American Viking

The last million threads I've read dealt with people having amp problems and didn't know why the amp is dying or just frying or burning up out of no where. This is the reason why 9 out 10 times.

STOP what you are doing and think for a moment...

MONO = 4 speakers STEREO = 2

The mono jack on the back of a 1960 cab is 16ohms, when you switch to stereo they are at 8 ohms. When you are running one cabinet your amp needs to be set to 16ohms and you need to be plugging into the 16ohm MONO side of the cab.

Doing different from this? Your amp is frying as we speak.

When you flip the switch to stereo each jack represents on half of the cabinet at 8 ohms each. Whichever side you plug into only that side will make sound. The stereo input is for if you have two heads running at the same time and want both of them to come out of the same cabinet.

When the switch is on mono the right side is 16ohms and the left side is 4ohms. Each side will run all four speakers. Stay away from the left side. Always go right, you don't need to worry about playing at 4ohms, doing that overworks your amp way too much anyway. That option is only there if you have a head that only runs at 4 or 2ohms.

The only reason you would vary the ohm setting on the back of your amp is if you have more than one cabinet. If you are running a full stack, you will continue to plug into the right/mono 16ohm side of the cab. Two cabs at 16ohms changes your impedance to 8ohms, so your 16ohm cabs will be plugged into the 8ohm jacks in the back of your amp.

If you do not follow these rules exactly you WILL FRY YOUR AMP. There are no exceptions to these rules when dealing with tube amps. If you do this wrong, your amp will work for a minute, maybe a couple days or weeks but you're burning up the insides and wearing out the transformers like there's no tomorrow and it's not a cheap fix and could result in a total loss. I hope this table makes things a little easier to understand.

Cab / Amp

16ohm Cab = 16ohm Amp load

16ohm Cab + 16ohm Cab = 8ohm Amp load

8ohm Cab = 8ohm Amp load

8ohm Cab + 8ohm Cab = 4ohm

4ohm Cab = 4ohm Amp load

4ohm Cab + 4ohm Cab = 2ohm Amp load


Questions or comments? Post them all here so every one can understand this. I hate seeing anyone fry their amp because no one told them before they bought all their gear how important and vital this is to the amps performance. If you have questions about tubes, biasing and anything else there are more than enough threads in this forum to help you along the way.

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Chadman

+1! these are the basic's one needs if they intend to be a tube amp player and understand the power that comes with it....nice post!

gtrman

I have a 6101 1x12 combo and want to use a 1922/1936 or 1960 cabinet for situations that might require more sound.
I am a little unsure of the Ohm settings when there a different combinations.

The 6101 combo amp impedance is 16 Ohm (4 ohm) - or 8 Ohm.
So what do I set the amp impedance (16 or 8) when using a 1922 2x12 ?
So what do I set the amp impedance (16 or 8) when using a 1960 4x12 (mono) ?

Using the same cabinets on my other combo - The 2554 1x12 Silver jubilee the amp impedance settings is different since it says 4 ohm or 8 ohm.
So what do I set the amp impedance (4 or 8) when using a 1922 2x12 ?
So what do I set the amp impedance (4 or 8) when using a 1960 4x12 (mono)?

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JCM800 2203X - 1923 85th Anniversary - JCM900 SL-X - VM 2266 - 425A - 6100LM - 6101LM - Silver Jubilee 2553 - Silver Jubilee 2554 - 1960A - 1960B - 1960Slash - 1922 - 1936V

American Viking

I'm assuming that the 6101 has a 16ohm speaker in it, correct? If that's true then when you add an extension cab that is at 16ohms then you need to change your amp impedance to 8ohms. It shouldn't change for either cab as long as they are both set at 16ohms. I'm not sure the options on the rear panel of a 1922 cab.

If the Jube only has 8ohm and 4ohm as an option is the speaker in the combo connected to the amp via a speaker cable or is it hard wired in? Can you look at the speaker and tell me what the impedance is?

I saw the back of a 6101 before and it had a speaker cable going into the back of the amp, if yours is like that then the information I gave you for that one should be correct.

