a 6 1/2" midrange, crossed over between 800 and 1000Hz.

Vertical speaker placement is becoming very popular, rather than side by side speakers. This is to improve horizontal dispersion and limit phase problems in the audience. The same trend has happened in PA's with line arrays being very popular. Same principal with bass. Two 2X10 bass cabinets stacked vertically is preferred to a typical 4X10 cabinet.

But I ramble.

[Roly]

I ramble.

But well.   :dbtu:

[Jim ONeill]

In simple terms - you can plug ***any*** electrical instrument into ***any*** amp.  Bass, mandolin, mic keyboard, banjo, etc, etc...  In the 50's & 60's it was common to plug a mic into a guitar amp for vocals.  An amplifier takes a small electrical signal (any signal) and makes it larger.

As explained in great detail by Roly, the problem lies with the speakers.  The speakers provided in guitar combo amps are not designed for bass.

If you like the sound of a guitar amp, get a properly matched (impedance - if you don't know what that is, look it up - you need to know it) bass speaker and rock on.

I regularly use a '79 Traynor Monoblock II amp with a Hartke 410 cab for bass or a Fender 112 cab for guitar.  I have also used my '72 Fender Princeton Reverb for bass at low volumes.

[mexicanyella]

Just checking in after being away for awhile, and what a great thread to check in on! Lots of info here, presented in a no-nonsense but articulate manner.

As a longtime guitarist and beginner bassist, this subject interests me too. I'd love to have one rig that sounded good enough to me to use with both instruments, so I could declutter my basement and feel less guilty about the various dusty items I don't use as much as I used to. I've been experimenting with a few different combinations in the past year, with one main goal being to look for guitar/bass dual capability.

It's beginning to look to me like there are some seriously different opinions about what constitutes good bass tone. To me, my P-bass copy, played fingerstyle with an ear more towards midrangier R & B-ish sounds, sounds pretty decent through any simple amp with a three-band EQ and just enough gain to achieve the threshhold of distortion, and the speaker configuration just limits how loud I can play. One of the tones I've liked best so far is a little Dean Markley K-20 guitar practice amp, on the drive channel but with the gain low. It's bright and gritty, but if you back off the volume and tone pots on the bass itself, it can sound natural and warm at low volumes...like, practice-along-with-a-smal-boombox-volumes. Of course, it's very easy to achieve "way too loud for the little 8" speaker to handle" levels with a bass, too, so you have to be realistic.

I've also liked the tone of the bass through my Fender Acoustasonic Jr. amp, which is basically a full-range PA-type amp feeding two 8" speakers and a single piezo tweeter. It's cleaner and more "modern" sounding, but it sounds good to me too.

I do not find that I can plug my guitar into any old bass amp and like the tone. But I have found that my Line 6 1x10 bass amp can achieve good sounds with either instrument (and my lap steel)...but it requires pretty radically different settings to do so. The factory model presets sound pretty decent with bass as-is, with some improvement possible, whereas it takes some pretty serious bass and low-mid cutting to get the guitar tones. But they're there.

Jim, I heard a bassist friend of mine get some pretty good (quiet) tones on a recording by running his J-bass into a silverface Fender Deluxe Reverb. I bet that Princeton does sound nice.

[Roly]

As if we didn't already have enough cans of worms open already, now you come along wanting to do both in one rig!   :loco

Well, as it happens, I think you can.  As a synth player I have a problem at both ends; "guitar" and even "keyboard" amps simply don't go low or high enough.  I want to kick punters in the chest AND shatter wine glasses.   8|

As a rule of thumb a driver will cut off around 80-times f₀, but you want a driver with a low f₀ to produce the bass, so the implication of this is that a single driver won't do it, you need tweeters as well.

If you look at my W-bin above you will see a couple of letterbox tweeters crammed down the middle.  Tweeters?  In a bass bin?  Am I mad?  :duh  Well yes, but that is beside the point that tweeters give the bass "snap" when it's required, something the horn alone won't provide.

This particular build taught me a few things, and I wouldn't do it that way again knowing what I know now.

What I have been looking at for a long time (and have a pile of scrounged wood around the place awaiting the attentions of saw and drill) is a rear-loaded, front-radiating, J-horn, with a low-reach tweeter.

The reason for going for a front radiator is that it acts like a pretty conventional speaker in mid-band; and for the J over the W because you can get a longer horn path in the same overall volume of cabinet, and they are simpler to build.

