Impedence Matching - a must?

Grandturk · 10-21-2009, 01:27 PM

So, talking about the taps on the output transformer to out different outs on the back on your amp - 16 ohm, 8 ohm, 4 ohm etc.

On all the Marshall's I've owned, and most common sense online and everywhere, the warning is pretty clear - don't plug into the wrong speaker out - you will blow up your amp. So I've never even tried it.

I picked up an old Boogie combo earlier this year (first non-Marshall tube amp I've ever owned) and its got a 100/60 Watt power select switch, and 1 8 ohm and 2 4 ohm outs. In the manual it basically says "if you're running at 100 watts, use the 8 ohm and if your running 60 watts, use the 4 ohm" but it then goes on to say that you will not damage your amp by running the 8 ohm out to a 4 ohm speaker, or vice versa. And it encourages you to mess around with impedence mismatches.

This seems incongruous to me. What's the deal?

American Viking · 10-21-2009, 04:22 PM

Quote:

Originally Posted by Grandturk

So, talking about the taps on the output transformer to out different outs on the back on your amp - 16 ohm, 8 ohm, 4 ohm etc.

On all the Marshall's I've owned, and most common sense online and everywhere, the warning is pretty clear - don't plug into the wrong speaker out - you will blow up your amp. So I've never even tried it.

I picked up an old Boogie combo earlier this year (first non-Marshall tube amp I've ever owned) and its got a 100/60 Watt power select switch, and 1 8 ohm and 2 4 ohm outs. In the manual it basically says "if you're running at 100 watts, use the 8 ohm and if your running 60 watts, use the 4 ohm" but it then goes on to say that you will not damage your amp by running the 8 ohm out to a 4 ohm speaker, or vice versa. And it encourages you to mess around with impedence mismatches.

This seems incongruous to me. What's the deal?

Well, Randall Smith is in a league of his own when he comes up with some of his hair brained ideas. He's one of the ones that say lower impedances will shorten tube life and in my experience I've found no one else to agree with that. Basically, you need to match impedance no matter what. They say the transformers in the Mesa/Booger stuff are more tolerant of a mismatch, but that only means they'll take a little longer to fry.

By running the amp at 8ohms to a 4ohm speaker you are less susceptible to damage, but the other way around is a definite NO NO. Match the impedance always, no matter what wattage is going on. UNLESS, when you flip the half power switch, do half of the output tubes cease to work? If that's the case you may have to adjust the impedance accordingly.

I wouldn't trust much written by Smith and his colleagues, they share a completely different view on things than the rest of the amp world. Ol' Randy doesn't even believe in tube amps with adjustable bias, he says it's a crock and it's unnecessary, BUT, it only works like that if you buy the tubes from MESA. Sounds like a scam to me. I like some Mesa/Booger stuff too, I even own one, but the president of their company is a loon.

bro blue · 10-23-2009, 10:01 AM

I always match. I don't care what anyone else says about it. I've been doing this for 20+ years and I have seen enough to know that if it was not that big a deal the amp companies would not clearly state the impedance on heads, cabs, in manuals, etc.

And what Viking said about not biasing is correct. You should always bias a non-cathode/non-self biasing amp. If, for no other reason, not to sound like a turd.

***Adwex*** · 10-23-2009, 12:02 PM

Some people say a 1:2 mismatch is tolerable. Not in my book. I lost an output transformer when I went from my 8 ohm cab to my 16 ohm cab and I forgot to switch the amp's setting. Cost me $300 and a month in the shop. Expensive lesson learned.

MartyStrat54 · 10-23-2009, 02:29 PM

I played hybrids and SS until 1983 or so and then I got an 800. I've had all tube Marshalls since then and I have yet to blow an OPT. I feel for anyone who has, because it is an expensive repair and it might take out other delicate components. 4 to 4, 8 to 8 and 16 to 16.

Vovka · 10-23-2009, 03:20 PM

I just have two questions. I bought an used amp with the 1960a cab and noticed that the previous owner was using the cabinet (only one cab) in stereo mode plugged in the left jack (which in stereo it supposed to be 8 ohm and in mono 4 ohm). The amp was set to 8 ohm. He was playing this way for the last 2 years (I asked, he though it sounded better). How bad this could be to the amp. Does this mean that he matched the impedance 8ohm amp to 8 ohm cab or this looks more like mismatch of 8 ohm amp to 4 cab? (I’m actually not sure if he used the left or right jack, there is a chance he used 16 ohm jack in the cab to 8 ohm amp). I hope that the stereo mode really cuts the impedance in half and that the amp was not abused.

and second question is: I have a Mesa Boogie DC-3 which has 8 4 4 Ohm option only. How should I set up/match impedance with a 1960a cab (which is 16 or 4 and 8 in stereo mode) Does it mean that I should use 4 cab to 4 amp?

