How long to ruin an OT running wrong impedance?

[wallythacker]

I did a search but my mind probably used the wrong terms.

Suppose someone has a 2203 and an 8 ohm cab but have set the head on 4 ohms. They gig and practice with it 4-5 hours a week and this has been going on about 7 years.

How long is it going to take before some form of OT damage happens? Minutes? Hours? Weeks? I suppose power (volume) has something to do with it but is there a formula ?

Can we determine the decay of an OT run with (x) watts at impedance (y) before sound (z) quality is reduced by (a) percent?

How do you measure the "integrity" of an OT? Are there any tests, other than listening, which is subjective, to determine if an OT has been damaged? Will a scope show anything of value?

I'm asking this because I have a chance to score a pair of 2203 reissue heads that look like new, one is a couple years old, one a few years older than that. To my horror, each head was set at 4 ohms and was plugged into a single 1960B cab. I asked the owner as tactfully as I could what he's changed over the years and has he always run the heads as they sit and he pretty much said what I see is how they've been run. oi vey.

WTF is wrong with what I perceive to be otherwise reasonably intelligent owners? One guy was demoing his 2210 using his Fender but of course his 2210 head was set for 16 ohms and most Fender twin speaker setups I know of are 8 ohms.

Back to the pair of 2203. They roar. They have a sound every bit a good as I remember my original 1983 used to make. I'd be happy to play through them but not so keen on replacing the OT on TWO 2203 heads.

 

[FutureProf88]

I know this isn't a helpful answer, but it depends.

First of all the guy may have been running the 1960B's at 4 ohms as well. On some of the older Marshalls the output taps were wired in such a way that the dampening factor was different for the different taps, and the lower impedance taps gave a slightly deeper sound. On the 1959HW Marshall actually made a specific point to duplicate this arrangement.

As far as I know the Master Volume series transformers were pretty beefy and could handle some abuse (not as well as Fender transformers, but decent). I believe if they are running and the amp sounds healthy, they will continue to run as long as you take care of them. A tube arcing may still take them out, but that can happen with a healthy transformer.

 

[mickeydg5]

Yeah, what impedance (setting) are the 1960 cabinets?

What type of Master Volume did he play at for 7 years?

If the amplifiers sounds fine then I would think all is ok.

 

[wakjob]

You're not going to see a problem or damage running a low amp load like 4 ohms into a higher impedance speaker load. Some people do this purposefully for a certain desired effect.

If it were the other way around, 16 ohm amp setting into a 4 ohm cab,
there will eventually be a problem.

 

[LAARS]

As long as the amps ohms is the same or lower then the cab your fine. You don't want a mismatch going the other way. This is from the Voodoo Amps website.

http://www.voodooamps.com/home/Links/TroubleShootingGuideInfoFAQs/Impedances/tabid/799/Default.aspx

1.) Matched Impedance - It is considered safest to match impedances.

For example;

2ohm into 2ohm
4ohm into 4ohm
8ohm into 8ohm
16ohm into 16ohm



2.) Safe Mismatch - You can go out of the amplifier into an equal or higher rated cabinet.

For example;

2ohm into 4ohm
4ohm into 8ohm
8ohm into 16ohm

Note - Running the amp in this manner doubles primary impedance on the output transformer and runs the tubes more "relaxed" or in other words is safer on power tubes than matching the impedance (4 ohm into 4ohm), which is due to the tubes seeing less power. The tubes are not "relaxed" per se technically but they are seeing less power. Over all, matching or mismatching impedances is all about the shortening or extending of tube life.

Tonal effects; An amps distortion characteristic will change and may become darker. IE: there is a slight loss of headroom/over all volume, smoother/warmer top end and a slight low end roll off. You will also notice the amp is smoother/easier to play.

 

[charveldan]

not long ...

 

[Micky]

It is a common misconception that mis-matched loads are OK if the load is less than the amp output.

This is simply not the case, and although I have no real evidence or proof that this is hazardous to do, or how long it takes to happen, I do in fact have a couple burnt-out output transformers from where someone did this.