To produce the lowest note on a four string bass requires a horn with a minimum mouth area of about a square metre.  You can compromise on this area but it will compromise on the f_low cutoff.  But with a J you can make it bigger simply by making it taller.

Horns reach low, but they also tend to cut off early too, in the mid band, and that is where the front radiation takes over, the horn effectively "going solid" at those frequencies.

Next you run into the times-80 rolloff of the driver itself, and that's where the low-reach tweeter comes in.  By low reach I mean around 1800Hz, and tweeters that go down that far ain't cheap, but worth every penny IMO.  For guitar it may be sufficient to have a secondary "top box" with a 10" or even a couple of 8's fed via a capacitor as a very basic crossover; or if you use a 12-inch driver it may go high enough to be satisfactory.  The tweeter must have at least a cap in series because it can't tolerate lower frequencies, and will generally need a tastefully chosen low value power resistor in series to balance it/them with the main speaker or it's likely to be shrill.

What do you drive it with?  Well, what have you got?  Once you have the reproducer right the amp is much less of a problem.  It needs lots of watts, obviously, and a heap of EQ will be helpful too.  Many bass rigs also have some form of compressor so they stay away from clipping, but a compressor and graphic EQ can be patched in the Fx loop and set as required for each mode.

The J-Horn design referenced above can be scaled down to a 12-inch driver (which is what my next speaker builds will be, two of), or scaled up to an 18-inch for really serious bass, but for dual use I'd suggest that the 12 alone would be sufficient unless you are playing large venues, in which case I'd go for the 15, but some tweeting would then be essential.

So if you want good bass on a beer budget you need to brush up some quite basic carpentry skills (and believe me, I ain't no carpenter).

[mexicanyella]

Roly, I "favorited" your link to the J-bin plans that you posted awhile back. That does look like a cool project, and one that would take my carpentry skills to the limit but might still be possible.

I have a recently acquired a Peavey M-3000 monaural power amp (210W into 4 ohms; 300W into 2 ohms) with DDT compression, currently sitting around idle looking for something to do. If I could decide on a preamp I liked and built a J-bin like you describe that I could still lift and transport...hmmmm.

In your keyboard playing, have you found tweeters that handle distorted textures particularly well? Not sure how best to phrase that, but I haven't spent much time thinking about tweeters or listening to them comparatively. I know the piezo tweeter in my acoustic amp is generally unpleasant with electric guitar--even electric guitar fed through a speaker simulator--unless you EQ the bejesus out of the signal.

But I too have noticed that it adds a nice crisp edge to a bass guitar at the low volumes I've been using.

Thank you for the information and suggestions. I'm going to be thinking about drivers, tweeters and keeping my fingers intact for bit, and will likely be back to ask you more questions.

[Roly]

Appended are scans of something almost identical from an Audio Engineering Society (AES) publication which have specific dims for 12, 15, and 18 inch drivers, and their respective sweeps.

There are basically two types of tweeter used in this sort of application, piezo (which are cheap) and voice coil pressure driver (which aren't).  I used piezos 'coz I'm cheap ('tho rather more up-market ones), and while they seem to work okay, particularly if they aren't over-driven, they have a rather lumpy frequency characteristic which I suspect is due to various resonances in the piezo element.  I've swept various piezo elements naked on the bench and they show a lot of resonant behaviour.

I can't really say much about how they handle "distorted textures", only that I haven't noticed anything that would demand my attention to fix, but perhaps a) I don't use voicing that might be a problem, and b) I don't push them hard at all; they are padded with power resistors that means they only add a taste rather than going flat chat.

You can get a couple (or four) "bullet" tweeters into the corners of the speaker baffle without modification, but if you go for a letterbox tweeter it may be better to mount it below the main driver back into the spare cavity space.

If you can afford pressure driver tweeters with large letterbox horns so much the better, but whatever you use go for those intended for PA use as domestic "Hi-Fi" units are pretty fragile, and avoid anything that has "soft" or "foam" in the description of the driver.  CTS seems like a brand to look for.

[mexicanyella]

More great info; thank you. I will add this to the Fane packet. I'm getting psyched up to try building one of these.

