Guitar head / cabinet impedence issues...please help!

C

copperandstars

New member
Hello,

I am quite confused about speaker/head impedence issues. I know that if you combine say two 8 ohm cabs the load becomes 4 ohms and whatnot...but here is my situation:
I own a Marshall AVT150H head. I have a Crate BV412SV cabinet, and a Carvin cab. The Crate has options to do 16 ohm mono, 8 ohm stereo, or 4 ohm mono and it has an output to go to another cab. I just got the Carvin cab, which is 8 ohms only, and I have never tried using the Marshall head with both heads until today. Here's what happened: after about 5 minutes the head shut off and smoke came out like crazy. The head does not turn on any more. Bummer! 12 days until my band tours the country and my head dies...
I am not sure what I did wrong. The Marshall head has two speaker outputs, one for using just one 4 ohm cab (the minimum impedence the head handles) and another for an 8 ohm cab. You can also plug an 8 ohm cab into each of the inputs so the load is 4 ohms total. Well here is how I had it set up, which was obviously wrong: I had the crate cab plugged into the 4 ohm jack. The Crate cab was set on 4 ohm mono mode. Then I ran a cable from the output of the Crate cab into the Carvin cab (which is 8 ohms). What did I do wrong here to cause my amp to die? Should I have used the 16 ohm mono on the Crate? And what do you think happened to my head...easily fixable or bye bye Marshall?
Ok...next issue. Now I think I will have to use the Carvin X100B head I just got on our tour. It has three choices for impedence on the back...4 ohm, 8 ohm, and 16 ohm. I am not sure of which to use. The Carvin is an 8 ohm cab, but the Crate cab is different. It is only 8 ohms stereo and I am not using it as a stereo cab. The mono choices are 16 ohm and 4 ohm. Which combo should I use on everything to make it work? I am pretty sure I will have to use the 16 ohm mono setting on the Crate cab, but then I am not of what to set the heads impedence too, because I don't think 16 ohms (Crate cab) plus 8 ohms (Carvin cab) add up to 4, 8, or 16 ohms.
I really appreciate your help on this issue. I know it may take a minute or two to explain but it will really help me out. Thanks!
Ryan
 
There is a lot of stuff in your post but basically, impedances add up when things are connected in serie and diminish when connected in parallel following the formula 1/R = 1/R1 + 1/R2 + ...

Or more simply, when you parrallel two identicals values, you divide by 2. And as a safety rule when combining unequal values, remember that the resulting value will be lower than the lowest. In you case, the 4 ohms in || with 8 ohms gave 2.67 ohms, a value below the minimum required. It is also uncommon to mix unequal ohm rating.

For a solid state power stage, this mean an easier path for the current, the worse case being a short circuit where the impedance would tend towards 0. You probably toasted the output transistors.
 
Thanks for the help. I think I get what your saying. I guess my most important question is this...how can I safely run the Carvin head and both cabs? And should I use both speaker outs on the head, or just use one and then connect the Carvin cab to the output of the Crate cab?

Thanks again.

Ryan
 
I believe that the safest way to connect both cabs is to match the resultant cabs impedance to the output of the amp. Whatever the way you connect your cabs as you described, they will be in parallel. Since the Carvin is 8 ohms, this limit your choices. You should use it in parallel with an 8 ohms cab for a resulting 4 ohms and this is half of your Crate cab in stereo mode.

There may be other acceptable combination and wether the power amp is valves or solid state does matter but I do not know enough to tell you. Ask on the alt.guitar.amps newsgroup; there are a few guys there that know their stuff.
 
OK, I'm not familiar with the AVT head specifically, but I gather it does not have an impedance selector. When you plugged into the "4ohm" jack, an internal switch in the jack engaged the output of the amp for a 4ohm TOTAL LOAD for ALL speakers connected to ALL jacks. This is standard for guitar amps. Since it is not a stereo head, it only has one output, all speakers connected make up its total load. Without getting out the formula, what you had connected was somewhere around 3 ohms, and my guess is that you had it cranked with a lot of distortion and bottom end in the signal, which also affects impedance -the lower the frequency of the signal, the lower the impedance the load will actually be to the amp. With a solid state amp, its ok for the load (speaker impedance) to be higher, but not lower.
Impedance ratings are often accompanied by the word "nominal" since the "actual" impedance varies with frequencies being reproduced in the program material. With the setup you had that fried the amp, if it had been say, a guy playing twangy country with a Telecaster, it might have been ok. Also, heavily distorted/compressed signals present a much higher duty cycle to the amp, so its working pretty hard to begin with. Here's what I would do:
Select 16 ohms on your Crate, connect it and your Carvin to the AVT like you did before, this will net something like 6 ohms, which will be OK for the 4 ohm output. If the Carvin head is solid state, set it for 4 ohms. Your sound output may be just slightly lower, I doubt if you would notice the difference, but this will be a safer setup for the solid state amp than even having an exact impedance match. One other suggestion would be that since you have 2 heads (when the AVT is repaired), just use one head with each cab, setup for the proper impedance. If one fails, the other is always there....
 
