What are the differences between bass and guitar heads? I have recently obtained a Hartke cab and I am looking at heads. I see that there are heads just specifically made for bass cabs. why can I not use a guitar head to power my cab?
The demands on an amplifier/speaker combo designed to reproduce the sounds desired for bass guitar are different than for a regular guitar.
To accurately and faithfully reproduce the low frequencies needed for bass guitar, the speaker surface must be able to move a greater distance and move more air. This greater movement demands more room in the cabinet and either a closed back or ported design. The larger overall volume of speaker surface used for bass cabinets combined with the large movement required to reproduce lower frequencies - results in greater acoustic pressure. Therefore the bass cabinets tend to provide a greater overall volume within the cabinets - and/or a porting mechanism to prevent the pressure from the back of the cabinet from interfering with the sound waves coming from the front of the speakers.
Getting that much real estate inside the bass cabinet and combining an amplifier head in the same box (as in a combo amp) would make the overall size of the unit unwieldy due to size and weight. That is why it is common for bass amplifiers to come in two pieces. It allows for large volume cabinets with heavy magnets to move large speakers a greater distance. Combo's exist for use in practice or small venues where less volume is required.
Historically, most guitar amps are combos - meaning the amp head and speaker cabinet are combined into one. Guitar speakers don't have to deal with the low frequencies, thus smaller speakers are more common (for cleaner reproduction of the higher frequencies a regular guitar produces), and open backed cabinets can be effectively used reducing overall weight further. Recently amp manufacturers have been inventing more low wattage "guitar heads", but the smaller wattage heads are primarily used for getting a tube amp sound in recording applications and the larger guitar heads are designed for getting bigger sound in large venues when coupled with an appropriate stack of speakers.
Getting back to the crux of your question. You could use a guitar amp head to power your cabinet but you must be careful to get a proper and compatible match for both power and impedance.
For one thing, you will need sufficient power to drive the speakers. But be careful not to get more power in your head than the speakers are rated for. Most bass cabinets have speakers rated for 100 watts or greater. A multi speaker cabinet will have a higher overall power handling capacity than each individual speaker. Ideally your power amp should provide less power than the maximum power the speakers are rated for. A good bet would be an amp head that provides about 50% to 75% of the power the speakers are rated for. But too little power may fail to satisfactorily drive your speakers. So if the speaker cabinet is rated for 400 watts and your head only provides 60 watts, your tone may suffer significantly.
The other thing that is critical in matching a head with a cabinet (includes speakers) is getting the proper impedance (measured in ohms) match between head and cab. You must get an amp head that is rated for an impedance level that is compatible with the impedance level of the speakers. If your cabinet is rated for an overall impedance of 4 ohms - you must be sure your head is rated for as low as 4 ohms. If the cabinets impedance is lower than the lowest one the amp head is rated for, you will damage the head. So if the cabinet was rated at 4 ohms and you used a head with a lowest rating of 8 ohms, your head will soon become a large paper weight. Bass cabinets often have lower impedance ratings than guitar speakers - so be careful here.
On multi speaker cabs - you must look at the overall impedance level of the entire cabinet, not the individual speakers. This is because the overall impedance will vary on a 2 or 4 speaker cab, depending on if they are wired in series or parallel or a combination of both (as is often the case in a 4 speaker cabinet).
Finally, if you intend to play mainly guitar through your stack, you might be better served with a guitar cabinet matched with a guitar head - or simply a guitar combo amp. The speakers and cabinets specifically designed for reproducing the lower frequencies of a bass guitar, are not optimal for reproducing the higher frequencies of a regular guitar. If you want to play bass guitar through a bass cabinet matched with a guitar amp head (maybe you want to add more distortion or other "guitar effects" to your bass?), that should work fine as long as the power and impedance rating between cab and head are optimally matched as described above.
Back in the day, there was more overlap between guitar and bass heads than there is now. Guitar amplification imparts a lot of the final tone that we love of electric guitars and most guitar amp designs are either similar to designs from the 60s and 70s or are meant to emulate the sounds of those designs.
We do also love the bass sounds of the 60s and 70s but there are newer bass sounds that are very popular. Only a few new popular guitar sounds have appeard in the last 40 years (most notably PRS guitars and Mesa/Boogie amplification and all the sounds inspired by those two), but many new bass sounds have become popular.
Bass players who are not looking for a vintage sound usually want full-range amplification. That means lower lows and higher highs than guitar amps can usually produce. Reproducing a wider range of frequencies requires more power to get the same loudness, so bass players today desire a lot more power than before. Concurrently, or possibly in response to these changing desires, bass speaker cabinets and amplifiers have taken advantage of modern circuit and magnet technologies to be able to reproduce a wider frequency range with more power for much less weight and size. Some bass players use PA equipment to amplify their instrument instead of bass equipment, and there is a lot of overlap in modern bass equipment and PA equipment design. Modern bass amplification is almost like a mini PA.
If we look at the lineup of current Marshall guitar amp heads, we see a range of 30W to 100W of rated output power, and most of the designs are 100% tube, hybrid, or meant to emulate tube amplification.
If we look at the lineup of current Ampeg bass amp heads, we see a range of 100W to 1200W of rated output power, and a significant number of the designs are solid state or hybrid (a little less than half, I think).
Here's a breakdown of things that are more common in bass heads:
Note that due to power output and handling, it's usually safer to power a bass cabinet with a guitar head than it is to power a guitar cabinet with a bass head. Clearly, plugging a 1200W bass head into a guitar cabinet that is expecting a 100W Marshall head could be very bad.
The main difference will be the frequency range the head is capable of controlling. The lowest note on a guitar is not far off the top string of a four string bass guitar, so an octave of lower notes needs to be treated with some equalisation that just isn't needed by a guitar. Likewise, a guitar uses far higher notes than a bass, normally, so the eq. on a guitar amp will need to cater for a different, higher, range in the sonic spectrum. Also, it's more usual for a guitar amp to have some sort of overdrive built in. If it's a valve amp that's intrinsic, but not so necessary for bass amplification. It's also more common for bass heads to have an XLR socket, to D.I. into a P.A.
The guitarist in a band I used to work with used a Hartke bass head - sounded lovely!
The obvious difference between bass and guitar heads/amps/cabinets is that each one is designed for a specific instrument. The bass guitar has a lower range than that of the guitar, so the related amplification equipment must be designed to express the lower ranges better than the higher ones.
This doesn't mean that you cannot connect a guitar to a bass head/amp or vice versa, but for the best outcome, you should stick with that there is for the instrument you want.
What I'd suggest is to try heads both for bass and guitar and see what sound you like better. You might prefer a guitar head for your bass cab.
An anecdotal answer: This one time at band camp (actually it was a wedding), our bassist arrived and realised he'd forgotten to bring his bass head. After we'd kicked him around the car park for a while, I suggested we try using a small PA amp which we use for our foldback speakers. It's about 150 watts (mono).
I was expecting it to crunch and struggle with the power but it was worth a shot. So we plugged it into his bass cab on gave it a go.
Actually, it was fine and our man reported, after his wounds had healed, that it had easily enough oomph to fill the room (a hall about the size of a tennis court). There was a bit of a difference in lower-mid response but with some tweaking of bass controls, it did the job.
It occurs to me that although there will be differences in a bass cab to a PA vs. Guitar cab, (others have no doubt covered this), the majority of the difference is in the speaker cabinet which will be designed to handle & brutal bass in a pleasing manner.
I have also tried playing a guitar through a bass head and although it was a bit 'chunkier' (which translated to a bit too bassy), with some tweaking of EQ it was quite useable.