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MartyStrat54

To: American Viking

I've been on the forum now for about three weeks and you are right. I read tons of threads dealing with mysterious amp problems. Folks, these tube amps ain't toys. You need to get real familiar with the ways of tube amps, or you're going to have a fryer. My advice to all who need help? Get a good tube amp manual that shows how to do the basic servicing of a tube amp. Become familiar with how to use a soldering iron and how to repair cords. Learn the basics of speaker wiring and speaker loads. This is all critical to keeping your amp happy and working. (Bad/improper power cords that are going between the amp and a cabinet are real amp killers!)

If you have a motorboat, it's pretty easy to operate. However, if you have a sailboat, it requires training and skill to operate. Same thing with a tube power amp. You need training and skill to operate it.

95 per cent of the amp problems are caused by heat related problems. If you are running your amp on 8 ohms and have it plugged into a 16 ohm cabinet, you are putting the cookies in the oven. As American Viking said, the cookies may be done in a few hours, a few days, or maybe even a month, but the cookies will get done and they will be very bitter to eat. A power tranny replacement is a major bench job and will cost you 300 bucks easy. Then after it is repaired, you still might have problems the tech missed because the heat got a resistor or two, or maybe it popped a little capacitor. It may take two or more trips back to the tech to finally get it right.

Conclusion: If you want to play a tube amp, you play it by the amp's rules, not yours.

Marty

philipeby

I have a Jcm900 that I run Into a mesa Cab my cab has an 8ohm in an 8ohm out and 2 4ohm stereo in's. I've always set my amp on 8ohms and ran in to the 8ohm input! Is that Correct? Thanks!

American Viking

That is correct, an 8ohm amp load going into an 8ohm cabinet is perfect. If you were to add another 8ohm cab, your amp impedance should be changed to 4ohms.

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philipeby

Thank you! One more Question! If you look at the back of my am the switch reads like this! (4)16 8 seems obvious that the one is 8! but what is the other! is it 4 ohms or 16 or both? never really understood that! thanks again!

philip

00jett

if im not mistaken on the jcm 900 series that (4)16 side depended on whether or nor the amp was the combo version or the head(dont quote me on this). the 4 ohm was when it was a combo amp and 16 ohm on the head model. i think its in the manual that way...someone else should know for sure.

MartyStrat54

Hey American Viking. Here's a good one. I had a customer bring me a pair of JBL D120's. He wanted me to install them and check the power tubes (Fender amp).

I installed the speakers and checked the tubes and adjusted the bias, but the bias reading was whacky. After some trouble shooting I made a surprise discovery. The speakers stated they were 16 ohm, but they had apparently been reconed as eight ohm speakers. The old OE speakers were 16 ohms wired in parallel and the output tranny was set at 8 ohm.

Now I was running a four ohm load!!! No wonder the bias was "off." I took the speakers and wired them in series and set the O/P tranny to 16 and everything was fine after that.

I used to be a service manager for a pro audio shop in Phoenix and we were an authorized JBL repair shop. I reconed hundreds of JBL's and a lot of those were 16 ohm speakers converted to 8 ohms. We would put a durable sticker on the speaker stating that the speaker was now 8 ohms. Not the case here. I know that from now on I will test speakers and not rely on what is on the speaker...especially on thirty year old speakers.

Marty

Stratman777

Hello, American Viking, First I want to thank you very much for this info, I really needed it.

Would you (and any others that don't mind helping), please clarify some things for me?

I have a Marshall cab that has 8 10" speakers [the speakers have the following stamped on them: 10BPGA8C 67 7429] I'm almost positive it is 16 Ohm's. It only has one input and here are the amp heads I have (all Marshall's):

JCM 900 SL-X 100 Watts

MK II Super Lead 100 Watts (looks like a Plexi)

Artist 3203 Valve State 30 Watts (appears to only have a 4 or 8 Ohm selector) Has this Ex on the back 4 = 1X4 or 2 by 8 -- 8= 1X8 or 2 by 16


Are you guys saying that I should set the selector for 16 Ohm's (on the JCM or the MK II) and only run ONE cable from the amps output to this cab? I was told years ago that you could not run a tube amp without BOTH outputs going to the cab and if necessary you should use a "Y" cord if the cab just has one input???