Thanks.
Thanks.

American Viking · 10-23-2009, 03:33 PM

Quote:

Originally Posted by Vovka

I just have two questions. I bought an used amp with the 1960a cab and noticed that the previous owner was using the cabinet (only one cab) in stereo mode plugged in the left jack (which in stereo it supposed to be 8 ohm and in mono 4 ohm). The amp was set to 8 ohm. He was playing this way for the last 2 years (I asked, he though it sounded better). How bad this could be to the amp. Does this mean that he matched the impedance 8ohm amp to 8 ohm cab or this looks more like mismatch of 8 ohm amp to 4 cab? (I’m actually not sure if he used the left or right jack, there is a chance he used 16 ohm jack in the cab to 8 ohm amp). I hope that the stereo mode really cuts the impedance in half and that the amp was not abused.

and second question is: I have a Mesa Boogie DC-3 which has 8 4 4 Ohm option only. How should I set up/match impedance with a 1960a cab (which is 16 or 4 and 8 in stereo mode) Does it mean that I should use 4 cab to 4 amp?

Thanks.
Thanks.

Well, if the guy had the cabinet set to Stereo he doesn't sound too bright... The Stereo switch cuts the cabinet in half so you are only using two speakers on either side depending on which input you select. When the Stereo switch is engaged both sides are 8ohms. When the MONO switch is engaged the left is 4ohms and the right is 16ohms. All four speakers are in use in MONO operation. Stereo is for if you were to plug two amplifiers in the same cabinet and each amplifier would only be driving two of four speakers. The stereo option is unnecessary in my opinion, if someone has enough dough to buy two amps, they should be able to afford two cabinets. This is only my sole opinion, some people dig it but I personally think it's stupid.

As for your Mesa, if it has a 4ohm and 8ohm tap only, then match the head and the cabinet both to 4ohms. Those transformers probably have more options than they have output jacks for. Slight and somewhat simple modifications can be done to give you more output options, but that isn't necessary if all you are going to be using is just the head and cabinet. They are compatible.

Grandturk · 10-23-2009, 04:04 PM

But you can always run a larger out into a smaller in right? 8 ohm out into 4 ohm cab is OK because the speaker load is more than enough for the out, right?

Hell - this is why I stick to combos!

Vovka · 10-23-2009, 06:27 PM

Thanks for the reply!

Wilder Amplification · 10-23-2009, 06:49 PM

The whole "in 100 watt mode plug the 8 ohm cab into the 8 ohm jack and in 60 watt mode connect an 8 ohm cab to the 4 ohm jack" tells me that the 100/60 watt switch takes two power valves out of the circuit so that you're only running two power valves. When only running two valves, the valves need to see double the impedance on the primary side of the OT, which is why you plug the 8 ohm cab into the 4 ohm jack in 60 watt mode (which is actually 50 but OK).

A common trick we do with 100 watters is pull either the two inner or two outer pairs of power valves out and set the impedance selector to the 8 ohm setting on a 16 ohm cab and setting it to 4 ohms for an 8 ohm cab. This effectively doubles the impedance seen on the other side of the OT, satisfying the double impedance requirement for two valve/50 watt operation. But this is the ONLY application in which you would run double the impedance that the amp is set for. This is essentially what the 100/60 switch on the Mesa is doing.

With all of the power valves installed, setting the selector too low (16 ohm setting on an 8 or 4 ohm cab for example) drops the reflected impedance on the primary too low and allows the valves to pull too much current (overcurrent condition) and will burn shit up. Setting it too high (in 100 watt mode setting it at 8 ohms on a 16 ohm cab) will increase the "flyback voltage" across the OT which can harm the OT and trash the screen grids in the power valves.

ALWAYS match the impedance.

pinto79 · 10-23-2009, 06:57 PM

Quote:

Originally Posted by American Viking

The stereo option is unnecessary in my opinion, if someone has enough dough to buy two amps, they should be able to afford two cabinets. This is only my sole opinion, some people dig it but I personally think it's stupid.

