# Guitar Amp Impedance booboo



## ne1roc (Mar 4, 2006)

My normal amp setup uses two 2x12 cabinets with the amp impedance at 4 ohms. Last week at practice I only took one cabinet and I forgot to set the amp impedance to 8 ohms. Over the last week, the amp is sounding and playing normal, so it looks like I'm lucky. 

What kind of damage can happen to your amp if using the wrong impedance?


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## Wild Bill (May 3, 2006)

*"It all depends..."*



ne1roc said:


> My normal amp setup uses two 2x12 cabinets with the amp impedance at 4 ohms. Last week at practice I only took one cabinet and I forgot to set the amp impedance to 8 ohms. Over the last week, the amp is sounding and playing normal, so it looks like I'm lucky.
> 
> What kind of damage can happen to your amp if using the wrong impedance?


Depends on the amp, and for only a few hours it's not likely you hurt anything.

It works like this. With tube amps, the output tubes need to see a high impedance load. Typically we see EL34s designed with a 3400 ohm load and 6L6s with 6600 ohms. So one of the functions of the OT is to transform this load down to something more suitable for the speakers.

It's important to understand that OTs work by fixed ratios and the transformation works both ways. This means that if you have a 3400 ohm primary and the secondary is set to feed an 8 ohm speaker if you put a 16 ohm speaker on it you then have 6800 ohms as the load to the tubes! Or with 4 ohms it becomes 1700 ohms.

Usually OTs are made with a couple of different taps on the speaker side, so that you change the ratio for different speaker loads to keep the primary side load the same. So you can choose different taps with a switch for 4-8-16 ohms but the tube side load stays pretty close to the same. Some modern amps find it cheaper to just use a different jack for each tap than to use a beefy enough switch.

So what happens when you've put a 4 ohm speaker on an 8 ohm tap with your Plexi? You've now got 1700 ohms to the plates of your EL34s but they're not going to immediately explode! :tongue: Things do change somewhat, however.

Tube data sheets will often show curves for typical circuit conditions that plot the optimum plate impedance load for maximum power out and distortion level. Surprisingly, that's usually not the same thing! The best load for the most power will not likely give the lowest distortion.

The curves also show that there's a broad range to the load where the power or distortion doesn't change a huge amount. Many early Marshalls ran EL34s with OTs that had 6600 ohm loads better suited to 6L6s. They still sounded great!

If the load is WAY out of whack bad things can indeed happen. If the load is too low the tubes will strain themselves, wearing out more quickly. If it's too high the voltage swings can get huge and scary, possibly enough to arc inside the tube or worse yet punch shorts into the OT windings.

A 2:1 mismatch is 99% of the time no significant problem. The tone will change but that's a taste thing anyway! :smile: Putting a 4 ohm speaker into an amp set to 16 ohms is more serious. That becomes 13600 ohms and if you're really cranking the amp I'd be worried, if I were you. The same with 16 ohms on a 4 ohm setting.

The short answer is that no, you probably didn't hurt anything. If you're still not certain and wish to dispose of the amp you can send it to me and I'll take care of things for you!:wink: 

(Unless it's a 'Boogie. I HATE 'Boogies!):2guns:


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## ne1roc (Mar 4, 2006)

Wild Bill, you are an amp tech right? 
My amp is a Carvin Legacy with EL34's. Yeah, I know your not a fan of Carvin but I love it! I wanted to change the power tubes and bias. Can I take it to you to do this?


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## Wild Bill (May 3, 2006)

ne1roc said:


> Wild Bill, you are an amp tech right?
> My amp is a Carvin Legacy with EL34's. Yeah, I know your not a fan of Carvin but I love it! I wanted to change the power tubes and bias. Can I take it to you to do this?


Sure! And I don't have anything against Carvin, I just like old amps better!

Besides, it's not important what WILD BILL likes but rather what the CUSTOMER likes!

My job is to make YOU happy with the sound! (And also to help guys break their lease with loud and Marshally sounds, but that's just a fringe benefit. None 

I'll PM you with my phone number.


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## Rumble_b (Feb 14, 2006)

Not related at all- but I love Bill's answers!!!! No simple yes or no, we all learn something. I also learned that he doesn't like boogies, I like boogies. But I don't own one.


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## Wild Bill (May 3, 2006)

Rumble_b said:


> Not related at all- but I love Bill's answers!!!! No simple yes or no, we all learn something. I also learned that he doesn't like boogies, I like boogies. But I don't own one.


