WARNING! Risk of Death with Mismatching an SVT's Speaker Load!!!

[bob p]

I've been looking for specs on breakdown voltage for a sound 6550. I haven't been able to find a good reference for just how much voltage a good 6550 will stand. Anecdotal experience tells me it's got to be at least a kilovolt.

This is in response to something interesting that I keep reading on the end-user sites, statements such as:

> "An SVT will destroy itself if you hook it up to an 8-ohm load."

> There is a "very real possibility of death by electrocution for the player
> from arcing voltages from the impedance mismatch alone."

Wow.

Some bad ideas just never die -- they only get worse. You know what I mean -- those situations where someone says something silly on the internet, and people accept it as internet lore, and fight to the death to preserve the ideas. And then they start mis-quoting references to further their position.

There's a thread at one of the musician user sites where a poor soul who bought a used SVT head wants to try it out with an 8-ohm cabinet, because he doesn't have a 4-ohm cabinet yet. He asked if it would be safe to hook up the new head to an 8-ohm cabinet for testing. Of course, he received the standard naysayer replies that preached to him that the amp would immediately begin the process of self-destruction upon hooking it up to a 2:1 mismatched load. They predicted that all sorts of bad things would immediately happen, including some very scary things that were meant for intimidation that weren't particularly clearly explained.

What's most interesting is that someone cut-and-pasted several paragraphs from R.G.'s FAQ on the subject of mismatching, and it served for the supporting reference for all the nay saying. Particular interesting was the fact that they selectively cut and pasted things from R.G.'s document, leaving out parts that they didn't like, which didn't support their position -- like the reference that a 2:1 mismatch is OK.

Some particularly scary side effects of a 2:1 mismatch were proffered, seemingly to create fear in the hearts of anyone who would consider mismatching a load on the SVT. This one was particularly interesting (fearmongering) though he wasn't quite clear in explaining his ideas.

A transformer is a coil with a primary and secondary connection. Any change to the secondary connection will be reflected back to the primary at the turns ratio of that transformer. Since speakers have impedance peaks all over the place, their connection ratings are the average of impedance across the entire frequency response of the speaker. The output tubes react to these impedance shifts by either increasing in plate voltage in the case of a high secondary impedance or lowering in plate voltage in response to a lower secondary impedance. The output transformer is wound to accommodate the operational limits of the tube and the average expected secondary impedance.

Putting an 8 ohm load on an SVT's 4 ohm output automatically shifts the operational voltages of the output tubes to a range potentially sustaining double the voltage of their idle state. Doing so across the 2 ohm tap is even worse. SVT's of all stripes generally run about 700 volts on the plates of their output tubes at idle. The operational limit of the 6550A and KT88 is 800 volts . Beyond that, the tubes themselves become prone to arcing internally and at their bases.

It is the output tubes to first feel the "pain" of a mismatched high load and they will demonstrate malfunction before significant damage occurs to the output transformer itself. All that is required is a single sustained impedance spike outside of the normal operation range of the transformer secondary to cause arcing in the tubes and output transformer.

With the SVT cabinet's original arrangement of eight 32 ohm speakers in parallel, all of the impedance resonances are averaged against each other, but in other multi-speaker cabinets, any series connection results in additive resonances increasing the chance of damage.

What this all means is that while an 8 ohm cabinet is not instant death for an SVT, it is certainly tempting fate. Considering the costs, voltages involved and the very real possibility of death by electrocution for the player from arcing voltages from the impedance mismatch alone, it is a situation best avoided.

We've hit an all-time high in the spreading of fear -- now there's "the very real possibility of death by electrocution for the player from arcing voltages from the impedance mismatch alone."

If that doesn't instill enough fear into musicians to properly match their cabinets, I don't know what will.

So what's the breakdown voltage for a 6550? I've never seen a good tube do what this fellow is describing. I guess I should consider myself lucky, eh?

