How long to ruin an OT running wrong impedance?

[chuckharmonjr]

What about if you're using an attenuator? Does the amp still "see" the cab or does it only see the attenuator? Say you set your amp to 8 ohms, use an 8 ohm attenuator, and hook up to a 16 ohm cab? Would that blow things up or create a hole in the space/time continuum?

THat actually is fine. I do it with my THC often. Their manual even states it is legal to do this.

 

[jross68052]

I think if your cabinet was lower impedance than your amp output, you would most likley blow a thermal or overcurrent fuse before you burned up the tansformer.

 

[LAARS]

I said I was done.

And LAARS, I am not singling you or anyone else out, my statements remain that it is just a matter of time before damage is done. Tube amps are different than SS amps where yes, you can run a higher load without problem.

The fact remains that the math (yes I am an Engineer) proves that a mismatch creates problems. Search Jon Wilder (Wilder Amplification) posts for the exact math, I am not going to attempt to recreate it here. I have posted links and other references time and time again, and he would back me up when I say the above that damage will occur, maybe not right away, but running a mismatch presents problems the circuit was not designed for.

Again, do what you want. My stuff works great, and has for a very long time. I really don't care about anyone else's stuff where it has been abused. (yet another reason to not buy used amps...) I have replaced enough OT's and finals to know that misuse causes damage. Sorry if anyone else disagrees with me.

I will say the same as others, show me the math where you can prove it does no damage...

No problem Micky,
I realize you said you were done. Wasn't looking to anger anyone. Thank you for responding.

I was just looking for real information as to why it can't be done when guys who I consider the tops in the amp field have all said otherwise. I could see if it was just one of them, but my list of amp guys is like a whos who of amps.

Paul Rivera had been modding my marshalls, and building my amps(I had one of the first TBR-1s) in the early 80s until 1989 when Evan Cantor from Bedrock took over with my amps. My Bedrocks had proto type modds in them, that I would test live for them. I had player endorsements (got them at cost) with both companies.

I'm now using Voodoo amps, and Trace has stated the same information when it comes to speaker impedence matching, and mismatching. Its even posted on his website under tech stuff.

This would be why I am stuck, not understanding where this other information is coming from and why I'm not so quick to dismiss what they have taught me over the years.

Thanks again......

 

[mott555]

My thoughts on mismatching a Marshall is it's like playing Russian Roulette. Speaking from a probability perspective, your chances of dying in Russian Roulette are actually somewhat low, but that doesn't mean I want to play.

Can you mismatch? Maybe. How long will it last? Hard to say, but why risk it?

 

[chuckharmonjr]

Q. If I am using just one speaker cabinet, can I still use the Hot Plate if it has a different impedance than my speaker cabinet?

A. ONLY if the impedance of the speakers is equal to or greater than the amplifier, AND
the impedance of the Hot Plate is equal to or greater than the amp. Again, this will lessen
the effectiveness of the Hot Plate’s tone controls.


From the THD Hotplate manual.

 

[Blacque Jacque]

Stock Marshall transformers are built DOWN to a price, many aftermarket parts are designed & manufactured UP to a spec. What this usually means is the expensive aftermarket parts can handle the abuse of a mismatch without exceeding their limits, whereas the OEM part is less likely to tolerate the overload conditions.

 

[Micky]

Laars - I know others have said this, and I think I will still stand by my statement, especially about Marshalls. Other amps may have different circuits, as well as beefier OT's and therefore can handle the heat and current.

Blacque Jacque stated it best I think, that you need to have good tubes as well.

In the Ham Radio world we deal with this all the time. Radios and RF amplifiers connected to antennas have the same types of problems. A load that is mismatched creates what is called a Standing Wave Ratio (SWR) where some of the power is reflected back into the amp. This power can be measured (can also be measured in a guitar amp...) and is generally referred to in a ratio format, such as 2:1, 3:1, 1.5:1, etc.

While a certain amount of mismatch can be tolerated ONLY IF the components in that circuit are beefy enough, we always strive for the lowest SWR we can get. This insures all the power is going out into the antenna and not back into the radio. When power is reflected back into the amp, it not only creates heat, but stresses the circuit in other ways, such as making the tubes draw (use) more or less current (depending on the amount of mismatch) and stress all the components in the output circuitry. Generally with Ham Radios this 'match' can be adjusted with a unit called a 'tuner'. Many of these types of units exist for guitar amplifiers as well.

My point is, that many amps are not capable of mismatch, especially Marshalls. This is why I stress that you should always match the load to the amp. Most amps are made to strict tolerances, using components that are designed for a specific load. Since the load is generally not adjustable in the sense where there is a control for infinite adjustment, we need to be satisfied with the selection the amp builders give us. You may own an amp where people say you can mismatch the load. I don't own an amp where the manufacturer has ever stated that. Sorry.

