Let's talk MOV's

[R.G.]

I'm happy with the use of MOV's for main transient suppression, but I thought it was a poor presumption that valve amp damage was a consequence (ie. that would be a rare occurrence), and a poor presumption given the rarity of an event where adding a MOV would be a good protective element to add to mitigate such damage - especially as the discussion had already indicated that robust transients such as from nearby lightning would likely damage a valve amp with or without an added MOV.

This discussion highlights the sort of thing you get into when discussing protective circuitry. The point of protective circuitry is that if done well, it either prevents damage, or minimizes it, possibly by sacrificing itself. Kind of like fire extinguishers. Fires are not nearly as common as they once were, but we still keep fire extinguishers ( I have one in my kitchen, one in my garage with the paint and solvents, one in each vehicle) even though we rarely if ever use them. But when we do need them, the cost of the $10-50 fire extinuisher looks very thrifty indeed. Like cheap insurance, it cuts the loss if the event does occur.

Then there's the necessity to guess about the relative likelihood of the causes. Lightning is a very potent event, but is very rarely near enough to damage an amp - much, much rarer than line transients. Nothing much will withstand a direct lighting strike with no damage. But lightning attenuated by being carried a few miles through power lines, that's more survivable, and of comparable intensity to motor transients and switching arcs. The problem with protective gear like MOVs and TVS devices is that you never know how many they protected you against. They don't have an internal counter you can access.

Tube amps are robust, and in that lies the issue that damage may lie latent. A line spike might puncture insulation, but not cause a direct short in the PT or OT until some more heat and life shorts the now-low-insulation, and you only know that one day it didn't work. It will never be clear that the actual cause was that spike six months ago.

MOVs and TVS devices are cheap, even compared to tubes. Even if they're only an amulet against the spike demon, they constitute good insurance. As a side note, the older (and possibly more valuable) the amp is, the more likely it is that it was constructed to a lower standard of safety insulation.

It is always best to suppress transients at their source. If someone's home is generating such transients as to damage valve amp equipment then going around adding MOV's to all household equipment is a poor remedial action by itself.

I agree - suppress transients at the source - if you can. Most tube amp players can't get back at the source of the line transients, though. So they can stand there with their fingers crossed, hoping they don't get hit, or do whatever is practical and affordable to protect against some transients. One of things I had to read and study as part of my training as a power supply designer back in TIC (Three Initial Corporation) was an in depth study of line surges and transients. The likelihood of a given voltage on the AC line starts at about 100% at the peak voltage of the line, and the probability declines dramatically from there on up. There are lots of blips and giggles just a bit over the peak line voltage, but at 2x, 4x, 10x the AC line peak, the likelihood declines dramatically. 20x, corresponding roughly to the 4000V safety spec level in 200V countries, are quite rare. So if you look at it in one way, with a $1 device, you can protect yourself against the vast, vast majority of chances to get damaged.

As to the home generating transients, I moved into my current home about 8 years ago. We were losing $100 smart thermostats about every fifth or sixth thunderstorm. I put in surge protectors consisting of **huge** movs on the incoming AC lines. Three years and counting for no damage to the thermostats, including massive thunderstorms. Anecdotal, but in my case, it protected the HOUSE.

Commercial environments are another matter, but again if there is a variety of electronic equipment being brought in then the best location for suppression is as close to the feed point as possible, and a commercial filter can be used. Without any other voltage clamping part in a building, or significant impedance, a sole MOV will be the first part to conduct current until insulation in another part somewhere else in the building breaks down - which is a concern to be noted and appreciated (similar to when a harmonic filter is added to address a local problem, but may end up pulling current in from elsewhere).

Yes - however, in a commercial environment, you're not worried about protecting the establishment, only your amp. A MOV or other device will clamp the amount of the transient voltage your own personal amplifier sees. Any surge currents going through will be of two kinds - short or long. Short pulses the MOV/TVS will simply eat and dissipate as heat. Long pulses will blow the line fuse, disconnecting your amp from the AC line, in almost all cases before the MOV/TVS dies. In either situation, you have protected your amp by clamping the voltage that could damage it, and shunted any currents through the protective devices, not through the operating parts of your amp. Again, I think that's good economy.

As a side note, I think that for bands working bars with dubious wiring, you might consider running the whole band from a ferroresonant constang voltage transformer. These will pretty much stop any of the local wiring junk from getting to you. Outdoor shows on generator power are another place. Modestly costly, but not in the class of your pristine Deluxe. In all of this, you are purchasing insurance with a one-time premium.

I agree with the failing short, and can add that if the resistive path through the failed MOV is somewhere between explosion and blowing an upstream fuse, then internal arcing and fuming of the internals over a second or two is sometimes observed.

Yep.

[mhuss]

+1 Great customer experience and great info gathering.

--mark

[printer2]

Used to know a little on this stuff long time ago and trying to remember will only make my head ache. The non-hurting stuff is that if you have serious concerns of transients you want to do a staged response to them.