[J M Fahey]

With the SVT cabinet's original arrangement of eight 32 ohm speakers in parallel, all of the impedance resonances are averaged against each other, but in other multi-speaker cabinets, any series connection results in additive resonances increasing the chance of damage.

FWIW *this* is wrong .

All speaker resonances "average", that's true, but that does *NOT* mean impedance peaks disappear, far from that.
All speakers are the same, all sub-enclosures too, do if a speaker has a 3X resonance peak at, say, 80Hz, *all* do, and the whole cabinet will show a 3X 80Hz impedance peak.

Same with impedance rise above 400 Hz.

[nickb]

Read the whole thing here
Any pros/cons about an Ampeg SVT-CL, 2010 model? - Page 3 - TalkBass Forums

[bob p]

Juan, none of what that fellow said is particularly clear to me. It all involved a lot of hand-waving, and sounded like it involved reciting bits of technical information that he had heard previously, but didn't fully comprehend.

I agree with you that when an array of 8 speakers is wired-up in parallel (like an SVT's 8x10) that none of the impedance peaks disappear; they just add in parallel to reduce the net impedance of the peaks compared to the impedance peak of an individual driver.

What I think that guy was trying to say was that if speakers are wired in series (perhaps as in a series-parallel cabinet) that the series connections would have impedance peaks that would add. Of course, in a series-parallel setup the series component is in parallel with another set of speakers, so that impedance peak would be reduced.

I thought he was making a point about speaker configurations that aren't likely to exist in any cab that gets plugged into an SVT -- that of a pure series connection of only two speakers. That's the only way that I could imagine purposefully mismatching the load to purposefully push the Z as high as possible. It's hard to imagine that anyone would hook up a series connection of a pair of speakers to an SVT, like two 4-ohm speakers in series, and then hooking that 8-R load up to the cabinet. That would be really dumb, as it would obviously be better to re-wire the cab in parallel and use the 2-ohm tap on the amp for an ideal match.

I think his point was to say that at the resonant peak frequencies, you might run into a situation where the Z of the load is high enough to get into a danger zone if you're already mismatching the cabinet. But I consider that to be an empty argument, as a bass amp isn't going to operate at one frequency for an extended period of time to create that sort of problem.

[bob p]

I didn't cite a direct link to the thread because I didn't want to encourage any sort of hit-and-run posting between forums. That's sort of pointless. I just wanted to discuss some of the technical considerations of mismatching, because that's a topic that we've talked about here several times already. I also wanted to limit the discussion to people here, who for the most part are technically adept engineers and circuit designers, rather than inviting comments from end-users who often have a rather poor grasp of many of the principles involved.

There is a prevailing "wisdom" on that end-user site that you can't run an SVT into an 8-ohm load or it will blow up. If you search, there are several threads over at that site where the prevailing wisdom is repeated over and over and over again, that an SVT will just blow up if you try to run it at 8-ohms. Now the naysayers have reached an all-time high in terms of fearmongering -- they're telling end-users that there's a significant risk of death by electrocution associated with running an SVT into an 8-ohm load. What's up with that???

What I really found more troubling than the comments that Juan cited was the second paragraph, which involves a lot of hand-waving and inadequate explanation. He was particularly nebulous and non-specific in terms of whether he was talking about AC or DC voltages, and didn't explain why he expected voltages in the amp to double, and he purposefully attempts to obfuscate references to the tubes' voltage specifications. It's not as if you can talk meaningfully about AC voltages exceeding the DC voltage specification for the tube. AC and DC voltage specs are different, and this guy doesn't seem to recognize that.

I'm also troubled by the way he's implying that a 6550 may start arcing uncontrollably when the output impedance is mismatched by 2:1. I've run a lot of 6550 at their specification limits in various high-power Class AB designs, and I've never seen a sound tube start arcing over just because there's a 2:1 impedance mismatch on the OT. IME if a tube starts arcing, that means that you've got other problems in the circuit that are causing tube elements to fail (like exceeding a component's power dissipation limit), or just a defective tube that's in need of replacement. I think it's false attribution to blame failing tubes on a 2:1 mismatch.

