[mistymountainhop]

Marshall Laws -- Impedance Settings

#1: Impedance settings 'n' tube heads

"What impedance do I set my head to?" This is one of the most common question that customers ask and is also one of the questions that invariably receives a wrong/misleading answer. Getting the answer to this question correct is vital due to the fact that setting a tube head to the wrong impedance can and WILL do damage to its output stage. In a nutshell there are two essential rules that HAVE to be followed when hooking-up a tube head to a cabinet or cabinets:

1. Always use speaker cable(s); never use a guitar (a.k.a. "signal" or "instrument") cable.

2. Always ensure that the impedance of the tube head is set to the EXACT same impedance of the impedance of the cabinet(s) being used.

With regards hooking tube heads 'n' cabs up, this small chart should clear up any confusion that a dealer or customer might have. I know the below is common sense to anyone who remembers elementary electronics from school but as it turns out, most guitar players have zero recall of those days - myself included!

If the cabinet is: Set the head to:

16 Ohms mono 16 Ohms

8 Ohms mono 8 Ohms

4 Ohms* mono 4 Ohms*

*IMPORTANT NOTE: Some Marshall tube heads (i.e.: 4100, 4500, 2100SL-X and 2500SL-X) need to have a modification performed on the transformer to run at 4 ohms - and this modification should ONLY be carried out by an authorized service technician. That’s the reason 4 is in parenthesis on the back of the chassis. The newer DSL and TSL models have the ability to run at this impedance without any internal modification as do the Plexi re-issues.

If 2 cabinets are used and they're: Set the head to:

16 Ohms (mono) each 8 Ohms

8 Ohms (mono) each 4 Ohms*

4 Ohms (mono) each This can’t be done…unless you like flames & smoke!!

"Does the same exact set of rules apply to Valvestate amps?" Do I hear you ask? The answer, in a word, is NO! Valvestate amplifiers have a solid-state power stage that doesn't obey the same laws. As it happens, these amps are self-adjusting so you can run them anywhere between 4 Ohms and 16 Ohms but NEVER below 4 Ohms. As you increase the impedance, the output wattage gradually decreases. Let's take a look at the VS100RH for example. This critically acclaimed brute delivers its full 100 Watts @ 4 Ohms. If you hook the VS100RH up to an 8-Ohm cabinet (using a speaker cable of course!) the head will be capable of delivering approximately 80 Watts of power.

SOURCE = http://www.smittymus...rshall_laws.htm

[quigs]

This really does need to be pinned. Good work!

[halfmoonbay]

OK, 'tis pinned.

[ericman197]

Standard rules:

Never overload a poweramp ( using a total resistance which is greater than that recommended for the head )
Never underload a solid state poweramp

To calculate resistance:

Parallel: 1/R1 + 1/R2 + 1/R3 + 1/R4... etc. = 1/Total resistance; to find the resistance, simply inverse that number.

For example:

2 8 ohm cabs:

1/8 + 1/8 = 1/4

1/4^-1 = 4 ohms

8 32 ohm cabs:

1/32 + 1/32 + 1/32 + 1/32 + 1/32 + 1/32 + 1/32 + 1/32 = 1/4^-1 = 4 ohms

1 8 ohm cab with 1 16 ohm cab:

1/8 + 1/16 = .1875

.1875^-1 = 5.3 ohms

Series: R1 + R2 + R3... etc.

For example:

2 8 ohm cabs:

8 + 8 = 16 ohms

3 4 ohm cabs:

4 + 4 + 4 = 12 ohms

2 4 ohm cabs:

4 + 4 = 8 ohms

When connecting multiple speaker boxes, series connections are rarely used. The most simple method is to dolly chain; to connect the head to one cab and connect the output of that cab to the input of the next, essentially connecting the two cabs in parallel. You will use series resistance if you ever plan on making or modifying a speaker cab, because most 4x12 cabs' speakers are connected in series parallel. For example:

4x12 cab with 16 ohm speakers:

Speaker 1 and Speaker 2 are connected in parallel to get an 8 ohm load
Speaker 3 and Speaker 4 are connected in parallel to get an 8 ohm load

Speakers 1 and 2 are connected in series with speakers 3 and 4 to get a 16 ohm load

OR

Speakers 1 and 2 are connected in parallel with speakers 3 and 4 to get a 4 ohm load. NOTE: This would be parallel, not series parallel.

Or using 8 ohm speakers we can get either an 8 ohm load using series parallel or a 2 ohm load using all parallel.

Some speaker diagrams can be found here:

Wiring Diagrams

This post has been edited by ericman197: 06 July 2004 - 10:33 AM

[pimp_vince]

i'm also curious about one other thing... say i had 2 100W heads and only had one cab. would an A/B box be able to handle that kind of current? Just curious because i saw a great deal on a JCM800, and just use and A/B box kind of like a stomper. to switch between the fender and the marshall.