I find the option to split the cab is great for running with a stereo pre/power amp setup. Especially since it's cheaper to buy a single 4x12 than a pair of 2x12s...

MartyStrat54 · 10-23-2009, 07:08 PM

There have been many threads like this one and there was even a long going thread about "Pulling Two Tubes" to turn the amp into a "quieter" 50 watt amp. Some say you can do it. Some say on certain models you can do it. Some say never do it.

I just love it when someone buys a 100 watt amp and then it's too loud and you get a dozen threads on how to tame a 100 watt amp. I've always said, "Buy a 20 watt amp."

However, I'd rather have someone ask about an impedance issue than to burn up a OPT or tubes or etc.

I honestly think that there are a lot of tube amp owners that do not have even the most basic knowledge about tube amps. Tube amps are not for beginners. You can ruin a good tube amp if you don't know the basics of operation.

Wilder Amplification · 10-23-2009, 10:03 PM

Quote:

Originally Posted by MartyStrat54

I honestly think that there are a lot of tube amp owners that do not have even the most basic knowledge about tube amps. Tube amps are not for beginners. You can ruin a good tube amp if you don't know the basics of operation.

X2

The whole "I think a 100 watter is too loud so I think I need a 50 watter" misnomer is another one you'll hear frequently too. You only get 3dB of volume difference everytime you double the output power, and you have to multiply the output power by 10 times to get double the volume, so a 100 watter is only twice louder than a 10 watt amp, and a 50 watter is only twice louder than a 5 watt amp. The difference between 50 and 100 watts is more a "headroom" thing than a volume thing and a 50 watter will break up earlier than a 100 watter and will generally have a "looser" sound and dynamic feel, but you won't be able to hardly tell a difference if any at all in volume between the two.

Volume has to do more with the frequency response and resonant frequency (the frequency that the amplifier amplifies the most) vs the frequency response/resonant frequency of the speaker vs the frequency response/resonant frequency of your own ears. Also, speaker sensitivity comes into play here as well. But the volume difference between 50 and 100 watts is hardly discernable.

Wilder Amplification · 10-24-2009, 02:44 PM

Another thing to consider -

Output transformers have been coming equipped with multi-tapped secondaries for impedance matching since long before the "different impedance loads and mismatching impedance affects tone" myth came out. If impedance matching wasn't that important and couldn't be harmful to a valve amp, why would transformer manufacturers have even gone through the trouble of creating a multi-tap secondary for different impedance loads years ago when they started doing it?

Bottom line, while I understand that most guitar players like to experiment it tone tricks, there are some things you just don't play with on valve amps and impedance mismatching is one of those things.

American Viking · 10-24-2009, 04:12 PM

Quote:

Originally Posted by pinto79

I find the option to split the cab is great for running with a stereo pre/power amp setup. Especially since it's cheaper to buy a single 4x12 than a pair of 2x12s...

Are you talking about using ONE stereo preamp and ONE stereo power amp? because even if you're splitting into two signals and two outs, it's still MONO because it's still the exact same tone. Unless you have two of something, like two amps or two preamps, it is completely unnecessary to split a power amp. This is why I still believe it's a dumb option, and again, if you have enough money to buy two amps and can carry all that stuff around, why not buy two cabs. One stereo pre and one stereo power running any way shape of form technically is still mono unless you add another amp with a different sound. It's just burning up a second set of tubes for no reason.

Wilder Amplification · 10-24-2009, 06:13 PM

Quote:

Originally Posted by American Viking

Are you talking about using ONE stereo preamp and ONE stereo power amp? because even if you're splitting into two signals and two outs, it's still MONO because it's still the exact same tone. Unless you have two of something, like two amps or two preamps, it is completely unnecessary to split a power amp. This is why I still believe it's a dumb option, and again, if you have enough money to buy two amps and can carry all that stuff around, why not buy two cabs. One stereo pre and one stereo power running any way shape of form technically is still mono unless you add another amp with a different sound. It's just burning up a second set of tubes for no reason.

Most stereo guitar rigs are ran with one side dry while the other side is delayed 15-25mS. On a stereo chorus pedal ran mono, the signal is split in 2, one left dry while the other is delayed, and then the two are electrically mixed back together. On a stereo rig, the same thing happens, except the signals are mixed back together acoustically through the two amplifier channels rather than re-joined electrically. This creates a "doubling" effect which phattens up your tone and makes you sound like you're playing through a whole wall of cabs.