Don't get me wrong, RB! 'Boogies have at least one good sound - crunch! They do a great job on it. Not surprising, since they appear to have swiped the circuit exactamundo from Mike Soldano's SLO100!  

You can't get a good clean tone outta 'em to save your soul but hey, it's all taste.

My dislike is more technical - they can be a HUGE PITA to work on! I've heard of shops in the States that charge a double hourly rate to work on 'Boogies. I can understand 'cuz 20 minutes into a repair of a 'Boogie and my blood pressure goes way up! They are as reliable as anything else nowadays but they were obviously made to keep manufacturing costs down and little or no care about ease of future service. 

This means that once the warranty is up they often cost more to repair. It just seems to me that it was a LOT of money to pay for an amp that really is only most useful to those who intend to play nothing but Metallica's "Enter Sandman" for the rest of their lives. And if that's the only tone you need then you could have saved a bundle buying a SLO100, an amp that's easier to work on.

This is all just the humble opinion of an old solder-sniffin' grunt and you can take it or leave it but just remember, it's grunts like me who might be totalling up a repair bill for you someday!


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## Ripper (Jul 1, 2006)

Santana plays a Boogie. The original Boogies were a different animal though. I had an old Mark I (and stupidly got rid of it) and it was a great amp. You could get some nice cleans and great crunch.

Bill is right though, they are a bear to work on and I whole heartedly agree about them causing blood pressure and language fluctuations. I've worked on a few and I cringe when I get a call saying someone is dropping one off.


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## Wild Bill (May 3, 2006)

*"Language fluctuations"?*



Ripper said:


> Santana plays a Boogie. The original Boogies were a different animal though. I had an old Mark I (and stupidly got rid of it) and it was a great amp. You could get some nice cleans and great crunch.
> 
> Bill is right though, they are a bear to work on and I whole heartedly agree about them causing blood pressure and language fluctuations. I've worked on a few and I cringe when I get a call saying someone is dropping one off.


Yeah, and now S plays a Dumble. I get a big kick outta Dumble. You don't get to ask how much or when he'll finish. He's like the amp version of Seinfeld's "Soup Nazi". If you ask him anything it's "No amp for you!"

I like your term "language fluctuations"! Great way to put it. I usually refer to "the magic word", meaning that you have to say the magic cuss word to make it happen. 'Course, the word keeps changing so you have to run through ALL of 'em!  

Anyhow, to each their own. Or "Chaque a son gout" as my friends in Pointe Claire might say. On my bench however, if I've got a backlog (and it seems there's ALWAYS a backlog!) and some guy comes in crying 'cuz he needs his 'Boogie fixed for a gig in 2 days I just tell him...NO! I can't and won't make other guys wait longer for a repair that will only put me in a bad mood anyway.

Hopefully it will be one of those people who champion putting smokers or fat people to the end of the health line. I'll get to send their 'Boogie down to the end!:2guns:


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## davetcan (Feb 27, 2006)

Just an FYI but you can drop in 6L6's or EL34's into the Carvin, just remember to change the switch on the back of the head. Bill, there's a bias trim pot on the board so it's pretty easy to adjust. I used to use a weber bias-rite and it was pretty straightforward. It's biased a little cold from the factory if I remember right. 
Great amp btw, wonderful cleans and a pretty good OD if you use the presence switch well. I preferred 6L6's for what that's worth. :rockon:


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## Rumble_b (Feb 14, 2006)

Wild Bill said:


> Don't get me wrong, RB! 'Boogies have at least one good sound - crunch! They do a great job on it. Not surprising, since they appear to have swiped the circuit exactamundo from Mike Soldano's SLO100!
> 
> You can't get a good clean tone outta 'em to save your soul but hey, it's all taste.
> 
> This is all just the humble opinion of an old solder-sniffin' grunt and you can take it or leave it but just remember, it's grunts like me who might be totalling up a repair bill for you someday!


I see where you're coming from, my friend has a Nomad 55 and all I know is that it does sound good. But like you say the clean is not the greatest, but in my(not very experienced) opinion I like it. He just got it not long ago and we were a/b-ing it with some other amps. Some had nicer cleans but could not touch the dirty side of it. I found a lot of amps got very muddy when the gain was turned up. Not the Nomad, you could crank it and still pick out every note. But that doesn't really matter anyways cause its his amp not mine. I'm still on my seach for what I want.


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