Hello, Simple amp question, maybe

[Ting Ho Dung]

So let's say I have two matched 20 watt heads. If I run them both on their own cabs at the same time do I have 40 watts? I know 20 + 20 = 40 but things are different when it comes to amps. At first I thought the equation was simple but then I thought it was still 20 watts just with 2 amps both pumping out the same volume. So two amps both pumping out the same volume; does it double the volume? I know to double the volume on a single head you would need 60 watts (20 x 3). Is running two 20 watt heads each on a 1 x 12 cab the same as running a single 20 watt head on a 2 x 20 cab volume wise? Is running two 20 watt amps each on their own 1 x 12 the same as running a 40 watt head on a 2 x 12 cab?

So let's say I have the same two heads, one on a clean setting and one on a dirty setting (these are both single channel amps). Can I A / B them into the same cab since there are two jacks in the back of the speaker cab? In other words, one at a time through an A / B switch. Then I would have what would simulate a two channel amp. I know the second jack is normally for another extension cab. If you say everything is okay so far let's say I accidently (or on purpose) A + B them together into the same cab. What happens? House on fire?

[triodecr]

Doubling power equals a 3dB increase in acoustic amplitude. An increase of 10dB is doubling the perceived amplitude. So running 8 amps would get you double the acoustic power (if I did the maths right).

Driving one speaker cab with two amps (assuming the cab is wired with both jacks in parallel) is bad idea. The amps are driving each other and I would bet eventually one or both will fry. To run two heads into the same speaker you want this: http://www.tonebone.com/tb-headbone-vt.htm

Driving a cab with multiple speakers each wired to their own jack would allow you to implement your A/B/Y idea safely. Each amp would drive one or more speakers separate from the other amp.

[cynic]

Do you have an objective in mind or are you just wondering how crazy you can get with two amps?

If you have something specific you'd like to accomplish it might be easier to make recommendations to get you there if we knew what it was.

[Ting Ho Dung]

Do you have an objective in mind or are you just wondering how crazy you can get with two amps?

If you have something specific you'd like to accomplish it might be easier to make recommendations to get you there if we knew what it was.

I do have an objective and was also wondering how crazy I could get. I have a Vox Night Train 15 which I love. I mean, I really love this thing. It has a thick switch that bypasses the tone stack and puts it right in an over driven (brown) state. Unfortunately there is no foot switch for this. So to gig it and switch between that and the "vox" tone I would need another amp and cab on an A / B switch. Then I got to thinking that 30 watts would be better then 15. That's where the brain short circuited because of all the options. Four amp heads instead of two. I know I'm overthinking this and all of you are wondering, "why?" But these are just things I wonder about. I was able to get another NT for $150 so I have my clean vox vs brown tone solved but thought that if 15 watts was good then 30 would be better.

[JohnnyB]

Actually, you get a 3 dB bump for having 40 watts instead of 20, and another 3 dB bump for having two cabinets instead of one. So that's 6 dB, but if you want twice the perceived loudness, you want to be able to increase iit by 10 dB.

OTOH, when you have 40 watts and 2 cabs for a +6 dB increase at the source, it may create a higher perceived loudness indoors as the extra SPL also increases the room gain from wall, ceiling, and floor reflections that you don't get outdoors. So if you add 3 dB more for room gain with 2 cabs over one, that gives you a total of 9 dB which is pretty close to double the volume at the audience's ears, though that partly depends on the reflection patterns of the room. You may also get more perceived loudness by stacking the cabs (or placing them side-by-side) to launch a bigger unified wave, especially if you place this stack where you get some room gain as well. If you put the cabs near the wall behind you it will likely increase bass response.

Edited to add:

I should have read your original post more carefully. I was thinking you intended to have two entirely separate amp/cab rigs with an a/b pedal to switch between the two input jacks. As TomTerrific says later on, you absolutely CANNOT run two amps into the same cab. You will most likely blow the speakers and you will certainly blow the output stages of both amps as each output will try to drive the other amp's output stage. In SS amps this blows out all the amps' output transistors. In a tube amp it probably also fries their output transformers as well as doing who-knows-what to the output tubes themselves. This would be a several hundred dollar mistake, not counting possible fire damage.

This would even be true if you turned one amp off before switching to the other unless you waited at least 10 seconds before switching to the other--hardly practical in the middle of a song. Most amps continue to produce output for a few seconds after the power is cut until its power supply capacitors drain. When I worked in a stereo shop I blew the output stages of two high end 150 wpc power amps (several hundred dollars damage) because of this latent output effect.