You can do the same thing with a stereo delay on a stereo rig. But unless one side is delayed, the rig is considered "dual mono" rather than stereo.

Grandturk · 10-25-2009, 11:00 PM

Thanks for the replies guys, interesting stuff - but other thank Amer Viking's "Randall Smith is a loon," anyone got the 411/opinions on why Mesa says that you cannot possibly blow up your Mesa amp by mismatching impedence?

... or is Randall Smith really a loon? lol

Wilder Amplification · 10-26-2009, 05:07 PM

Quote:

Originally Posted by Grandturk

Thanks for the replies guys, interesting stuff - but other thank Amer Viking's "Randall Smith is a loon," anyone got the 411/opinions on why Mesa says that you cannot possibly blow up your Mesa amp by mismatching impedence?

... or is Randall Smith really a loon? lol

He may very well have his transformers built in such a way that allows them to handle it. Who knows? I know the valves don't like extreme flyback voltages from the speaker and those will still feel that (the screen grids in the valve take a beating with excessive flyback voltage).

But I'm more willing to bet Randall Smith is a loon.

Ken · 10-27-2009, 11:37 AM

Quote:

Originally Posted by Adwex

Some people say a 1:2 mismatch is tolerable. Not in my book. I lost an output transformer when I went from my 8 ohm cab to my 16 ohm cab and I forgot to switch the amp's setting. Cost me $300 and a month in the shop. Expensive lesson learned.

That's a surprise but obviously it can happen.

I will say I plug my 15 ohm and 8 ohm cabs into my DSL. I'm loading 5.217 ohms and I set the output to 4 ohms. I'm not losing any sleep over it.

Ken

RachelMorgan · 11-03-2009, 06:17 PM

I asked an amp tech about this once... his reply? "What's your budget?"

Wilder Amplification · 11-03-2009, 06:22 PM

Quote:

Originally Posted by RachelMorgan

I asked an amp tech about this once... his reply? "What's your budget?"

That pretty much sums it up right there.

10-21-2009, 01:27 PM	#1 (permalink)
Grandturk Senior Member Join Date: Sep 2009 Location: NYC Posts: 110	Impedence Matching - a must? So, talking about the taps on the output transformer to out different outs on the back on your amp - 16 ohm, 8 ohm, 4 ohm etc. On all the Marshall's I've owned, and most common sense online and everywhere, the warning is pretty clear - don't plug into the wrong speaker out - you will blow up your amp. So I've never even tried it. I picked up an old Boogie combo earlier this year (first non-Marshall tube amp I've ever owned) and its got a 100/60 Watt power select switch, and 1 8 ohm and 2 4 ohm outs. In the manual it basically says "if you're running at 100 watts, use the 8 ohm and if your running 60 watts, use the 4 ohm" but it then goes on to say that you will not damage your amp by running the 8 ohm out to a 4 ohm speaker, or vice versa. And it encourages you to mess around with impedence mismatches. This seems incongruous to me. What's the deal?

10-23-2009, 10:01 AM	#3 (permalink)
bro blue Senior Member Join Date: Aug 2009 Location: Southern USA Posts: 112	Re: Impedence Matching - a must? I always match. I don't care what anyone else says about it. I've been doing this for 20+ years and I have seen enough to know that if it was not that big a deal the amp companies would not clearly state the impedance on heads, cabs, in manuals, etc. And what Viking said about not biasing is correct. You should always bias a non-cathode/non-self biasing amp. If, for no other reason, not to sound like a turd.

10-23-2009, 12:02 PM	#4 (permalink)
*Adwex* Super Moderator Join Date: Sep 2006 Location: Long Island, NY Posts: 1,631	Re: Impedence Matching - a must? Some people say a 1:2 mismatch is tolerable. Not in my book. I lost an output transformer when I went from my 8 ohm cab to my 16 ohm cab and I forgot to switch the amp's setting. Cost me $300 and a month in the shop. Expensive lesson learned. __________________ http://www.youtube.com/MarshallLesPaul

10-23-2009, 02:29 PM	#5 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Impedence Matching - a must? I played hybrids and SS until 1983 or so and then I got an 800. I've had all tube Marshalls since then and I have yet to blow an OPT. I feel for anyone who has, because it is an expensive repair and it might take out other delicate components. 4 to 4, 8 to 8 and 16 to 16. __________________ MARTY 2009 Gibson Explorer-Hell Raising Machine 1989 Fender Strat Plus USA 2008 Fender Tele USA 2004 Schecter Elite w/DiMarzio's TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers. TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends." DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker. Tubes: Let's just say they are NOS that kick ass.