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[bob p]

As to the home generating transients, I moved into my current home about 8 years ago. We were losing $100 smart thermostats about every fifth or sixth thunderstorm. I put in surge protectors consisting of **huge** movs on the incoming AC lines. Three years and counting for no damage to the thermostats, including massive thunderstorms. Anecdotal, but in my case, it protected the HOUSE.

Hijack: What kind of specs did you use for the MOV that you put on your house? I used to rent a house that was on a supply line that was a lightning magnet. My current house loses power all the time when the subdivision's line fuse gets taken out by a high voltage surge during an electrical storm.

Another application that would be interesting to think about -- DSL and Cable connections. I've recently had lightning storms take out ethernet routers on more than one occasion.

[Steve Conner]

I see MOVs and TVSs mostly as protection for keeping solid-state equipment alive. Tube gear is much more robust. It's a lot harder to punch through some transformer insulation than a diode or transistor junction.

I've used TVSs on a few projects, but they were all power electronic things that ran SCRs and IGBTs straight off the line.

I once had my ass saved by a MOV when I was wiring up a high voltage exhibit at a science park. The guy who did the control panel decided to use a mains filter with a built-in MOV. I accidentally connected the control power between two phases instead of between one phase and neutral, so it got 400V instead of 230. The MOV exploded so violently that it punched a hole in the filter casing, but it took out the breakers in the process and saved the rest of my circuitry.

[R.G.]

Hijack: What kind of specs did you use for the MOV that you put on your house? I used to rent a house that was on a supply line that was a lightning magnet. My current house loses power all the time when the subdivision's line fuse gets taken out by a high voltage surge during an electrical storm.

Another application that would be interesting to think about -- DSL and Cable connections. I've recently had lightning storms take out ethernet routers on more than one occasion.

GREAT BIG ones. I was going to put them in one at a time myself and found I could get an add-on suppressor unit as cheaply, and not have to trust my electrician skills as much.

They were Cutler-Hammer Surgetrap CHSPULTRA. They were about $70 each, and I bought four. I have a complex and extensive AC power setup, including always-on routers and some servers for the house network, so I probably overprotected. It worked. No losses to lightning since, compared to four ~$100 losses of thermostats and 24VAC utility transformers in a similar period before the protectors. The box blathers about 3500 Joules and 175,000 amperes tolerance for the protectors. They are on boxes that stick out of the breaker box and have indicator lights for "still have voltage across me", so they haven't shorted or opened the breakers yet, and are still in operation.

I see MOVs and TVSs mostly as protection for keeping solid-state equipment alive. Tube gear is much more robust. It's a lot harder to punch through some transformer insulation than a diode or transistor junction.

Probably so. Tube amps are tough. Still - how can one draw the line that says "this amp would not have died except for those two transients a couple of years ago in combination with two shorted power tubes and a hours-long run with a power soak" ? And we do know that tube amps have survived since the 50s. Some of them, anyway. How many more would be here if they'd had MOV/TVS protection? Who knows?

I have a strong reluctance to say that a $200 gold-and-titanium amulet would be better for an amp, but a $1 one that can be shown to do SOMETHING useful - why not? Even if they only act as amulets most of the time, they are cheap amulets. I have a fire extinguisher in my car. I've been driving for 46 years and never needed one. But I'm sure the day I don't have one in my car is when I'll have an electrical fire under the hood. We all have our little superstitions.

[bob p]

http://www.eaton.com/ecm/idcplg?IdcS...ILE&dID=460726

[R.G.]

Yep, that's the one. I had the electrician put one on each side of the split-240Vac lines coming in, as I remember. It's been a while. Looks really funny, seeing two sets of mildly-yellow glowing "eyes" in the night when I walk by them, but I've had no funny electrical breakdowns since.

[trobbins]

I always put a smallish MOV on the PT primary of any valve amp I restore. I do it primarily to suppress PT induced transients, but also with mains born transient protection in mind (for that very reason of an irreplaceable old PT).

To round out the discussion, it needs to be appreciated that unless the MOV has its own associated fuse in an amp, then a random mains swell or surge could end up causing the amp's AC line fuse to blow as a result of adding the MOV. Some such surges may not be a stress for the PT, and have no consequence for the amp itself, but could stop one's performance abruptly half way through a gig - so there may be other unforeseen consequences for the client that has just had a MOV added 'as a service'.

[R.G.]

I always put a smallish MOV on the PT primary of any valve amp I restore. I do it primarily to suppress PT induced transients, but also with mains born transient protection in mind (for that very reason of an irreplaceable old PT).

Yep, good practice.

To round out the discussion, it needs to be appreciated that unless the MOV has its own associated fuse in an amp, then a random mains swell or surge could end up causing the amp's AC line fuse to blow as a result of adding the MOV. Some such surges may not be a stress for the PT, and have no consequence for the amp itself, but could stop one's performance abruptly half way through a gig - so there may be other unforeseen consequences for the client that has just had a MOV added 'as a service'.