What bothers me most is the way he's trying to intimidate people, telling them that they run the risk of death by electrocution, simply because they mismatched the Z selection for their cabinet. Think about this -- what he's saying is that you could die by electrocution if you have an SVT hooked up to a 4-ohm fridge cabinet, and you mistakenly have the impedance selector set to two ohms. (Some models of SVT do have a slider switch for impedance selection, rather than jacks).

Setting the electronics considerations aside for a moment, think about this from a product liability standpoint -- if it were possible for someone to die by electrocution just because the SVT's impedance selector switch were slid to the wrong setting, that would create a *HUGE* product liability problem for the amp manufacturer -- it would create a product liability problem so extreme that the product would never be able to receive UL or CE safety certification, and the company's lawyers would demand that the product be pulled from the market until it could be re-designed for safety.

To paraphrase one of Chucks one-liners of wisdom: "All things considered, I think that killing customers would be really bad for business."

Interestingly, the guy who made that post goes by the name "psychobassguy."

[nickb]

I didn't cite a direct link to the thread because I didn't want to encourage any sort of hit-and-run posting between forums. That's sort of pointless. I just wanted to discuss some of the technical considerations of mismatching, because that's a topic that we've talked about here several times already. I also wanted to limit the discussion to people here, who for the most part are technically adept engineers and circuit designers, rather than inviting comments from end-users who often have a rather poor grasp of many of the principles involved.
....

I see. I put the link in as I think context is important. However I'm sympathetic to your position - no argument from me. Anyway, moving on.

The way I see it is he's trying use inductive reasoning, but taking it ad absudum yet neglecting the limiting effect of voltage rails and rate of change of current in inductances.

[bob p]

I don't have any objection to your posting of the link to provide context. I had just decided that I didn't want to do that, for the reasons I mentioned earlier.

[loudthud]

Setting the electronics considerations aside for a moment, think about this from a product liability standpoint -- if it were possible for someone to die by electrocution just because the SVT's impedance selector switch were slid to the wrong setting, that would create a *HUGE* product liability problem for the amp manufacturer -- it would create a product liability problem so extreme that the product would never be able to receive UL or CE safety certification, and the company's lawyers would demand that the product be pulled from the market until it could be re-designed for safety.

To paraphrase one of Chucks one-liners of wisdom: "All things considered, I think that killing customers would be really bad for business."

Bob p brings up a valid point. Where could one look at the data on how many electrocutions and fires were caused by guitar amplifiers? How many suits were filed, won or settled?

[The Dude]

Let's get a lab rat, a 1/64 scale bass, an SVT, and see if we can prove the "theory" true.
Of course, one may want to use a chicken in the event of an actual electrocution........... They're much tastier.

[jazbo8]

There were some good discussion on this thread. I think if anything that can kill the tubes, isn't so much the high plate voltage that results from doubling the load, but rather the increased screen dissipation. Does anyone still have a copy of ES345's worksheet (the link doesn't work anymore)? I think it shows the plate and screen dissipation over a single cycle and may provide some clues.

[bob p]

If someone wanted to do legal searches, I think the searching service that they'd need to look into is called Lexis. I don't know anyone who has the service.

I think if anything that can kill the tubes, isn't so much the high plate voltage that results from doubling the load, but rather the increased screen dissipation.

Screen failure is one of the things that I had thought about when I mentioned tube component failure due to over dissipation in a previous post.

What I'm still having a real problem understanding is that guy's allegation that voltages will double with a mismatched load to the upside. Where in the heck does that idea come from? I'm waiting for someone who has a better knowledge of magnetics than I do to explain what I'm obviously missing.

And even if a tube started arcing, where does he get that hair-brained electrocution idea? I've had tubes arc over internally, and it's never caused a dangerous situation. With a 6550 there are no exposed connections on the bottle's exterior, like the cap/plate connection on a 6146B. On a 6550 the arcing is always confined to the interior of the bottle, or to carbon formation at the socket. And even if arcing weren't confined to the envelope, the end-user is isolated from the power supply rails, plate, screen, control grids, cathode and socket... all of which are mounted inside of the closed chassis.