It appears I could NOT run the 3202 with with the 8 10" cab because it doesn't have a 16 Ohm selector.

Have I understood correctly so far?

Part of the reason I ask, is because I'm getting a Marshall VNTG MOD 42B 4X12 SPKR CAB STR delivered tomorrow and I don't want to screw everything up!!

I truly thank you for all this great info.

American Viking

More than likely it is 16ohms but to be sure you can open the back and see the ohms of the speakers and how they are wired. You can also test the jack with a multimeter or ohm meter and it will read numbers very close to it's actual impedance.

Who ever told you that you had to run both outputs out of the amp to run one cab is crazy. lol. All you need is one cable.

Yeah, it seems like you've got a pretty good grasp on the subject. When in doubt it never hurts to ask and always refer to the table above if you aren't sure about something.

If you need any more help just ask. There's a lot of people on here with some great knowledge they're willing to share. I've learned a hell of a lot from surfing through these forums.
Rock on, brother!

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Adwex

HA. I would've chosen a different word than "crazy". Whoever said that doesn't understand basic electricity, and has no business giving advice.

If you have one cab, you only need one cable. If you have 2 cabs, you need 2 cables. The speaker output jacks out of the amp are wired in parallel.

Stratman777

Hi All, Thanks so much for you feedback.

I'm really embarrassed to admit this, but I now think I know why I had to have my amps repaired so much over the years....
Hopefully this will be a thing of the past. Over the years I DID ask numerous people about what settings the Ohm's selector should be set at, but I almost always got unclear or differing advice; You WOULD think at least ONE of the music stores that I bought the amps from would at least know to tell me to use a speaker cable instead of a guitar cable, but no, I learned that HERE TOO.

At one time I had a Sound City SMF head that sounded great, but I really fried it, more than likely because I had the wrong ohm setting and was using a "curled" guitar cable running from the head to a "Y" cord into the amp; after learning the things you guys have shared here, I feel like such a moron for doing this, but I just didn't know..

You truly are a GREAT group of guys and I thank you so very much

steelhorse

Takes a big person to admit something like that Strat. Hopefully your amps live long and prosper!

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American Viking

I'm pretty sure most of the people in music stores know nothing about how to properly use the products they sell. The information really should be made more available as these amps are not toys and require a basic knowledge of how electronics work just be be able to use them properly!
I didn't know much about this until I baked my first Marshall. Lucky for me the amp didn't go down completely but one of the tubes got so hot it melted a solder joint on the tube socket!

I still hurts to think about it...

But to you Stratman, have much good luck in the future, rock on!

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Stratman777

Thank for your kind remarks and help guys, you're true brothers!!!

gtrman

I don't know if the speaker inside the 6101LM is running at 8 or 16ohm. I assume it's 16ohm since the selector in the back of the amp is 16ohm or 8ohm. (i can't tell from looking at the speaker in the back and the handbook says nothing about the internal speaker) My 2554 jubilee has a 8ohm speaker and the selector in the back is 8ohm or 4ohm. So logically the largest selector is the ohm for the internal speaker.

So if I understand this correct:
The 6101 combo amp impedance is set at 8 Ohm if I connect an external 1960 cab (internal speaker running at 16ohm and 1960 at 16ohm)

But when connecting a 2x12 1922 cabinett I am unsure. The 1922 says Mono 8 ohm. So if the internal speaker is a 16ohm and the 1922 external is 8 ohm what do I set the amp selector to ? (To add an extra confusing element the amp selector on the 6101 says: 16ohm (4ohm) - 8ohm... dont know what the 4ohm is all about)

The 2554 Jubilee Combo has an internal speaker connected to a cable and the speaker is running at 8 ohm.
So if I understand this correct:
I set the amp impedance to 4 ohm when running the internal speaker 8 ohm together with the 1922 mono 8 ohm.
But now I am unsure about how to connect a 1960 4x12 cabinett ? My guess would be to set the amp to 4 ohm and connect the 1960 to the mono 16 ohm. This based on this taken from the Marshall site:
"In situations where you are using cabinets of different impedance's the most likely configuration would be one 16 Ohm cabinet and one 8 Ohm cabinet, in which case you would set the amp to 4 Ohms"

Now I am pretty close here ....

