What makes a Bass amp a Bass amp....or can I use a guitar amp?

[TerryD]

I teach guitar and have an absolute beginner that wants to learn bass and figure I can keep ahead of him.  I have a bass guitar coming.  How can you screwwww a guitar amp up with a bass...or what gets screwed up?  Here are the amps I have with their speaker configurations.

68 Fender Bassman head completely reworked with Dan Torres stuff to be guitar oriented.  With this I have two cabinets that I use seperately.  One with 2-12s JBL D120Fs and one cabinet with 2-15 beefy/35 watt? magnavox stereo speakers and very nice.

Vox nighttrain head with one open backed with 2-12 celestion greenbacks.

Epiphone jr with one 25 watt 15 cabinet

last but not least a 135 Acoustic Amp.

Can I use any of these for bass?    Could I modify any?  The Acoustic is a beast at church, so another would be better, especially the Epi.   Thanks ahead.

[HotBluePlates]

Well, think about the mods done to the Bassman to be more guitar-friendly...

You might have made coupling caps, cathode bypass caps and/or tone circuit caps smaller to push them into the higher guitar range or to strip out bass to keep things from getting really muddy when you have a lot of distortion. So sometimes bigger caps are used.

You might use larger diameter speakers in a bass setup (think Ampeg B-15), but then again you might use small speakers for a punchy sound (think Ampeg SVT 8x10 cabinet, or an Eden 4x10 cabinet).

A guitar amp might use an under-rated speaker, whose power handling just equals the amp's output power, so that you can be assured of driving then speaker until the cone physically twists and bends during a note, giving you "speaker distortion". That's not to rough on the speaker unless you play full-power square waves by blasting the amp with a RAT pedal in front of it. Square waves heat the speaker more than sine waves of the same peak voltage, and can burn up a speaker. But you likely also know if you play cleanly through a guitar amp, the speakers generally last forever.

Well, in a bass amp, you almost always play cleanly. But those low notes make the cone jump really far. The bass' notes are down around the bass resonant frequency of the speaker, and make it jump so far forward and back on the initial transient that the speaker literally tries to pull itself apart. So part of the genius of the 10" speaker in the bass cabs is to have the resonant frequency a little higher so the cone doesn't jump through the grill cloth, while at the same time you use much more speaker power capability than amp-power capability. Yes, you don't get the gut-wrenching low end that a 18" or 15" speaker will give, but the roll-off of the lowest tones gives the bass a punchier, more authoritative sound that really jumps through a mix.

And there's a psychoacoustic effect where if you hear all the harmonics of a note, even if the fundamental is missing, your brain fills it in as though it were there. So you feel as though you're hearing all that low end even though you're not. Just recall all the recorded music where the lows of a kick drum are rolled off, but you still know you're hearing a kick drum.

But in the end, you're gonna be in a teaching situation, playing fairly soft. I'd use at least a 30-50w guitar amp with a lot of speaker cab, if in a setting where the amp can be left in place all the time. You won't have to worry about speakers blowing out with the amp set on 2-3, and the sound will be plenty clean. You might prefer keeping the treble knob rolled back a bit, or viewing it as a presence knob, while adjusting the mid knob for the high end of the bass notes. Should work out fine.

[bigdaddy]

It really doesn't matter what head since you are not going to be gigging or rehearsing at volume. It's the cabinet design, any of those heads will produce enough clean low end to be heard. It's about the cabinet being able to reproduce those low frequencies. The Low E string is 41.5 HZ and most guitar speakers will do 70-80 at best according to their specs which is debatable.

So I would try to get a cheap bass cab with a descent bass 15, 1-12, 2-12 or a pair of 10's. or even an empty one and fill it with some Eminence bass speakers like the BP102 10" or one of the many 15 inch speakers they make for bass and low end. There's stuff on ebay all the time, get an empty cab and fill it. It's all about the ability of the speakers voice coil to travel a certain distance to reproduce those low notes. They call it XmaX. By the way you do not need one of those horns in there that every bass speaker manufacturer puts in. It just adds distortion to the sound and adds expense, totally useless. Unless you're playing a certain type of music and need very clear highs, which you don't get with those cheaper horns and crossovers which add distortion.

[triode]

>>It really doesn't matter what head since you are not going to be gigging or rehearsing at volume. It's the cabinet design, any of those heads will produce enough clean low end to be heard.

I beg to differ, slightly. I can tell you for fact that all guitar amp makers (that I know of, and I have spoken
to some major manufacturers personally) use "cheap" or "guitar" iron, one way or another. What I mean is,
if you ask any tranny maker, which basically is the most expensive thing in the amp, frequency response costs
money, particularly at the low end. It is not a small insignificant difference... a tranny that can do full or half
power at 60hz costs a lot less than the same iron that can do that same power level at 30hz.

In a nutshell, my comment is: guitar amp output transformers are in fact different than bass output transformers.
Guitar OT start to roll off at say 100hz, so that at 70hz or 60hz, its "enough". Bass guitar OTs _should_ start to fall
off at 50-60hz so that the 40hz is "enough". Personally, I use iron that starts to fall at 30hz, minimum on a bass rig.

Real world example: compare a Dynaco A431 (MkIII) OT (60W, same as the Sunn), to a 60W guitar OT.
I am pretty sure you will find a nice multi-lb difference... The A431 weighs 12lbs (I have several), a Fender 60W
weighs 4.7lbs, a Marshall 100W weighs 7.8lbs. Not to make a direct comparison by weight, but iron and copper
weigh a lot, and it takes more to get lower in frequency. The A431 will do full output at 25hz. I doubt the Marshall
would do 1/2 output at 25hz. So, if you want a 60W bass amp, you need a tranny that is not going to roll off that
low end very quickly. I use the A431 in my bass amps, and I can tell you that it does 40hz with major authority.

My $0.00002.

[bigdaddy]

Like I said just to be heard in a room to practice or jam at a low volume or teach it doesn't matter at all what amp you use. It's the cab and speaker, believe me I put together many bass rigs and sound systems.

You would be amazed at how loud a little guitar tube amp can be with a high efficient bass cab and speaker. Like those folded horn cabs.....old Perkins bins, scoop bins, stuff you don't really see much anymore because of their size and weight. But a good speaker designed to reproduce low end in a descent cab made for low frequencies will be fine with a guitar head as long as you do not turn it up, it will distort then.

[Jim Coash]

I make no distinction between amps for guitar, bass or anything else for that matter.  Sure, each sounds different and there are reasons why a dedicated bass amp might have advantages for that use, but all things considered, the amp sure doesn't care what you use it for.  many times I have used my PR for bass and often I have played my Ampeg B-15S with my guitars.  Just be a little careful not to clip the amp or drive the smaller speaker too far.  If there is one thing that makes any good amp a nice amp for bass it is choosing a large diameter speaker that will generate deeper bass more easily.  When I use one of my E/V 15" speakers with a small amp, the sound is glorious!  Jim