(this probably wouldn't work though... just might be cool if it did)

[quigs]

You would be more than fine. A/B/Y boxes dont care what you send through them. Thats exactly what I have been intending on doing with my amps for a long long time now.

[pimp_vince]

that is amazingly cool, i was either thinking a JCM800 (mind you it is BEAT UP) or a '63 Bassman Head. they're pretty much the same price (give or take 150$)

[metalhead204]

$150 is a pretty big margin for error

[KaZu]

This really does need to be pinned. Good work!

Yes, one of the best...

[ericman197]

You would be more than fine. A/B/Y boxes dont care what you send through them. Thats exactly what I have been intending on doing with my amps for a long long time now.

I hate to doubt you, but are you sure about that? I don't think most A/B boxes are really meant to handle those currents. Most of their resistor values are more in line with guitar signals than they would be with an amp's.

[tapelator]

oh this makes my head spin...

i might buy a fender amp next spring, do i really need to know all of these stuff?

[halfmoonbay]

i might buy a fender amp next spring, do i really need to know all of these stuff?

Nope, all you need to know is whether you like Fenders and whether it's going to do the job for you.

[halfmoonbay]

By Ericman:

Does Wattage = Volume? Wattage is only half the equation. The other half, of course, is the speaker. But to understand these two monsters, you must first understand how sound works. Like most things in nature, sound is logarithmic ( the inverse of exponential ). It works like so:

1 watt = Standard
2 watts = Twice the power, but only somewhat louder
10 watts = Twice as loud
100 watts = Four times as loud
1,000 watts = Eight times as loud

This contradicts everything you may have been told about loudspeakers, but it's the way physics works. Twice the input does not give you twice the output. Here's how we calculate how loud an amp will be, the scientific way:

(Log(input power)*10 + sensitivity of speakers = output in Db

Db, or decibels, is how we measure sound. Here are some random sound levels:

20dB = Quiet whisper
60dB = Conversation
80 ~ 90dB = Loud conversation, such as in a crowded restaraunt
115dB = Yelling in someone's ears
120dB = Pain threshold. Sound can no longer be louder, only more painful
135dB = Police sirens at 1 meter away
147dB = The level where your inner ear will implode
153dB = Jet engine at 1 meter away

So on and so forth! As you can see, with a relatively small increase in decibels, things get a LOT louder. This is because a 10dB change represents a tenfold increase in wattage. Speakers work the same way. The efficiency of a speaker is measured by it's SPL, or Sound Pressure Level. The SPL is how loud the speaker will be with 1 watt, at a distance of 1 meter from the baffle board. So if you have a speaker with a sensitivity of 99dB, and compared that with 10 of those speakers, the 10 speakers would be twice as loud ( sensitivity of 109dB ). If you then compared those 10 speakers to 100 speakers, the 100 speakers would be twice as loud as the 10 speakers ( now a sensitivity of 119dB ).

There are a few random numbers that are helpful:

2dB = The smallest appreciable difference.
3dB = The additional volume gained by double speakers OR wattage
6dB = The additional volume gained by doubling speakers AND wattage ( or 4x wattage or speakers )
10dB = Twice the output

Let's do some sample problems to help solidify this concept:

100 watts into a speaker with a sensitivity of 99dB:

(Log(100)*10 + 99 = 119dB... pretty loud.

50 watts into a 4x12, in which each speaker has a sensitivity of 100dB:

1 speaker has a sensitivity of 100dB, so 2 would be 103dB, and 4 would be 106dB. The same logarithmic ratios that apply to wattage also apply to speaker area.

(Log(50)*10 + 106 = 123dB... now we're getting louder

So on and so forth. As you can see by these sample problems, an amp with twice the wattage is not necessarilly any louder than a smaller amp. Although wattage remains a constant, speakers can vary quite a bit. Also, keep in mind that manufacturers do lie. Fender's 50 watts may be 60 watts to Ampeg, which might be only 30 watts to Mesa Boogie but 70 watts to Marshall. Then there's solid state vs. tube. While solid state amps have the same clean headroom as their tube equivolents, tube amps can be overdriven fairly hard without sounding bad, whereas many solid state amps tend to crap out before they hit their maximum rated power rating. This is why you will oftentimes find that a 200 watt tube amp will sound just as loud as a 500 watt solid state amp. The solid state amp might really only be putting out 300 watts before it begins to distort, and the 200 watt amp can really push out 300 watts without sounding bad.

[mistymountainhop]

this topic thus proves that the difference between a 50watt head and a 100watt head is what 1/3 DB(i need to re read it ) difference! through the same cab...

merely noticable...

[ericman197]

this topic thus proves that the difference between a 50watt head and a 100watt head is what 1/3 DB(i need to re read it ) difference! through the same cab...

merely noticable...