I have in fact replaced several Fender and a couple Marshall OT's because they were damaged. I think I even did an Ampeg years ago as well... But anyway, it is always best to match the amp to the load, I have always said that and I always will. It is just a matter of time in many respects, heat and overload are the largest contributors to component failure.

And again I will state that I don't really give a shit what anyone else does to their amp. You can pull 2 tubes trying to reduce power and it does the same thing. You can run 40 4-ohm speakers for all I care. If you think you can hear a difference, then good for you.

 

[LAARS]

Thanks Micky

 

[zachman]

My point exactly. I don't know Randall personally, but the others who's amps I have used live, and use now I do know. They all have said to me the same thing. You can match the amp and cabs ohms, or run the amp lower in ohms then the cab. It's their amps, and they know what is safe and not safe.

I would like to hear what Zachman has to say about this topic. We seem to know a lot of the same people and use the same gear. You out there Zachman?

Okay-- generally you're correct. You can mismatch within 1 tap, but only in one direction. You don't want to have the cab be a lower impedance than the head is set for or you will have current blowback and potentially blow your output transformer. (This is for tube heads. SS heads don't care.)

Mesa/Boogie has a patented circuit (which is a load resistor-- at least they do on the MK series)-- basically, which allows the safe mismatching of their heads, but any other amp, I've always treated as stated above, and have never had a problem. That being said, it's BEST to match the impedance of the amp to the cab.

When you mismatch ex 16 Ohm cab/8 Ohm head, you're no longer operating the amp at max efficiency, and may (will likely) experience a reduction in overall output volume and clean headroom.


The story about lower impedances being unsafe is an urban myth in gear circles that comes up. The myth is based on the behavior of solid state amps. Tube amps have an optimum impedance, and they are tolerant of mismatches in either direction, but low loads are generally safer than overloads.
Solid state amps have a minimum impedance. You get maximum efficiency at the minimum impedance. But if you go below the minimum impedance, bad things happen. (There's no output transformer between the main power transistors and the speakers in a solid state amp.)

The myth is probably due to the extreme urgency of never going below the minimum with solid state amps. It got generalized to tube amps by people with no understanding of the difference between tube and solid state amps.

The transformer is designed to balance the internal resistance of the tubes with the resistance of the speakers (and it's an approximate match-can be out of spec by a factor of 2 from the factory without real danger to the tubes). If the amp has a lower impedance from the speakers, then the "flyback voltage" generated by the speaker coil returning to center thru the speaker magnets field and the magnetic field of the output transformer collapsing on it, will be quite weak when it hits the amp and can't do any damage. The resistance to flow of electrons thru the tubes in the active phase will be lower, resulting in potentially higher output from the tubes, which could burn them out quicker. Still, they're replaceable components (unless we're talking GEC KT66s or KT88s

). If the speaker impedance is too high, the flyback voltage is amplified to the point that it can fry components or arc between components in the amp-destroying anything from tube sockets to the OT-very expensive to replace.

When you forget to plug a speaker into a Fender head, the output of the amp is shorted thru the jack (eg speaker impedance of 0 ohms) which is much safer than an open circuit. Also why there's no sound if you plug into just the extension jack (all the amp output is being shorted thru the main jack).

That's my understanding. Hope that helps

 

[Kunnz]

The 'maximum power transfer theorem' better clarifies this issue.

Maximum power transfer theorem - Wikipedia, the free encyclopedia

 

[RickyLee]

I would not stress on it too much. And actually, for all anyone knows those two 2203 amps might now have just a touch more of that extra special tonal vibe going on from running them the way he did. Try to look at it from a positive point of view.

 

[jimmyjames]

Several attenuator models are claimed to be able to handle any mismatch, ie 4,8 or 16 ohms selectable for input and for output. Can these be relied upon? I'm matching head and cab at 16 ohms with my Aracom DAG, but may try other heads and cabs later.

 

[Joshabr1]

Match the friggin load!!!!!!!!

 

[Ken]

. You can mismatch within 1 tap, but only in one direction. You don't want to have the cab be a lower impedance than the head is set for or you will have current blowback and potentially blow your output transformer. (This is for tube heads. SS heads don't care.)

This is nonsense.

You can short a tube amp output which means zero load, the OT won't blow right away although it's obviously not a good idea to try. It's when you have an infinite load (i.e. an open circuit) where the OT will fry the quickest via power tube failure. So on a tube amp, a 4 ohm cabinet in an 8 ohm output is far better than an 8 ohm cabinet in a 4 ohm load, although BOTH will fry your tubes and OT given time.

On a SS amp, too low a load WILL FRY THE POWER TRANSISTORS IMMEDIATELY! Even a momentary short and poof. A 2 ohm load on a 4 ohm output on a SS amp is certain to blow them in short order. OTOH, you can have no load at all (infinite ohms) or mismatch high with no damage on a SS amp without an OT. You just lose peak power.

Ken

 

[zachman]

This is nonsense.