Where are these mythical electrocution voltages and currents supposed to come from, and how are they supposed to get out of the chassis and into the user? His warnings make no sense.

[frus]

Interestingly, the guy who made that post goes by the name "psychobassguy."

AND has been banned.

Regarding the "double voltage": the idle plate voltage doesn't change. It's the DC voltage that you put on the tube plates directly via power supply, and it doesn't go anywhere, regardless of the load (or even lack thereof). However, when you apply some signal, the maximum voltage (being the point where the tube goes into cutoff) is greater than idle. For instance, this is the typical class AB push-pull EL84 amp:



The idle point Q (green) is at idle plate voltage (320 V). The blue load line represents typical 8k OT (4k for one tube). Of course, the left side of the load line is drawn wrong because the load will halve as the other tube reaches cutoff, but we're not interested in left side right now. On the right hand side, we can see that the maximum voltage plate can (theoretically) reach is 440 V, and that's possible because of the flyback action of the OT (which is an inductor). Now if we increase the load to double mismatch (red load line), the maximum voltage will increase to some 580 V.

However, it's not increased 2 times. It's clearly visible that the higher the bias current, the higher the voltage increase under mismatch. I haven't found any relevant 6550 plate characteristics, but I presume that idle current at 700V is pretty low, so the max plate voltage shouldn't be that much higher than idle voltage, even at mismatch. If anyone could chime in with real numbers, we can estimate the real situation

[cbarrow7625]

Full disclosure - I really do not know a whole lot about load lines; just the rudimentary stuff. That said, my recollection of how the load line is drawn is that the end of the load line on the x-axis is placed at the rail voltage for the stage. Is that remotely close to accurate? So, if the rail is 440V how does it increase to 580V when the load is doubled? Would that come from transformer fly back voltages? In my mind, the rail is the rail is the rail and it can't be increased unless a different power supply is used. Yes, I understand that a given mains transformer will increase it's output voltage somewhat based on load, but this example of load dependent voltage increase seems excessive based on my experience with transformers & loads.

I thought the real problem with higher load impedances is that the transformer can get damaged more easily (from higher flyback voltages?). With double the load, doesn't the current through the tube actually decrease? i.e., the x-axis end of the line stays where it is (more or less) while the y-axis end of it moves down (lower current) as indicated by the red line on Frus's graph? Higher impedance, same voltage; ohm's law says current decreases.

Forgive me, I am sure I am forgetting a number of critical pieces of information. Would love to be better educated on why Frus's post is either totally accurate or inaccurate. Thanks in advance.

[frus]

And what's a rail? Sounds like transistor-talk to me

Actually, in the above example the B+ voltage from the power supply is 320 V. Transformer, as a reactive element (inductor) can and will generate additional voltage as the current through the coil decreases. So the maximum voltage does reach 440 V in the first case, and 580 V in the second case

As for the current, you're right, although, as I said, left side is drawn wrong. I should have drawn it like this in the first place (but was lazy). There is a "knee" in the load line where the other tube goes into cutoff



The current is evidently lower so it does produce lower output power. There is a load of information on how to calculate the power here: loadmatch4-pp-beamtetrodes

Also, the plate voltage where the tube goes into saturation (where it crosses the topmost curve) is much lower with higher load. That is dangerous because the screen current rises exponentially under certain point, so it can melt. For instance, the dash lines here:

http://www.triodeel.com/6550ap4.gif

If the plate voltage falls below some 70-80 V, the screen current rises steeply

[R.G.]