How long, or how loud it takes to do this is the question here, and with tube amps there are several factors that can affect the time it will take to burn out or damage an output transformer. Time, temperature, voltage, windings and God only knows what else can and will contribute to premature OT failure. If a transformer is spec'd to perform with a specific output and no 'extra room' for mismatch error, then I think the MTBF will be shorter than expected. On the other hand, if an OT is over-spec'd and can better handle the mismatched load then premature failure may not be so premature.

Bottom line is, it may not happen right away, but if you continue to do it (mis-match) with tube amplifiers, OT failure will happen.

 

[mott555]

Depends on a lot of things. I think the most important is volume, if you're running a mismatched load but are playing fairly quiet, like master volume below 1, there just isn't enough power in the output section even with the mismatch to overheat the transformer. Overheating is the primary cause of OT failure.

It also depends on the brand. The general rule with Marshalls is you never mismatch the load, but the instruction manual for my Mesa says it's perfectly okay to mismatch a Mesa and that you should experiment and see if you happen to like it mismatched.

 

[Ken]

Micky is right: NO MISMATCH IS SAFE unless the OT is spec'ed high enough to handle it, like on Mesa amps. Marshalls? Nope. The OP must have a pretty good OT in there that the amp is still running because this is the exception. I've read OT's blowing after just an hour or two with an extreme mismatch on more modern Marshalls.

Anyone who says a small mismatch is safe on (most) Marshalls is wrong. It's like cigarettes: one won't kill you, but smoke all the time and people will die. You can run a small mismatch for a gig and (probably) be okay, but if you make a habit out of it you're asking for trouble. Yes, some 100 year olds still smoke, and some amps last a long time with a mismatch. But why chance it?

Ken

 

[LAARS]

I don't mean to disagree with you guys, and I don't want to get anyones panties in a bunch here, so I say this with respect, please........

Can anyone actually offer "real technical information" that you "can't" safely mismatch guitar amplifiers? I'm talking from actual amp techs, or articles, not personal opinion here.

Every amp tech I personally know, (Paul Rivera-Rivera amps, and Gundolf Roy from Kitty Hawk, Evan Cantor from Bedrock amps and Trace Davis-Voodoo amps, ) has told me you can safely mismatch without any damage to the output transformer.

Been doing that since my first Marshall 1971 SLP in the early 80s. This includes JMPs, JCM800s, I'm talking a lot of my amps. 3 of these guys are the ones that have taken care of my amps since the 80s until now. So why would these guys all say its ok if it really is bad?

 

[Micky]

If Jon Wilder was here, he would.

But then again, it is your amp, do whatever you want.

 

[chuckharmonjr]

I'm just speculating....but in mono mode.....the right hand (looking at the back of the cab) jack is 4 ohms while the left hand one is 16 ohms. Is it possible he was running in the 4 ohm jack all along?

 

[Ken]

There's been technical discussions before here on exactly what happens when you mismatch. You can search for it if you have the time.

Ken

 

[MorePaul]

think of it like a pipe the diameter of the pipe is the ohms
lower ohms means bigger pipe = less resistance

if your output transformer tap(Amp ohm setting) is 16 (expecting high resistance from small pipe) it will send a stronger signal that has less resistance.

What this does: over heat (fry) your output tranny because the extra cycles of current that keep spinning around in the transformer exponentially create heat.

Going the other way around and going 4 into 8, 4 into 16 etc..could also have electrical spike backs (from the current running into too much resistance)
that can damage components in the amp.

best to go matched.

 

[Micky]

No one has even mentioned the load placed on the output tubes because of mis-matched impedance...

But again, it is your amp, do whatever you want.

 

[LAARS]

think of it like a pipe the diameter of the pipe is the ohms
lower ohms means bigger pipe = less resistance

if your output transformer tap(Amp ohm setting) is 16 (expecting high resistance from small pipe) it will send a stronger signal that has less resistance.

What this does: over heat (fry) your output tranny because the extra cycles of current that keep spinning around in the transformer exponentially create heat.

Going the other way around and going 4 into 8, 4 into 16 etc..could also have electrical spike backs (from the current running into too much resistance)
that can damage components in the amp.

best to go matched.

I agree, its always best to match......

Just to be clear here, I'm not looking to bash, insult anyone, or argue opinion on this. I'm looking for facts only.

I'm not asking for anything more then you would want if someone made a statement that contradicts what you believe to be true. Which I have here.

I'm giving you information from experts in their field (they own and design guitar amps) information and advise that say you can do it.