You can short a tube amp output which means zero load, the OT won't blow right away although it's obviously not a good idea to try. It's when you have an infinite load (i.e. an open circuit) where the OT will fry the quickest via power tube failure. So on a tube amp, a 4 ohm cabinet in an 8 ohm output is far better than an 8 ohm cabinet in a 4 ohm load, although BOTH will fry your tubes and OT given time.

On a SS amp, too low a load WILL FRY THE POWER TRANSISTORS IMMEDIATELY! Even a momentary short and poof. A 2 ohm load on a 4 ohm output on a SS amp is certain to blow them in short order. OTOH, you can have no load at all (infinite ohms) or mismatch high with no damage on a SS amp without an OT. You just lose peak power.

Ken

Thanks Ken,

It's nonsense, so you then agree w/ what I said would result? weird... anyway

I explained it in detail, in the rest of the post... At least I thought I did. If I have a Marshall head that I want to use w 2 cabs I wouldn't use (2) 4 ohm cabs giving an approximate 2 ohm load, I'd go higher and use (2) 8 ohm cabs and set the head to 4 Ohms, or (2) 16 Ohm cabs, and set the head to match at 8 Ohms -- but you go right ahead and do what you prefer.

 

[LAARS]

Okay-- generally you're correct. You can mismatch within 1 tap, but only in one direction. You don't want to have the cab be a lower impedance than the head is set for or you will have current blowback and potentially blow your output transformer. (This is for tube heads. SS heads don't care.)

Mesa/Boogie has a patented circuit (which is a load resistor-- at least they do on the MK series)-- basically, which allows the safe mismatching of their heads, but any other amp, I've always treated as stated above, and have never had a problem. That being said, it's BEST to match the impedance of the amp to the cab.

When you mismatch ex 16 Ohm cab/8 Ohm head, you're no longer operating the amp at max efficiency, and may (will likely) experience a reduction in overall output volume and clean headroom.


The story about lower impedances being unsafe is an urban myth in gear circles that comes up. The myth is based on the behavior of solid state amps. Tube amps have an optimum impedance, and they are tolerant of mismatches in either direction, but low loads are generally safer than overloads.
Solid state amps have a minimum impedance. You get maximum efficiency at the minimum impedance. But if you go below the minimum impedance, bad things happen. (There's no output transformer between the main power transistors and the speakers in a solid state amp.)

The myth is probably due to the extreme urgency of never going below the minimum with solid state amps. It got generalized to tube amps by people with no understanding of the difference between tube and solid state amps.

The transformer is designed to balance the internal resistance of the tubes with the resistance of the speakers (and it's an approximate match-can be out of spec by a factor of 2 from the factory without real danger to the tubes). If the amp has a lower impedance from the speakers, then the "flyback voltage" generated by the speaker coil returning to center thru the speaker magnets field and the magnetic field of the output transformer collapsing on it, will be quite weak when it hits the amp and can't do any damage. The resistance to flow of electrons thru the tubes in the active phase will be lower, resulting in potentially higher output from the tubes, which could burn them out quicker. Still, they're replaceable components (unless we're talking GEC KT66s or KT88s

). If the speaker impedance is too high, the flyback voltage is amplified to the point that it can fry components or arc between components in the amp-destroying anything from tube sockets to the OT-very expensive to replace.

When you forget to plug a speaker into a Fender head, the output of the amp is shorted thru the jack (eg speaker impedance of 0 ohms) which is much safer than an open circuit. Also why there's no sound if you plug into just the extension jack (all the amp output is being shorted thru the main jack).

That's my understanding. Hope that helps

Hey Zachman,
Thank you for going over that. Thats the same information that I've been taught by some of the best amp guys in the business over the last 30 years who have worked on, and built my guitar amps.

 

[zachman]

Hey Zachman,
Thank you for going over that. Thats the same information that I've been taught by some of the best amp guys in the business over the last 30 years who have worked on, and built my guitar amps.

Sure thing, any time. It's the way I've been doing things for the past 30+ years, way longer than I've had an electronics engineering degree -- 20 years, and I've never once had it cause a problem.

 

[zachman]

Several attenuator models are claimed to be able to handle any mismatch, ie 4,8 or 16 ohms selectable for input and for output. Can these be relied upon? I'm matching head and cab at 16 ohms with my Aracom DAG, but may try other heads and cabs later.

Yes, internally they are actually matching the impedance for you (to a degree). IF the attenuator fails though, potential for amp repair is high.

Aracom is great!

 

[Marshallhead]

Here's a test of whether suggesting mismatching is a good idea -

Anyone proposing that mismatching is fine, would you reinforce your advice with a personal guarantee that you will cover the repair costs if the person you're advising blows their amp?

 

[zachman]

Here's a test of whether suggesting mismatching is a good idea -

Anyone proposing that mismatching is fine, would you reinforce your advice with a personal guarantee that you will cover the repair costs if the person you're advising blows their amp?

Mesa/Boogie already does that w/ the warranty. Tada