Full disclosure - I really do not know a whole lot about load lines; just the rudimentary stuff. That said, my recollection of how the load line is drawn is that the end of the load line on the x-axis is placed at the rail voltage for the stage. Is that remotely close to accurate? So, if the rail is 440V how does it increase to 580V when the load is doubled? Would that come from transformer fly back voltages? In my mind, the rail is the rail is the rail and it can't be increased unless a different power supply is used. Yes, I understand that a given mains transformer will increase it's output voltage somewhat based on load, but this example of load dependent voltage increase seems excessive based on my experience with transformers & loads.
...
Would love to be better educated on why Frus's post is either totally accurate or inaccurate.

The concept of frus' note is pretty accurate, but applies more to single ended amps or Class A-ish push-pull amps as shown. Here is my understanding of what happens in an amp that's much closer to Class B as most high power amps are.

To start, consider a class A SE amp. It is biased and operates much as frus shows. For an amp with a 330V B+, biased at - what? 35ma? - about 10V might be dropped across the resistance in the output transformer primary. The grid voltage is adjusted to keep it at that current for zero-signal conditions.

When signal is applied, it swings the grid both positive and negative with respect to the bias point. When the signal goes positive, the tube is allowed to conduct more current because the grid is less effective at shutting off the current that the tube "wants" to let flow, and current goes up in the tube. This increased current is forced to follow the load line toward high current, and this forces the voltage across the tube to fall by Ohm's law. The load line is the line with a slope of minus delta-V/delta-I. Note that the illustration shows a very simple purely resistive load line, and the reality is that there is always some reactance from the primary and reflected reactance from the secondary to unfold it out into an ellipse, but that's not germane to the simple situation.

Positive going signal can allow the tube to conduct until the grid voltage goes to the cathode voltage, at which time the grid starts sucking a LOT of current and most drivers can't pull it any more positive. So the signal stops increasing in current because the tube is conducting all it can for those conditions. Voltage across the tube is at its lowest, and current highest.

Negative-going signals turn off the tube current. This forces the tube to conduct less, and the stored energy in the primary and leakage inductances force the voltage on the plate higher. The tube refuses to increase current, so the stored energy is shunted into the reflected secondary impedance. The secondary impedance current goes up because the voltage across it increases, forced by flyback (i.e., release of stored energy from the primary inductance) and following the load line to high voltage. The tube current goes down along the same load line, and the combined tube/reflected load follows the load line toward the highest voltage/zero current.

Zero current is where things get different for SE and Class A PP versus Class AB PP. The region from the bias point (320V, 35ma) to the turnoff point is the crossover region. This is where the one tube shown has the signal voltage turning it off compared to the DC bias point. The other tube is seeing the opposite polarity of the same signal from the PI, and is being driven further on. When the first tube turns off, the second tube is turned on, and drives its half of the primary (this is PP, remember) towards low voltage.

The tube that is now turned off does not change its current. But autotransformer action from the active tube drives the plate voltage of the inactive tube above the DC B+. So for the diagram shown, it looks like the minimum voltage on a fully-conducting plate might be in the 40-50V region. The active tube pulls down on its half-primary until the voltage across its half-primary is 320-50 or 270V. The opposite, non-conducting tube sees the DC voltage on the CT of 320V *plus* the 270V of autotransformer action, or 590V.

So for a P-P setup, the off-side plate voltage can go up not quite to two times the DC B+.

The diagram shown here is not quite what you'd use for a real Class AB amp biased near Class B, but it can illustrate the case. In fact, it's a good place to understand what Class A, AB, and B do for you. For Class A operation, the single output tube conducts for the full signal time. For that to be true, the maximum current it can put INTO the output transformer when it's being turned off by the signal is just the bias current. You can't turn a tube any more off than zero current. So for negative-going signals, the peak current into the OT primary is equal to Ibias. And for positive-going signals, the peak current is when Vgrid = Vk. This current has to be 2*Ibias (minus funny stuff for imperfections, of course). So the tube operates with at DC point of Ibias and a plate voltage of B+ (minus resistor losses in the OT primary). You'd like to make B+ huge and Ibias huge to get more power out. But the no-signal power dissipation of the output tube is Ibias times the plate voltage with no signal. You can't make that more than 100% of the tube's ability to get heat out (i.e. it's power rating) without melting it down, so you can take those limits and figure out what the plate impedance for the output transformer is, and from your speaker load what the transformer impedance ratio has to be.

