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DeFrag
Moderator
USA
3537 Posts |
 Posted - 02/14/2008 : 21:59:29
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Over on http://www.orangeamps.com/forumtest/topic.asp?TOPIC_ID=8703 Andy H. claims for tube amps, we should be running higher impedance outputs into lower ohm loads which is opposite of what we have all been doing these many years. He makes a good argument. I'd like to hear what you guys have to say on the subject after reading the following:
Power is always reduced by running mismatched in either direction, but the tone change is different. Mismatching low (8-ohm amp into a 4-ohm cab) produces a flubbier, thicker, smoother tone; mismatching high (8-ohm into 16-ohm cab) produces a flatter, more complex, more midrange tone. Matching always produces the most power & most even frequency response.
If you are going to mismatch VALVE amps it is actually less bad, (for the amp or more specifically, the Output Transformer), to mismatch to a lower impedance, (16 ohm into 8 ohm cab), rather then higher. Unlike solid-state amps, valve amps are basically self limiting current wise (the valves!) into a lower impedance, though the valves will take more wear. Into a too high impedance the risk is different - potential very high flyback voltages can fry the OT. But most valve amps with strong output transformers will take a 1/2 to 2x mismatch without complaining.
Although not recommended, technically you can short circuit a valve amps output (0 ohms) without frying it - it is trying to put a signal into an open circuit (virtually infinite ohms) that is a real killer. Total opposite from solid-state amps of course - short circuits will kill the power transistors pronto, but they'll sit happily all day with no speaker load applied.
Solid State amps: Safe with rated load or with any higher impedance speaker load (up to & including an open circuit ie infinite ohms.) Develop less power as impedance increases. Do not short circuit (0 ohms) as this is sudden death for the output transistors.
Valve amps: Match impedance if possible. If mismatching, it is safer for the amp to mismatch low. This will wear the valves but the amp shouldn't suffer. A short circuit (0 ohms) is normally survivable.
If mismatching high there is a risk to the OT, which increases with the severity of the mismatch, & with how hard you are pushing the valve output section. The ultimate 'high' mismatch is no speaker load ie infinite ohms. If trying to pass a signal into this then there is a severe risk to the OT from high flashback voltages which can arc through the insulation layers & burn out the tranny. Mismatching between 1/2 & 2x the impedance the amp 'expects to see' is normally problem free for most amps with healthy OTs. It is never guaranteed safe though, & being manufacturer specific Marshalls fail much more often when doing this then Fenders do. There is a lot of misunderstanding about impedance & mismatching issues, but I repeat that the advice I'm giving is correct for valve amps. (The exact opposite is true for solid state amps, but they work very differently, usually having no output transformers for starters!)
For why Marshalls are extra sensitive, could be the transformer design, could be that selector switch. I personally would not worry too much about a 2:1 mismatch too low, but I might not do a mismatch high on Marshalls with the observed data that they are not all that sturdy under that load. In that light, pulling two tubes & leaving the impedance switch alone might not be too bad, as the remaining tubes are running into a too-low rather than too-high load.
Yes running 8 ohm amp --> 16 ohm cab is probably within normal safe limits (within 1/2 to 2x impedance range) for most valve amps. In fact you'll often get away with playing Russian Roulette by running a 4 ohm amp to a 16 ohm cab, but the risk to transformers is definitely greater when going into a higher then intended impedance. You really are always better to have the speaker impedance lower if mismatching! If you value your amp that is. Going lower strains the valves more then normal, but they are disposable in a way that the output transformer isn't. (Which is after all why valves are removable from their sockets.)
The thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, & acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer & short the windings. I would not go above double the rated load on any tap. & NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.
It's almost never low impedance that kills an OT, it's too high an impedance. The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all. If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.
There is magnetic leakage from primary to secondary & between both half- primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike. This voltage spike can punch through insulation or flash over sockets, & the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, & eventually erode the insulation to make a conductive path between layers. The sound goes south, & with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, & now you have a dead transformer.
So how much loading is too high? For a well designed (equals interleaved, tightly coupled, low leakage inductances, like a fine, high quality hifi) OT, you can easily withstand a 2:1 mismatch high. For a poorly designed (high leakage, poor coupling, not well insulated or potted) transformer, 2:1 may well be marginal. Worse, if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper & hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break before make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted & the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.
Too high impedances on the speaker outputs are much more dangerous for valve amps then too low impedances are. You can short circuit the + & - speaker output connections by connecting them together. (This gives very low impedance, close to zero ohms.) Valve amps can survive this. But if you don't believe me then try this: power up your own valve amp, unplug your speaker, take it off standby & crank that baby into an open circuit very high impedance load. (Fresh air) You stand a very good chance of frying your amps OT!
Speaker load impedances and reflected loads to the output tubes are all "nominal". An 8-ohm speaker may actually look like anything from 6-ohms to 100-ohms, depending on the frequency, since the reactive impedance changes with frequency. This means that the reflected load to the tubes is varying widely over the frequency range.
A nominal 8-ohm load may reflect 4k to the plates of the output tubes with a given transformer. The amp might be designed to produce its maximum power into this load, with a designed frequency response. This is the "power bandwidth". If we change the load to 16-ohms, the reflected load doubles and the frequency response shifts upward. We lose bass but have a brighter sound, and also lose power. If we change to a 4-ohm load, the reflected impedance drops to 2k, into which the tubes produce less power, and the bandwidth is again narrowed.
The reason for the confusion, I believe, is that people think tubes will try to behave the same way transistors do. Into half the load impedance, a transistor will try to deliver twice as much current. The device may overheat and destroy itself in the process. Tubes, however, simply don't behave like transistors.