Your comment about running the tweeters padded down for just a taste, rather than full out, made me think that I'd benefit from devising a way to turn my acoustic amp's tweeter up and down relative to the main drivers. I'd have a better idea then of what a tweeter can add before it gets too "present," with different sound sources.

I've seen speakers for sale with so-called "whizzer cones" protruding from where the voice coil dust cap would normally be. Is this to allow a conventional driver to sort of fudge its way into approximating an upper-range driver? Does this practice sound especially good or bad for any specific uses?

[Roly]

I'm cheap, so I just use a power resistor, but you can get suitable variable controls like this;


http://www.altronics.com.au/index.asp?area=item&id=A2110


...which is just a low resistance high power wirewound pot that you can use to control the level going to the tweeter.

Nothing wrong with dual-cone speakers - if you can find something suitable.  The ever popular Rola P-series was available with a whizzer cone and they are as rare as hens teeth these days; standard kit in many early Aussie PA's and a few guitar amps, not to mention many thousand domestic "radiograms".

Just a point about the drawing above.  While it's complete it could be a bit of a dog to build from directly and I strongly suggest that you generate your own working drawing from that before you start to save you having to measure from here to there and then back to over there.

If you are going to build more then one take a piece of Masonite or three ply and make a drilling template.

Drill both sides at once by aligning and clamping them together.

Take a side and build from the inside out, putting the other side on last.

Ply is better than particle board/MDF, and if you are going to cover it you don't need the expense on "marine grade" ply - 18mm for bass I'd suggest.

Hardwood 1"x1" for the framing; I use garden stakes and other recovered timber.  The first horn I built was entirely out of scrounged materials.  There may even be some advantage in using warped ply 'cause you can spring it square with clamps (you can never have too many clamps) and it ends up pre-loaded and less likely to vibrate.

Particularly with bass, everything must be rigid, no possibility of flapping bits/areas.

I also add sections of poly drainage pipe to round corners for smoother air flow, but that's optional.

I first assemble a part screwing it in place (with an old cordless running off a computer power supply), when I'm sure it's right I take it off, give it a good squirt of Liquid Nails along the mating edges, then re-screw it.

I'd make the top removable so the speaker can be rear-mounted to the baffle, and you can still get at it.  Alternatively make the baffle itself removable.

Tricking out with covering, corners, recessed jack plate, handles, etc., to taste.  I'd suggest that XLR's are much superior to standard jacks for speakers, and handles should be positioned by balancing the build on a round dowel (e.g. broom handle) to find the CoG, and well-handles fitted on that line, and lowish.  Bigger cabs can be cut at an angle at the bottom rear and wheels fitted just inside the cab outline.

http://www.jaycar.com.au/ShowLargephoto.asp?id=1110&PRODNAME=Cabinet%20Handle&IMAGE=

I also include a rodent guard of 1/4 gal mesh back up the throat a bit, just far enough to be out of site but still in the low pressure area near the mouth.  It's surprising how many people mistake a horn for a rubbish bin - bottle tops, ring-pulls, ciggie butts, even whole bottles.  Or you can put speaker fabric or mesh over the mouth (but why hide your creation?).

Something I did on some bins was to fit an "OverLED" so the mouth glows yellow on peaks and yellow+red at full chat - looks great.

[mexicanyella]

Thanks as always for taking the time to think about this and offer suggestions.

Why do you recommend rear-mounting the speaker onto the baffle board, whether through a removable top or onto the back of a removable baffle? What is the disadvantage of just mounting the speaker's flange onto the front of the baffle board? Is it just to make grille coverage of the cone easier and more flush? Or is there a mechanical/structural reason I'm unaware of?

Second question: is the length of the horn calculated to add in some time-of-arrival difference, so that the sound coming off the front of the cone and the sound coming off the rear of the cone and through the added horn length emerge in phase with one another?

Third question: Is there any reason why a rear horn-loaded J-bin wouldn't work on its side, in horizontal orientation? Is the stood-up vertical arrangement required for some acoustic reason, like, maybe what's coming out of the horn is strong in the lower frequencies but limited up top, and all the 500 Hz and above (or whatever range) is coming only off the front of the cone...and having it standing up gives better distribution to all possible listening positions?

More and more I'm liking the idea of covering a cabinet with a coating like truck bedliner compound, or that industrial Durabak paint that has the rubber crumbs in it for traction...instead of tolex or animal-hair-and-chaff-collecting live PA carpet stuff.