I'm thinking of buying a preamp (Rocktron prophecy / voodu valve) - does anyone know what difference it would make as to whether I put it through a valve power amp e.g. a mesa boogie 20/20 or solid state e.g. a 1500 watt cloud power amp?

To say they're similar pricewise, the difference of 20 watts per channel and 750 watts per channel seems quite a lot... any idea why this is? Would it make any difference soundwise that the power amp doesn't have valves in and isn't specifically designed for guitar? (Considering that the preamp contains a valve and produces a guitar tone)

Would the power amp simply amplify the guitar tone produced by the preamp or would it add its own, presumably non-guitar frequencies / distortion?

Many thanks, Chris

This looks interesting.

http://en.wikipedia.org/wiki/Valve_sound

Hope it helps,

Joe

Thanks, that does have some interesting facts! Looks like getting a more modern power amp would be a definite advantage.

I'm still confused about whether the tube in the preamp would mean that the overall sound would be 'warmer' even after its run through a solid state amp or whether I'd have to use a valve driven power amp to get this effect, however!

ie, would the partially valve driven preamp initially remove the harsh higher frequencies and produce the right harmonic response, and if it did would putting it through a solid state power amp then re-add these frequencies to the signal and cancel out even-order harmonics again?

Hope that makes sense, I know its a difficult question but any help or experience with this sort of set up would be appreciated!

Actually, for anyone else pondering this sort of stuff, one of the links at the bottom of that page is very helpful! (A report comparing transistor, hybrid and valve amps under studio conditions)

Hybrid amps (containing both valve and solid state components) actually seem to fare pretty badly... though as I play death metal I might not be looking for the same characteristics as for instance a jazz trumpet player or singer might be, which seems to be the point of view that they're looking at it from.

I might just have to go and annoy local music shops by trying out lots and lots of their gear then buying a 6 quid cable (again!)

Crayfish,
is the wattage you are stating peak or RMS?
Ignore anything which says peak or PMP or PMPO. The only value that matters is RMS.
Valve power amps in my experience always sound better, clearer and much louder for guitar. Hybrid amps like valvestate marshall are ok for a classic metallica sound as they have a heavily overdriven crunch sound.
750w is much much to powerful for a guitar setup, unless it is also the PA for the band.

As a rough guide an ideal set up would be a balance like this for an average live show for a rock band at say the boardwalk or similar size venue.

all power ratings expresses as RMS
1000-2500w vocals
50W-100w valve or 100w transistor Guitar amp
250-400w Bass Guitar amp
100w for keyboards

Guiitar amps are designed to amplify a narrow range of frequencies very highly and use the characteristics of a stiff coned guitar speaker, and it's interference patterns.
A PA amplifier and speakers is designed to operate in what is called a "full range" of frequencies, which as an industry standard is 20hz up to 20khz, without dropping 3db[or sometimes 6db it doesn't matter]. If you were to use equipment like this for a loud guitar sound you would mask the other frequencies which are reserved for all the other instruments, and probably cause extreme ear pain to anybody who was in the way of the loudspeakers.

Even if you were to operate an amplifier like this at a low volume, the [power headroom] would be too high, and you would probably still get extreme feedback problems.

Okay, that makes sense! Didnt realise that too much power could be a problem. Saw high numbers and thought louder must be better :) That does make the whole thing cheaper, which helps

Looks like it'd be safest to look out for a 50/50 valve amp or something then... though they seem to come pretty expensive. Failing that a 20/20 might even do the trick as they're always miced up at gigs anyway

For my next question.. (the other end of the setup!)

Is there any reason why people use 4x12 guitar cabs? Would there be any problem in using a 1x12 cab with a more powerful speaker instead? (I have my eye on a couple on ebay, a 100 watt 1x12 and a 200 watt 1x12... though if I'm only using a lower wattage power amp i don't suppose there'd be that much difference between those?)

It seems like it'd sound the same or even a bit clearer, as the microphone would only be picking up a signal from one speaker - and would be a hell of a lot easier to cart around!

(Though not as physically iconic in a 'my amp's taller than I am!' sort of way)

Yeah, the question part being: why does everyone use 4x12s? what does a 4x12 give you that a 1x12 wouldnt?

Thanks, Chris

Right,
If you are buying guitar speaker cabs, make sure they are proper guiar cabs loaded with guitar loudspeaker cones and not PA cabs.
Celestion G12s are common ones G = guitar 12 = 12inch.

Guitar speaker cabs have many design characteristics. Usually they are set up so that adjacent cones are out of phase. So that you get interesting troughs and peak interference patterns,remember light boxes and water in physics at school?

Also guitar is essentially a mid range instrument so there is no tweeters and the back is often enclosed back of the cab to give a boxy sound and a better bass responce for a guitar. Compare to bass guitar cabs which have baffles and often 10 inch tops.

The more speaker cones you are driving from one amplifier, the louder the guitar sound will *appear* to be. Even though having more cones is less efficient as you will have losses due to inertia of cones and voicecoil, eddy currents hysteresis etc..

Plugging in multiple cabs into a single amp.
Right techy bit. If both cabs are 8ohms
then you will present 2 ohms in total at the amp.

If both cabs are 16 ohms then you will present
8 ohms in total at the amp

If the amp is rated at full power 4ohms, then adding more speakers will short out the output stage and cause damage.

The lower the impedence of the speakers, the harder the amp has to work. If the amp is working really hard it's more likely to blow a fuse or overheat. So don't drve an amp really hard for a 2 hour gig.

Don't use guitar cables for speaker connections, connecting amps to cabs, get special speaker leads, these have more copper, and can handle the power and your sound will be better, they are tougher and less likely to fail. I make my own up from mains flex or professional speaker cable from a hifi shop

If you line up a 4x12 marshal cab next to a 2x10 marshall combo amp, the 4x12 will always sound better. It's ritcher and bigger and appears louder, because there is more interesting physics going on between 4 cones than there would be from 2.

If you have the transport, and the funds, and the space get the 4x12. Although people often love the indivivual sound of smaller amps, I'm guessing you need the magic which a 4 x 12 gives to the sound of a rock or metal guitar.

If you can go to a shop and try to hear the difference yourself.

Thankyou :) That was a very thorough answer

Funds are uncertain, but if I manage to raise them I probably will go for the 4x12 to play with live, but also a 1x12 because our practice studios are up 4 flights of stairs and i definitely can't afford a 4x12 for practice and a 4x12 for home!

That has made me clearer about what I'm shopping for anyway, thanks! Just got to decide if I want a preamp+poweramp or a head now :)