Major Tom, that really helped a lot....thanks! I really appreciate the responses. I think I am starting to understand this a lot better now. The Carvin head is all tube but from what I understand it would still probably be right to set it for 4 ohms so I stay above the minumum. The Carvin head has two of the same speaker outputs...do you suggust plugging each cab into them or plugging the Crate into one, then take the output on the Crate cab into the Carvin? Thanks again.

Ryan
 
Major Tom, I have heard before that when the power amp. is all tube, it was then unsafe to have a higher impedance. Is this true? And if it is, what happen? Is it a problem with the output transformer becoming the load? I think an answer here would help Ryan too.
 
Most tube amp gurus state that the speaker impedance should be an exact match for the output impedance - neither higher nor lower (I don't think I have ever seen an explanation tho). I have also read that a higher impedance is OK...The load of the output transformer's primary winding to the tubes is dependent on the load connected to the secondary winding. If the higher impedance mismatch is a problem for tube amps, it is due to the nature of the output tubes. In your case it would be in between the exact match and the "next step up". not a full step up. Again, the ACTUAL impedance of the circuit is dependent on the frequency content of the signal, the more low frequency content, the lower the actual impedance. Having an understanding of ohms law, if I were to go one way or the other, I would opt for the lower impedance (4 ohm) setting on the head for the slightly higher impedance of the speakers. More impedance (resistance) means less current flow in the circuit. I would also try to avoid running the thing full out, maybe backing it off just a little...
As far as how you connect the cabinets, either way will yield the same result since they will both be connected in parallel. I would connect each to the amp to divide the current draw between 2 connectors, less likely to have a problem.
Another idea if you are going to be touring, install a cooling fan in the head(s). Get a 110 volt type, easy to hook up inside the amp. You will probably need to block the existing air vents, rig it to where it will blow air into one end, and exit at the other. Electrical components in general will last longer, including your tubes. Increasing the cooling (dissapating more heat) also will, in general, increase the power handling capacity of components. Even a small fan from Wal Mart set up behind the amp blowing into the back will help a lot. Cheap insurance.
 
I ran an old Randall RG100 head at 2 ohms for about 5 years of hard abuse and it never blew. It was by accident at first, then I figured it out after a few years. I figured I would blow it up. Never did. It finally got some strange noises when turned up though.

I now have a Peavey Transtube head and a Peavey 4x12 cab in mono at 16 ohms and I get more output than 8 or 4. As I recall I should get less output the higher the ohms. Maybe I should put some batteries in my ohm meter and check it out-lol....Probably shouldnt trust the writing on the switch. It appears that the Cab has been tampered with. I bought it used..........
You never now until it happens. In Coppers case it happened at a real bad time.
I dont do marshall tube heads anymore. Just not reliable and they didnt sound so great to me anyway. I have a Marshall Combo solid state in my studio though:)
Myx
 
Hey,

Thanks a lot for all the responses. I took my Marshall into our local guitar shop to get looked at by the repair guy. He seemed to know his stuff...and gave me a different answer. He said that with tube heads if are you unsure about the impedence to always choose a higher setting on the head (i.e if the cabs add up to be 6 ohms, put the head in 8 ohm mode). But Major Tom's explanation definitly makes sense. Hmm.

Ryan
 
Yeah, I have read that also somewhere about tube heads, but most of what I have seen from the "gurus" say not to go higher or lower...I have also read that it is ok to go the other way... I'm sure that it has a lot to do with how hard you are pushing it. What's a girl to do? One other thing you could do to stay within the manufacturers rated load is to use yer Crate's stereo 8 ohm setting, using only 2 of the speakers. 6 speakers can move a lot of air.
 
hey, i just read your post about the marshall avt150h... i actually had the same problem when i first got it.
what i did was (stupidly) plugged one guitar cord into my digi recorder, and plugged the other end into the back of the amp's headphone output. instead of line out.......... BUT, (again, stupidly)..... i did this when the amp was turned on, literally short-circuiting it.
so yeah, amp shut off and shot some crazy smoke and refused to turn on again. fried it real good. i took it to an authorized repair shop and they explained that there had been a problem with the model so they replaced it cuz it was under warranty.

i'm thankful i didn't have to buy it over again, as this was my last purchase before i married my wife, and didn't expect any large purchases for a while to come.
but she's awesome (must be the s*x), so now i'm buying a PRS singlecut brand new. still have another month and 1/2 to wait before i get it though, that is the only hard part.

copper, i'm curious to know what the exact problem was with yours, what part got wrecked when you did that?
 
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