[carfish7]

2 separate amps at 15 watts will not "equal" the output and headroom of a 30 watter to the best of my knowledge. Radial makes a tool to switch between 2 amps into one cab and another to switch between 2 cabs with a single head. They are spendy but well-built.

There is also the 50-watt Night Train from Vox if "more louder" is the goal.

[crunchee]

I barely know enough about this stuff to be dangerous (one amp at a time is enough for me!), but it seems to me that there was a reason that guitarists used to 'jumper' or 'daisychain' amps together (Jimi Hendrix comes to mind)...I think it was in an attempt to be heard and get louder, because venue PAs used to suck. As in really, really badly. But, as what has already been mentioned, two amps don't double your loudness, which is why players used to use walls of amps. They still do I suppose, but more for the cool factor than anything else, because PAs are much better now, and regardless of how many amps they use, they're still gonna get miked up to the PA...unless (maybe) your next gig is in a garage or a parking lot. IIRC, seems like there was a brief market for power amps with no or minimal preamps, for guitars and basses back in the '60's and early '70's. They'd get jumpered to the primary amp, the intent being to increase the volume, not color the sound. Of course, you're gonna need speakers/cabs that can handle that, too.

Edited to add: I guess my point is, is this a practical solution to getting more volume? My answer would be, probably not very, and there are probably easier ways to get more volume...like adding more speakers/cabs. Just my two cents.

[tomteriffic]

So let's say I have the same two heads, one on a clean setting and one on a dirty setting (these are both single channel amps). Can I A / B them into the same cab since there are two jacks in the back of the speaker cab? In other words, one at a time through an A / B switch. Then I would have what would simulate a two channel amp. I know the second jack is normally for another extension cab. If you say everything is okay so far let's say I accidently (or on purpose) A + B them together into the same cab. What happens? House on fire?

As for the house on fire: In a word, yes. If you run two amp outputs into the same load at the same time (even if you are A/B-ing the inputs), something is going to let all of its smoke out. If those two output transformers see each other, even through a load, it's not likely to be pretty.

The only safe way I know of is to use an amp head switcher, something like the Radial Engineering Headbone. It uses a "break before make" switching logic and has some other safety features (ground loop lifter, etc.) too.

Alternatively, if you had, say, a 2 x 12 cab, you could wire it for stereo, with each input going to one speaker only and A/B-ing between the amp inputs with a normal A/B footswitch.

[jaberwock]

I have been using two Orange Tiny terrors at our rehearsal space for the last six months, each running through two 1x 10 cabs; ignoring the science, two amps running at the seven watt mode , sound a lot fuller, and "bigger" than one running at 15 watts. If it's any help, the two amps running at 15 watts are way too loud for our rehearsal room, which is pretty spacious.

Jaberwock

[Ting Ho Dung]

I have been using two Orange Tiny terrors at our rehearsal space for the last six months, each running through two 1x 10 cabs; ignoring the science, two amps running at the seven watt mode , sound a lot fuller, and "bigger" than one running at 15 watts. If it's any help, the two amps running at 15 watts are way too loud for our rehearsal room, which is pretty spacious.

Jaberwock

Thanks Jaberwock.

I guess with all the input given it is best for me to get another cab and run each amp head through its own cab. That leaves lots of options open for stereo or A / B between clean and dirty settings. Thanks for your help. #2 should be here tomorrow. Very cool. Thanks for all the information, tomteriffic, crunchee, carfish7, JohnnyB, Cynic, and triodecr. It really helped.

--MT

[Hamer95USA]

Hey Thorn,

Get a 2 channel amp with EL-84 power tubes or use an A/B box like the Lehle Dual SGoS.

Guitar George

[jaberwock]

Power aside, the true joy of using two amps is the stereo effects; delays, chorus, rotary simulators, sound stunning.

Jaberwock

[JohnnyB]

Power aside, the true joy of using two amps is the stereo effects; delays, chorus, rotary simulators, sound stunning.

Jaberwock

I have a tube-driven trem pedal and an Alesis Nanoverb with stereo output with several effects including many reverbs, flange, and rotary speaker. I tried them both out with two combos, one facing front and one rearward, and the results were a lot of fun and could prove useful. Running stereo reverb into two cabs spaced wide puts out a big enveloping sound.