I'm looking for the same expert advice, that says "you can't mis match", at any time and why.

I realize its always best to match it perfectly (which isn't actually perfect, no speaker cab that says 16 ohms is really 16 ohms).

I've seen this topic brought up before, and I've never seen actual technical information showing you can't run the amps ohms the same or lower then the ohms on the cab...... Only not to run the amps ohms HIGHER then the cab..... That puts stress on the OT.

The only thing I've read that says you can't is from a few of you that just says you can't, and because.....

 

[Micky]

Here is one of many posts:

http://www.marshallforum.com/other-...-best-use-16-ohm-speaker-load.html#post762080

Do the math.

 

[LAARS]

Here is one of many posts:

http://www.marshallforum.com/other-...-best-use-16-ohm-speaker-load.html#post762080

Do the math.

Thank you Micky..... This actually backs up what I have posted.

Jon points out that when you mis match the ohms, it can put stress on the tubes (just like Trace stated as well) Not the OT. EL-34s are not great for durability. If you loose a power tube, you risk taking out your OT. This can happen when your Ohms are matched.... You lose a power tube, you risk taking out the OT.

He also makes a statement that its not a tone tweak..... Some guys swear they can hear a difference. That is really gonna be someone's opinion as to what sounds the same or different. I can't hear a difference in Throbak PAFs pickups over Bare Knuckles Mules vs real 1959 PAFs, but some guys swear they can and pay double the price for Throbaks.

No where does it say that you CANT mis match amps and cabs running the Ohms lower in the amp then the cab. The mis match doesn't take out the OT.

If there are other techs that support the "NEVER" mis match, I'd like to read about it.

Thank you,

 

[Appetite4distortion]

I don't have technical answers but I have an experience:

I've had in the past a 2204 Swiss version, which I guess it's similar to Canadian: these motherfu..ers do not have the 16 ohm tap on the OT. I guess it's due to some local safety restriction or what hell.
So I've run the amp for 7/8 years 8 ohm tap on 16 ohm cab. And, believe me, most of the times it was wide open.

When I sold the amp, it was kicking asses to 99% of other amps in comparison. Talk about sound degradation? I don't think it happens..I believe maybe one day they sudden die and stop, just like a cable which does contact or no contact, no "in between" situation. Either on or off. So...for my experience, I don't think it will hurt too much then. These old amps were oversized and built to last.
BUT..on the other side I run all my plexis with no mismatch....because no need to do, and maybe I was just lucky, who knows. I don't want to discover.

 

[ampmadscientist]

I did a search but my mind probably used the wrong terms.

Suppose someone has a 2203 and an 8 ohm cab but have set the head on 4 ohms. They gig and practice with it 4-5 hours a week and this has been going on about 7 years.

How long is it going to take before some form of OT damage happens? Minutes? Hours? Weeks? I suppose power (volume) has something to do with it but is there a formula ?

Can we determine the decay of an OT run with (x) watts at impedance (y) before sound (z) quality is reduced by (a) percent?

How do you measure the "integrity" of an OT? Are there any tests, other than listening, which is subjective, to determine if an OT has been damaged? Will a scope show anything of value?

I'm asking this because I have a chance to score a pair of 2203 reissue heads that look like new, one is a couple years old, one a few years older than that. To my horror, each head was set at 4 ohms and was plugged into a single 1960B cab. I asked the owner as tactfully as I could what he's changed over the years and has he always run the heads as they sit and he pretty much said what I see is how they've been run. oi vey.

WTF is wrong with what I perceive to be otherwise reasonably intelligent owners? One guy was demoing his 2210 using his Fender but of course his 2210 head was set for 16 ohms and most Fender twin speaker setups I know of are 8 ohms.

Back to the pair of 2203. They roar. They have a sound every bit a good as I remember my original 1983 used to make. I'd be happy to play through them but not so keen on replacing the OT on TWO 2203 heads.

4 ohm amp into 8 ohm cab = 1/2 power.
Not as bad as 8 ohm amp into 4 ohm cab = 2X power demand.

The tubes draw excessive current, and the sockets can arc or burn.

Tube amp "will" run into the wrong improper load....
depends on how hard you push it, before it blows.

It's merely bad practice. You are asking for problems to erupt.