the new guy

[alphasound]

hey all. I was discussing some tube amp questions and got a little confused... if a speaker output is rated at 4Ohms, can you use a peaker cabinet that is rated at 2 Ohms? That's what the discussion was about, the other guys said you can use a cab rated below the Ohm rating on the chasis, but I always thought you should stay the ssame or higher..... I thought it was a rule of thumb to use a speker with the rated or HIGHER resistance rating than the ouput... I've worked in pro audio for years, so I may be confusing some things in my head... any help would be appreciated.

[Amp Kat]

It's really more of a dynamic thing then a harmful thing. What you don't want is -negative ohms but 2 ohms on a 4ohm tap would be ok but louder than a 4 ohm setup and harder on your tubes. OT don't like big mismatches in impedance swings or at least may not sound as good with large swings but IMO won't harm them unless it's very large or open. Running 2 ohms on a 4 ohm tap or 4 on a 8 or 8 on a 16 is ok and some will argue that runnning 4 ohm speakers using the 4 ohm tap will fry your tranny but I'm not buying it. It's opens that the OT doesn't like IMO.

[Chuck H]

I agree, but...

It's worth noting that most Marshall amps run lowish on their primary impedance and are much more prone to failure with a speaker load at half their rating than they are at double their rating. With a Q like this it would be good to know the OT primary impedance in question.

Chuck

[alphasound]

I guess my confusion is that a speaker is not XOhms at all times, with different frequencies being reproduced the resistance changes, with the nominal impedence (what the man. put on the spec sheet) is never the lowest (for example:A speaker rated at 8 ohms impedance might range between 4 and 50 ohms, depending on the frequency of the signal. )

I was just curious because if matching impedence is so important with tube amps, how do Marshall and THD sell their attenuators? wouldn't that present more resistance to the amp (thus causing a 'turned up' sound with the volume lower.... again, I may just be having a brain fart....

[Enzo]

Attenuators are not simple resistors, they show a steady impedance to the amp, and then split the power between the speaker and some dummy loads, usually resistors.

Speaker impedance is a nominal thing measured at one freq. Yes the impedance varies across the freq spectrum, but if you move from a 4 ohm to an 8 ohm speaker, that whole curve moves up. Best not to consider the impedance curve thing when matching impedances. STick with the nominal.

Look at it like guitar strings. If you get a set of "10s," only the high E is a 10, the other strings vary in size. If you go to 11s, then all the other strings are a little fatter too. I am sure many could , but I sure couldn't tell you what size the D strings are in either set. I just order 10s, and the rest fall into place. Think that way about speakers, it will be more relaxing.

[Amp Kat]

It's worth noting that most Marshall amps run lowish on their primary impedance and are much more prone to failure with a speaker load at half their rating than they are at double their rating.
Chuck

No this is not really true and if you can proove it I'm listening. Because the impedance is lower on the primary only effects the tone in the frequency domain but has little to do with the current draw and wattage. The ratios are the same and the impedance reflected back into the primary the same but just different values. A higher primary impedance tranny may match up better tone wise or have a closer impedance to say a 15k EL-34 plate impedance. The current draw is the same for 4,8,16 ohms as a ratio for 4,8,16 ohm speakers and I agree that the speaker can vary from 8 to 50 ohms in frequency response. Theres 3 factors that contribute to some Marshall OT woes and one is that they mostly use the EL-34 tube that has 5 times the failure rate of a 6L6/6V6/5881 and draws way more screen current than the other tubes do. Another factor is the wire of these JCM900 type small low budget OT's are thinner than the older type Drake 50 and 100 watt stout trannys and IMO are underated for EL-34 operation. The third thing is coicidence opens caused by bad speaker cables,tube shorts and connections or opens when the amp is cranked will blow easier than a more stout tranny will because there isn't any leeway left when failures do occur. I have a JCM900 small type low budget OT running 4 ohm speakers wide open on the 16 ohm tap and in 6 months has not had one problem but I check the tubes periodically and the dB meter for lack of volume. One thing else also true is if you keep the speakers and taps as close as possible you'll have less problems when failures do occur.

[Don Moose]

One key thing to remember - tube amps are happy at their rated impedance or BELOW (one or two steps); solid-state amps are happy at their rated impedance or HIGHER. One of the things that falls out of that is that one should NEVER turn a tube amp on unless a load is connected - signal or not.

Within reason, the load impedance matters more at higher output levels and less at lower levels -- if you're gonna run dimed, match it good.

[Amp Kat]

Totally agree with that Don !

[Chuck H]

I can agree that there are specific factors like wire guage and the like that contribute to the Marshall OT failure rate. I also know well enough that any tube amp is better off with a dead short than an open load (in fact, I wire my mains with a shorting jack on the amps I build.) but I have read that the lower impedane of the Marshall OTs is something to keep in mind when mismatching impedances in a few different articles, interviews and books.

As for proving the lower impedance...Well you cited that the scale simply moves with the winding count and reflects the same in the end, or something like that. But the wind count is independant of the reflected primary impedance which is strictly a matter of turns ratio, not number of turns. Most Marshalls from the JCM800s forward run 3350-3400 primary impedance for 50 watters and 1900 for 100 watters. MOST that is. No need to site some specific model that differs...I know. That is about 20% lower that comparable Fender models and MUCH lower than many others that run a more common tube mfg recommended impedance of say 5000 or 6600 for 50 watts. I know that apples aren't oranges and EL34s aren't 6L6s but many Parks, for example, run a 5000 primary for a pair of EL34s. To finish I'll just say that if you cut that impedance in half, as you would running, say, the 8 ohm tap into a 4 ohm load, then a 50 watt Marshall amp will have a 1700 ohm primary impedance as compared to Fender and other brands that would reflect 2000 to 3300 primary impedance. Running into a dead short may be better than an open load, but it still aint good. And you can bet that the higher plate voltages found in many Marshalls just excaserbate the problem.

If what you mean by "prove it" is that I should prove that the lowish impedance of Marshall amps is absolutely what is responsible for their failure rate, well... I can't. And so I digress to your many other very good points. For all I know the info commonly disseminated about Marshall amps is just "lore". God knows ther's plenty of that floating around.

Chuck

P.S. I'm glad the Marshall your running the 16 ohm tap into a 4 ohm load on hasn't blown up... Yet

[Amp Kat]

Why thank you on the Marshall sympathy as it may go any day, it just hasn't yet. I guess my proove it point was for a formular to show the reason it would fail as opposed to running matched up impedance wise but didn't mean anything argumentive. You have some good points on reflective impedance and I think these trannys are maybe a combination of weak and abused but they are smaller and the wire is thinner and it just seems like the cores and laminations are cheap iron and don't possess the permeability of the old type Drakes that are much larger EI type. I'm sure you can make a case for 4,8,16 into any load under certain situations but all of the test I've run show that under normal operating conditions with good tubes,speakers,caps ect..... it won't fail at any mismatch of speakers and taps of the OT. It's when one of those things goes bad is when the problems start happening then the mismatch thing takes the blame for it when in reality that may not be the whole story but it gets the blame. That was the proove it point as a math formular but then you could also ask me to proove it's not which I'm working on but the math is very complex and I don't have the computer skills to insert the symbols used in the text but I know it's possible.

[Richard]

I have heard that running a higher impedance speaker with a lower impedance tap can cause the "flyback voltage" to become multiplied on the primary side causing arcing, burned through insulation on the windings, etc. Is this not a factor?