__________________
JCM800 2203X - 1923 85th Anniversary - JCM900 SL-X - VM 2266 - 425A - 6100LM - 6101LM - Silver Jubilee 2553 - Silver Jubilee 2554 - 1960A - 1960B - 1960Slash - 1922 - 1936V

Podrides

Hi everyone!
Some question for you guys:
Just for being sure... I just bought a Marshall JCM800 with a 1960 lead cab that was loaded with 4 Celestion G12L-35. Each speaker at 16 ohms. The cabinet doesn't have any impedance selector and the amp was set at 8 ohms what I thought was strange. It must be wrong? Anyways I checked the amp on another 1960 cab at 4 ohms with the amp set at 4 ohms. I was afraid of checking the cab... Someone could help me please?

Ps: Sorry my english is not very good, hope it just make sense.

Adwex

You are correct to think that 8 ohms was wrong. It is impossible for four 16 ohm speakers to equal 8 ohms, it has to be either 4 or 16, depending on how they're wired.

The best way to determine the correct value is to measure it with an ohmeter, or open the back of the cab and see how the speakers are wired. If they're all wired in parallel, then it's 4 ohms...if they're wired in series/parallel, then it's 16 ohms.

If you take a picture of the wiring, I can help.

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MartyStrat54

Has anyone seen American Viking???

Viking, if you are out there, we sure would like to have you back, ASAP!

Marty

Podrides

Hey thanks a lot!
I didn't have a camera so I did a drawing. Hope you can understand 'cause I know nothing about electronics.

Attached Images

jcm800leadcab2.gif (19.2 KB, 32 views)

Adwex

Good drawing. The total impedance is 16 ohms.

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woodddj

i asked marshall the same question about by 900 head, they sent me a reply and told me my head could be set up for 4ohm, by a sevice tech. that is why it has the (4) . but the 900 head is actually a 8ohm and 16ohm head only. the combo's are 4 and 8 ohm only. it confused the hell out of me to. i had a mg100hdfx and a mc212 cab. the cab is 4ohm mono and 8ohm stereo. now i can only use one side of my cab, because the 900 is not a stereo head. so now im looking for some 16ohm speakers to replace the two 8ohm's in my cab.

Podrides

Hey thanks Adwex.

hmstrat

Well done, American Viking... Your post should remain as a "sticky" for a PSA here...

It's amazing to me how many guys will knowingly plug a 16 ohm cab into a high-power tube head set to expect 8 ohms. Where, exactly, do they think that other 8 ohms of resistance is going to finally turn up?
Well, the correct answer is this: the unaccounted for 8 ohms is alive and well - it's residing in your audio output transformer - heating it up "real good" and melting the plastic off of it's wires.

P.S. - and don't forget that when you boogie your OT, you likely will change the internal "values" of several other amp components (resistors, capacitors, etc.); i.e., your amp will NEVER be the same - it will effectively be unreliable from that point going forward...

The moral is: "MATCH YOUR IMPEDENCES!!!" - and do not ever listen to ANYBODY who tells you that it's absolutely OK to plug a 16 ohm cab into an 8 ohm amp; for they do not know what the hell they are talking about... PERIOD...

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steelhorse

This really should be a sticky and Viking had the right idea and was attempting to stem the tide of the same old question being asked repeatedly sometimes on the same day regarding the ohm factor!

Just the past few days I can think of 3 threads of the same issue regarding ohms

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Adwex

I must've answered similar questions at least 20 times.

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Procter2812

genius thread American Viking!

+1

Procter2812

Exactly what happened to my TSL... excuse my french but its now fucked! haha