This is the state of affairs if you have the *perfect* impedance load on the plate. If the impedance is too high, the plate swings through voltage faster than current, and runs out of voltage. If the impedance is too low, the plate runs into current limits before it reaches voltage limits. In either case, the available power output to the secondary goes down.

To get more power out of the same tube, you have to give it some relief from the heating. Using two tubes in push-pull helps. Class A push-pull helps a little, because although the tubes have to have the same bias currents (can't let plate current go to zero and still have class A), you can use a smaller OT, and you have two tubes dividing the heating. So you can get more power out, at a cost of using two tubes.

To get more power out, you further turn the DC bias current down. Now each tube turns off for a part of the signal cycle, and so the heat it generates is no longer Ibias times the DC Vplate, but rather there are periods where the opposite tube is conducting and the tube is not dissipating any heat at all at the instant. So on average, for the same Vplate and Ibias, heating goes down. The tube is cooler. And you can turn up the B+ till the tube is again nearly melting, but now the output power is increased.

The limit of this process is where each tube is (theoretically) biased just at cutoff. Each tube turns on for **exactly** one polarity of the incoming signal. The B+ is set to the value where the tubes just barely don't melt down for their duty-cycled portion of the output swing, and you'd have to go to class C to get any more power out of the setup.

OK. With that as a background for what happens, you can understand what's going on. In an SVT - which I am NOT terribly familiar with - if B+ is 700Vdc
and it's biased closer to Class B than to Class A, the tubes are going to be swinging from nearly the B+, perhaps 680Vdc, down to about 50V at Vg=Vk [if you use followers to drive the grids of the output tubes, you can pull Vg above Vk for a little bit] and the "off" side swings perhaps 620-630V above B+, to maybe 1400-1500V.

How does this electrocute the user? Well, to start with, it doesn't in any kind of normal operation. The OT has to be able to withstand at least 1500V normal operation, so you probably have to design it to 3-4kV to withstand "normal" transients. Changing the load line doesn't change this unless changing the load line makes BOTH SETS of output tubes turn off at the same time, letting stored energy in the primary go into flyback. Even then the secondary voltage is limited by transformer action unless the spike punches through the primary-secondary insulation to the actual secondary copper.

If that happens, things get more uncertain. An "8 ohm load" is probably about 6 ohms resistive, so the loading on the primary goes from open circuit to 6 ohms plus some inductance. The spike energy, although substantial, is probably not deadly to a human holding the speaker wire, much as a 20kV electric fence pulse won't kill you - but it will make you sorry you touche the fence. If and only if the punch-through melts wires and leaves a solid copper trail from primary to secondary wires will the situation turn possibly deadly. If and only if a conductive trail is established from the B+ side of the OT to the secondary wires, then this could put B+ on the secondary wires.

... for a while. What the power supply does when B+ is loaded with 6 ohms resistive is the next question. There will be one incredible pulse of current as the 6 ohms of speaker empties the main filter caps. At some point, the mains fuse will blow, as the steady state power of 700V and 6 ohms is 2.94 megawatts, and neither the rectifier, the wires from the rectifier, the OT, nor the speaker load will sustain that. Nor will the input power. If they could, the incoming power line would need to supply 24,500 amperes. The mains fuse would likely pop under those circumstances...

But I'm wandering.

The net is that changing to an 8 ohm load will make things unpleasant for the amp because of the high voltages and currents it handles, and demand some sincere high voltage design in the power transformer, but to be dangerous to a human on the secondary side, a whole chain of thing have to go bad just the right way and stay that way long enough to kill the human, which is tough for the setup to sustain for any time. People are more abused by Tasers.

An open secondary might be a worse problem, but it is possible it also might kill the amp internally before a human had a chance to die. Things get hard to predict when you're dealing with multi-kV arcs that melt copper and iron.