The design issue for impedance matching comes into play when a designer takes the approach that "everything is critical". In some circuits, this may be the case. Tubes don't really care. There is no optimum load for a tube unless you are going for minimum THD, and this then depends upon the other operating conditions. For guitar, criticality is purely aesthetic. The designer says "this is good", "this is bad" and in that decree believes it to be so. He is correct in his subjective impression, but should not confuse the subjective and objective.
Good tube amp FAQs:
http://www.londonpower.com/faq.htm
http://aga.rru.com/FAQs/general.html
http://aga.rru.com/FAQs/technical.html |
Edited by - DeFrag on 02/14/2008 23:19:53 |
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FRANZONI
Double Platinum Member
Ireland
4207 Posts |
Posted - 02/15/2008 : 21:00:34
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Wow.... your really getting into the valve/tube amp thing DeFrag bigtime..... .. i'm 100% with you on the marshalls...i blew the speaker lead out of the back of the cab(this can happen sometimes with closed back cabs,i should do what dr bob suggested and fit speakon connectors) on a gig a while back,and was lucky with getting away with only frying a fuse..... i know it's off topic a little bit but the point i am making is that marshalls are one of the more sensitive amps i've come across with any sort of mismatch...having said that my early 70's superbass mk II is still getting thrown in the back of the van week in,week out and is still going strong..i look after it and don't leave it in van or storage room in cold or damp conditions...home to the spare room after every gig these days.... |
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DeFrag
Moderator
USA
3537 Posts |
Posted - 02/15/2008 : 22:42:37
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Heh, yeah there sure is a lot to learn. Having a only a 40W solid-state since 1983 will do that to you I guess 
What do you think about the technical issue of a higher-ohm output into a lower-ohm speaker load for tube amps? I hope it wasn't too much of a read but the physical reasons are intriguing. I only expect a certain few to respond.
PS. I wish I gigged as much as you do.  |
Edited by - DeFrag on 02/15/2008 22:50:33 |
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pedals 4 pv
Platinum Member
Canada
1354 Posts |
Posted - 02/15/2008 : 23:58:06
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Hi DeFrag; Andy H. has written a good acticle and he makes some good points. The most relevent one is this; "Valve amps: Match impedance if possible." Engineers must must consider the complete machine in their design. For example, the EQ and boost circiuts are designed for RATED load and should work best under that circumstance.
I don't believe that the subject of combo amps has been considered fully in this article either, as the permanently mounted main speaker is put in parrallel with the extention speaker and according to Ohm's law the total resistance will be less than the smallest resistance in the circuit. Adding an extention speaker activates a different tap on the output transformer to allow for this change in load.
Andy H.'s article is good lore to know, especially if you own an amp and no speaker, but if you are looking to buy a speaker cab, buy the proper one for the amp according to manufacturers specs. Then all you have to do is remember to bring your speaker with your amp. |
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jack
Platinum Member
USA
1438 Posts |
Posted - 02/16/2008 : 00:31:10
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| I don't know...just play, and if it sounds good and doesn't blow up, you're okay |
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FRANZONI
Double Platinum Member
Ireland
4207 Posts |
Posted - 02/16/2008 : 11:02:04
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[quote]Originally posted by jack
I don't know...just play, and if it sounds good and doesn't blow up, you're okay
[/quote
I hear ya bro'....when you start getting blue flashes out of the speaker cone and smoke is coming out of the back of the amp..i always tell the audience its all part of the show..... not being sarcastic but this actually happened to a bass player in a band i was in one time just after the start of gig he plugged his amp into a 15" speaker cab he got a loan of without changing or checking the impedance on the cab or amp..the same guy used to think it was ok to use a guitar lead to connect heads and cabs....... |
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DeFrag
Moderator
USA
3537 Posts |
Posted - 03/15/2008 : 22:48:04
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*bump*
Where's lauries2? I thought he might have something for this thread. Assuming he has a tube amp, that is. |
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Laurie
Double Platinum Member
Canada
5666 Posts |
Posted - 03/17/2008 : 01:56:55
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Haven't posted for a few daze. Been away for the weekend... I can certainly think of things to add (even if just for comic effect )
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Laurie
Double Platinum Member
Canada
5666 Posts |
Posted - 03/17/2008 : 03:22:55
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Ah, yeah, g'day Yep, I have a tube amp DeFrag - a Marshall JCM600 head and 1960A cab.
So I've read all the thread and first of all I have to make the suggestion that it is never (ever) a good idea to intentionally mismatch the amp and speaker cab in a tube amp. Seems like a no-brainer to me if my choices were:
1) Amp with a higher impedance than cab = risk of early burn out of tubes.
2) Amp lower impedance than cab = real risk of frying something in the output circuit.
3) Amp matched to cab = maximum power transfer, optimal operation (to design specification), and minimum risk to components
But there is possibly an even better reeason to match impedances - "damping factor".
Every time a speaker cone physically reaches the end of travel during a cycle and starts to move back the other way, it takes some amount of energy to make that happen - think of having to brake your car to a stop, then go in reverse. This where the concept of "damping factor" comes from - an amp with a high damping factor can turn the speaker around quickly ("damping the overshoot"), and it has more "control" over the speaker - meaning less of that "flubbier" sound mentioned in DeFrag's original post. Conversely, an amp with a low damping factor has less control over the speaker - think of shooting past the turn-off in your car... every time you try to turn.
So, to get a clearer sound, it's better if the damping factor is high. And here is my point (finally!)... mismatching the impedance between amp and cab alters the damping factor of the system, making the sound muddier. One more reason for me to not do it - I'd rather hear the guitar, pedals, and tubes.
Just my 25c worth. As always, I could be full of 
PS: if anyone has ever used JBL PA speakers (I have some I've collected over the years) - you need an amp with huge damping factor to drive them or they will sound like mud.
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Impedance Matching
