We learn early on that the speakers should match the amp's output impedance, a mismatch could result in damage to the amp's power transisters in the case of solid state amps, or the output transformer in tube amps.

What I'd like to know is, is there a tolerance to how much mismatch an amp will withstand? I'm sure it would depend on the amp...

Specifically, I'm interested in knowing about how it affects the Fender Bandmaster Reverb TFL5005D amps. As you would know, it's got a 4-ohm output transformer. I've read in some websites that the Fender amps could withstand a 100% mismatch and that running the amp at 8 ohms would cause no damage. It would seem that running an amp at a higher impedance would be less dangerous than running it at a lower impedance than its' rating, but that's just a guess. What do you say?

Also just as a side note, I bought a Gibson/Moog Lab Series L5 amp in about '79, rated 100 watts, all solid-state, and I had the store put two new JBL K120's in it, I don't think anyone noticed it was supposed to run at 8-ohms, because the JBLs were 8 ohm speakers of course in parellel at 4 ohms. I used it that way for many years, and playing in some huge clubs LOUD with no instruments onstage mic'd into the P.A. I never had any problem with it. At another website it says an owner had problems running his at 4 ohms, others say they haven't.

As I said I know you should run the amp at its rated impedance, and that impedance is often measured using a tone of 1000 hz since the impedance measurement would be something different for every different frequency, and that actual music will have many different frequencies, so how rigid is this, really?

Any info from knowledgeable individuals especially on these two amps would be greatly appreciated.

[This message was edited by Jim Phelps on 04 November 2006 at 10:07 PM.]

Single-ended amps like Champs are not as tolerant, although they get hooked up to wrong speakers all the time. Marshalls - most JCM800 and earlier models - do not tolerate mismatches well, and a wrong setting on the impedance selector switch can lose an output transformer and other expensive parts.

Solid state amps vary - some are designed simply to "adjust" their power/tone to the impedance they see; others see a mismatch and trip a breaker. Depends on the design. Most bass SS amps use the power-variance system. I don't know about SS steel amps as I don't own one and have never seen one for service....just not many around here.

As far as your other assumption, that a higher impedance is safer than lower, it's exactly the opposite. Lower impedances cause a mismatch and some strain on components, but higher impedances add a phenomenon called "flyback" to the mix, where, in layman's terms, power gets shoved backwards into the output transformer. This is NOT a good thing, and is very expensive. In fact, even on the 100%-mismatch tolerant Fenders I recommend either removing two power tubes (from a 100-watt amp) and rebiasing, unhooking one speaker, or whatever can be done to match the impedance. Obviously, "don't try this at home" - you can really foul things up in a hearbeat.

And speaking of heartbeats - do not EVER open an amp chassis if you do not have a solid knowledge of electronics. If you are not aware of the stored charge in filter caps, you have a nice chance of electrocution - from an amp that's been off and not plugged in for a month.

I hope that helps. My simple rule is: Spend the money on the right speakers. It's a LOT less expensive than fixing a dead amp. Or a dead human.

PS - this came up last week - a resistor will NOT help you balance impedances, as it would have to be an expensive, foot long, wire-wound resistor to handle the power. A "normal" resistor is nothing but a fire hazard used this way.

And yes, impedances "flex" depending on power and frequency, but you can ignore all that - use what the manufacturer recommends, and save yourself some grief.

This comes up regularly on the Amp Workshop and Weber Forums. Someone needs to set up a "FAQ" page and add this to it, as it's so commonly asked...and so commonly answered wrong!

Hope that helps -

Yes, I'm aware of the dangers of high-voltage and electrolytic caps, etc. but thanks for the warning. Been a ham radio operator since age 13, advanced class license, had to pass a test in electronics and morse code.... I've built radio transmitters and such since I was a kid, they use voltages that would scare the he<< out of a guitar amp...we used to have the "one hand behind your back" rule....but as far as any real experience with guitar amps, other than using them, I don't have that much. That's why I asked you guys.

The flyback effect, now that you mention it I've heard of it, in fact it sounds not too different than one of the bad things that happens when you try to transmit on an antenna that's not matched to your frequency. I should've realized that would apply to audio too.

Well I do have enough speakers to use the correct impedance matching, just wanted to know more about this. I do wish I could use my Bandmaster with my one 12-inch JBL cab though, it's so much lighter than two JBLs! I have a small cab with 2 8-inch Jensens too, I built it just for the Bandmaster, but believe it or not it has more bass than the 12-inch JBLs, just a hair more than I like, and the JBL is so much more efficient, has much more volume. Maybe if I redesign the cab I can get rid of some of bass...still won't have the volume of the JBL....oh well.

Thanks again for the info and your concern, Jim, I appreciate it, and it's a good reminder for all of us to keep in mind when we're messing with our electronics.

[This message was edited by Jim Phelps on 05 November 2006 at 02:37 AM.]

Not to discount anything Jim has said, but, rather to clarify, if I may...

A SS amp will have an increasing risk of damage the LOWER the load gets below that for which it was designed...at zero, or close to it, it will likely blow (unless it has serious protection built in).

A TUBE amp will have an increasing risk of damage the HIGHER the load gets above that for which it was designed...with no load at all, it will blow the output transformer. (meaning, for instance, having a speaker cable plugged into the amp with no speaker on the other end)

Many amps are fairly forgiving if you're only going up or down one step...from 8 to 16, or from 8 to 4 ohms. BUT, BUT, BUT, having said that, you MUST understand that you ARE putting a strain on the amp however slight you may think it is, and regardless of how many people say they have been doing it for years. (I just LOVE hearing that one )

The amp manufacturers know their market and how much pickers love to switch things around, so they try to create designs that are as tolerant as possible, but there's only so much any design engineer can do in this regard.

So, the best approach, as Jim and many others have said, is to use the load impedance your amp was designed use...knowing that if you vary from it you COULD damage your amp.

[This message was edited by Mike Wheeler on 05 November 2006 at 09:53 AM.]

quote:

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As I said I know you should run the amp at its rated impedance, and that impedance is often measured using a tone of 1000 hz since the impedance measurement would be something different for every different frequency, and that actual music will have many different frequencies, so how rigid is this, really?

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Greg

By the way, there's one type of player that puts even more strain on things than Chalker, and that's the heavy-metal crowd. Distortion causes power levels to rise above their intended maximums real quick! Give a rocker a Strat, and have him start "windmilling" loud chords through a fuzz, and it can be a disaster in short order. Most of the amps that I've seen speaker and transformer failures in can be directly attributed to the guy doing the playing, or rather the "thrashing and wailing".

I must admit though, that as Donny said, "Lastly, I'll just add that if an amp has an extension speaker jack, it's probably been designed to run at a lower impedance, and power an extra speaker." ... I do find it strange that Fender would put an extension speaker jack on their amps, knowing that it could possibly fry them... Honestly I am still tempted to wire the Bandmaster's 2 - 12's in series for 16 ohms and run the head into it and the other 4 ohm cab together, which would be a 3.2 ohm match total...

"Honestly I am still tempted to wire the Bandmaster's 2 - 12's in series for 16 ohms and run the head into it and the other 4 ohm cab together, which would be a 3.2 ohm match total... "