3dB difference, yes. It's a noticable difference, but not earth shattering. In the bass world, we have a wide variety of poweramps available. If 100 watts isn't enough, it's usually recommended that you go up to a 200 to 400 watt amp. If that's not enough, you need to get 1,000+ watts. It's generally assumed that when you add wattage, you also add speakers, so doubling the wattage can be significant; 6dB is a very appreciable difference. I'd recommend a 3dB increase if you just need a bit more headroom ( your clean amp is distorting a bit or you're getting too much tube overdrive ). If you need a bit of oomph and aren't getting enough cut, a 6dB increase is the way to go. Then if you want to step up to the plate and take it to the next level of loud, a 10dB increase is necessary. You can use the properties of speakers and wattage to your advantage. For example, say you're using a 2x12" cab with Celestion Greenbacks and you need just a little more volume. You could do one of three things:

1. The most obvious; get a more powerful head.

2. Another obvious option; get a 2x12" extension cab.

3. A less obvious option; replace your cab with a more efficient cab.

Any of these options could add 3dB. Going from a 50 watt head to a 100 watt head will get you 3dB, as would doubling your speaker area. However, the same end can be accomplished by upgrading to efficient speakers. Greenbacks are not very efficient guitar speakers; they're rated at 97dB sensitivity. You could switch them with G12H30s which are rated at 100dB sensitivity. The G12 Century is even more efficient; it puts out 102dB. Conversely, if you're using very efficient speakers, you can downgrade to less efficient ones to work your amp harder. After all, why use an efficient 106dB rated 4x12" cab if you can get by with an inefficient 100dB rated 2x12"? The 2x12" will take significantly more amp power, but it'll sound a lot better granted you didn't really need the 4x12". That's the theory behind the 1x12" cabinet; it'll work your amp a lot harder than a 4x12" cab would, which can be very desireable for a tube amp.

[nobulator]

Never overload a poweramp ( using a total resistance which is greater than that recommended for the head )

putting too great a load on a transistor based output section will just reduce the output power, as less current will flow through the load, this will not damage it, it will just make it quieter.
however with a tube based output section a higher load will cause the power not dissipated in the load (via current limiting by the load impedance) to be dissipated in the tubes and output transformer, casusing damage or destruction to ur amp.

i'm also curious about one other thing... say i had 2 100W heads and only had one cab. would an A/B box be able to handle that kind of current? Just curious because i saw a great deal on a JCM800, and just use and A/B box kind of like a stomper. to switch between the fender and the marshall.

do NOT use an ABY box on the output of a tube amp, this will destroy it very quickly, specially designed head switchers can be obtained however. using a standard aby box on a tube based power section will cause one amp to be completely unloaded, as with the previous example an open circuit power section is the most extreme case of too great a load impedance possible. this will destroy ones amp with great immediacy. the head switcher must not only change which amp the cab is conected to but it must also remove the input signal from the other amp, and connect the power stage to a dummy load to protect it.

[mistymountainhop]

Tube Amp Care

Ground rules:
*Never run an amp without a speaker load (i.e. dont run the tube amp without a speaker connected, this will fry the output transformer!)
*Try not to bash the amp about as much as possible!
*Never Touch Capacitors in an amp!!! EVER!
*Never use a signal lead to connect speakers to head, ALWAYS USE SPEAKER LEAD!
*always match impedance (ohms) - see impedance thread!

Turning your amp on
*after checking everything is connected and all proper!
*first, turn your power ON, leaving the standby OFF (both switches start off the same, so lets say when standby is off, its in the power off position!!!)
*let the amp warm up for about 3-5 minutes (the longer the better)
*Now you can turn the standby on, and let rip.

Turning your amp off
*finished playin? good!
*turn the standby to off position and just strum your guitar til it stops sounding from the amp.
*now you can turn the power off.

and you are done!

my suggestion is this gets pinned, and locked, but added to as and when good suggestions are given

This post has been edited by mistymountainhop: 04 October 2004 - 12:27 PM

[bobdingwood]

is it ok to turn off off the stand by and then come back later and play, you know, like i turn the amp on the right way and everything, play for a bit then leave and switch to standby and come back in like 15 minutes and switch it back on and keep on playing?

[trickyfingers]

is it ok to turn off off the stand by and then come back later and play, you know, like i turn the amp on the right way and everything, play for a bit then leave and switch to standby and come back in like 15 minutes and switch it back on and keep on playing?

It's perfectly alright to do so...that's the purpose of "standby"...

[halfmoonbay]

Ladies and Gentlemen, I give you the new FAQ on Amps. Basically I've merged a bunch of older posts and pinned them up at the top. If there's anything that needs to be added on the subject, but I want to try and keep this topic free from clutter and discussion so I'm going to lock it. If there's anything you feel needs to be added, PM me and I'll reopen it and add further information.