10-23-2009, 03:20 PM	#6 (permalink)
Vovka Junior Member Join Date: Aug 2009 Location: Arlington, Virginia Posts: 13	Re: Impedence Matching - a must? I just have two questions. I bought an used amp with the 1960a cab and noticed that the previous owner was using the cabinet (only one cab) in stereo mode plugged in the left jack (which in stereo it supposed to be 8 ohm and in mono 4 ohm). The amp was set to 8 ohm. He was playing this way for the last 2 years (I asked, he though it sounded better). How bad this could be to the amp. Does this mean that he matched the impedance 8ohm amp to 8 ohm cab or this looks more like mismatch of 8 ohm amp to 4 cab? (I’m actually not sure if he used the left or right jack, there is a chance he used 16 ohm jack in the cab to 8 ohm amp). I hope that the stereo mode really cuts the impedance in half and that the amp was not abused. and second question is: I have a Mesa Boogie DC-3 which has 8 4 4 Ohm option only. How should I set up/match impedance with a 1960a cab (which is 16 or 4 and 8 in stereo mode) Does it mean that I should use 4 cab to 4 amp? Thanks. Thanks.

10-23-2009, 04:04 PM	#8 (permalink)
Grandturk Senior Member Join Date: Sep 2009 Location: NYC Posts: 110	Re: Impedence Matching - a must? But you can always run a larger out into a smaller in right? 8 ohm out into 4 ohm cab is OK because the speaker load is more than enough for the out, right? Hell - this is why I stick to combos!

10-23-2009, 06:27 PM	#9 (permalink)
Vovka Junior Member Join Date: Aug 2009 Location: Arlington, Virginia Posts: 13	Re: Impedence Matching - a must? Thanks for the reply!

10-23-2009, 07:08 PM	#12 (permalink)
MartyStrat54 Senior Member Join Date: Feb 2009 Location: Formerly Phoenix, now north of KC Posts: 2,583	Re: Impedence Matching - a must? There have been many threads like this one and there was even a long going thread about "Pulling Two Tubes" to turn the amp into a "quieter" 50 watt amp. Some say you can do it. Some say on certain models you can do it. Some say never do it. I just love it when someone buys a 100 watt amp and then it's too loud and you get a dozen threads on how to tame a 100 watt amp. I've always said, "Buy a 20 watt amp." However, I'd rather have someone ask about an impedance issue than to burn up a OPT or tubes or etc. I honestly think that there are a lot of tube amp owners that do not have even the most basic knowledge about tube amps. Tube amps are not for beginners. You can ruin a good tube amp if you don't know the basics of operation. __________________ MARTY 2009 Gibson Explorer-Hell Raising Machine 1989 Fender Strat Plus USA 2008 Fender Tele USA 2004 Schecter Elite w/DiMarzio's TSL100 with custom 4x12 cab. TSL122 with Man 'O War Speakers. TSL602 #1 with JBL D123 Speakers and 602 #2 with Altec 417 Speakers. "Oh so sweet my friends." DSL401 #1 with Red Fang Speaker and 401 #2 with JBL D123 Speaker. Tubes: Let's just say they are NOS that kick ass.

10-25-2009, 11:00 PM	#17 (permalink)
Grandturk Senior Member Join Date: Sep 2009 Location: NYC Posts: 110	Re: Impedence Matching - a must? Thanks for the replies guys, interesting stuff - but other thank Amer Viking's "Randall Smith is a loon," anyone got the 411/opinions on why Mesa says that you cannot possibly blow up your Mesa amp by mismatching impedence? ... or is Randall Smith really a loon? lol

11-03-2009, 06:17 PM	#20 (permalink)
RachelMorgan Senior Member Join Date: May 2008 Location: Seattle Posts: 207	Re: Impedence Matching - a must? I asked an amp tech about this once... his reply? "What's your budget?" __________________ Guitars: ESP LTD Alexi-600 V; Gibson SG Standard; Schecter C-1 Hellraiser (JB/'59 w/ JB coil tap) Amps: Randall Lynch Box; Peavey 6505+; Fender HR Deville. Board: Typically a tuner and TS808 Speakers: Two Avatar Contemporary 212s

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