Yep, there is no substitute for understanding what you're doing. MOVs in particular need a fuse between them and any source of continuous power. The very manly MOVs in my whole house protectors are mounted after the breakers, and it's pointed out in the installation literature that this must be so. This is for two reasons - one so that the MOV intercepting a 100,000A surge will cause the breakers to clear instead of exploding, and two, so that after shunting many transients that cause its withstand voltage to drift down, it will trip the breaker instead of starting a fire - or exploding.

The issue then becomes - if there is a separate fuse for the MOV, what should its value be, compared to the real mains fuse? No point in making them the same, as then it becomes ambiguous which opens on a transient. So should it be bigger or smaller?

Since it's not intercepting surge currents from the PT primary charging the core, or from rectifiers charging the filter caps, you're free to make it smaller. However, if you do that, a surge might clear the MOV fuse, then have its way with the PT. I think you have to make the MOV fuse rating bigger than the primary fuse. If the primary fuse clears on a transient, and you put another back in, all is well. In fact, the only time the MOV fuse is likely to clear is when the MOV itself is shorted, which is what you want the individual fuse to do.

[trobbins]

I didn't look at your house unit's specs, but MOV 'maintenance' was always a concern and so many industrial units have at least failure indicators, or auxiliary contacts for alarming. It is indeed a very complex design issue to work through all the coordination sequences of protective parts, and the impact of different surge profiles, and the different types of impedance that can be added and the behaviour of that impedance to the surge profile, and the likely levels of prospective current that could be achieved, and throw in EPR issues, and marry that with the withstand limits of the equipment you are trying to save.

[loudthud]

What about using outlet strips or those rack mount "power conditioning" toys. I put power conditioning in quotes because real power conditioners have isolation transformers.

[R.G.]

I did a series of articles on power quality for amps, and the pitfalls of poor power setups, as well as power conditioners for Premier Guitar a ways back. I think they may still be on line.

The power strip "conditioners" are usually MOVs or similar line to line, and both lines to ground, with fuses. Some of them contain L-C filters for EMI.

You want clean power? Get a 2KW isolating ferroresonant/constant-voltage transformer. Ebay has great deals on them sometimes - or did back when ebay wasn't just about selling new goods for dealers.

[km6xz]

Sola and other brands can be seen surplus but constant voltage transformers are large and generate noise(acoustic sound). We had all the control rooms and studios wired from large constant voltage transformers which needed their own ventilated isolated sound proof room. They work great without maintenance, handle over loads well(but losses increase dramatically) and lower line distortion but do cost a lot new, are heavy and might not be suited to carrying to stage performances. Generator powered shows really need them however and are often used due to riders for at least the stage instruments.
Later, I got one surplus for my work shop that was cheap but I had to remove it from its installation and rent a truck to haul it. It took 4 people to lift it onto a pallet so I could use a fork lift to put it in the truck. Around Silicon Valley they were plentiful surplus, essentially free to haul them away from the many high tech companies that went out of business during the switch from electronics to software in the 80s. They do need a load on them however so planning a protected workshop has to consider that. At the studio it was not an issue since the consoles were on 24hr/day but in my shop it meant wiring primary power contactors to shut it off.

Ferroresonant transformers(constant voltage transformers) are not very efficient compared to modern electronic double conversion regulators which generate very low distortion sine wave power from rectified mains unregulated power. They top out at about 80% efficiency but can easily get below 50% depending on load and voltage.
An alternative is use a UPS in constant on mode, or modify one to run continuously by beefing up the AC supply, which normally us used only to charge the batteries. You could use am unmodified UPS with high enough rating to just run the amp throughout the whole show then you and the drummer might be the only instrument on stage still working if there is a bad spike. Large UPS's that use stationary cell batteries can supply a lot of clean power. The little ones they sell at computer stores would not last long on a charge however.
To protect one piece of valuable vintage gear, it is possible to make your own low distortion stable supply by using a solid state power amp feeding a step up transformer. Set a high stability 60 or 50 hz oscillator as its source signal and power your amp from that combination. We did that with tube power amps before DC servo motors for capstans in tape decks to get vari-speed where we could use a HP 200CD as the stable source and vary the tape speed over a wide range.

A spike might get the solid state amp but would not damage your vintage amp.
Power strips with MOVs should not be trusted at all. They do not work usually after the first spike but there is no indication of the demise of the MOV. They are usually sold as high profit add ons by computer and home entertainment equipment dealers.
The biggest advantage for the Constant Voltage Transformer is that it is always working and you just forget about spikes and even sags.
The other regulators that can be found surplus are feedback driven motorized variacs or automatic switched tap transformers but they do not offer spike protection so they should not be introduced into this discussion.

[R.G.]

Here's an example:
500VA, enough to power an amplifier. Weighs 42 pounds crated for shipping. New, about $200 plus shipping, available used/ebay for $80 plus shipping. Do not ignore shipping on these things. There were several 200-250Va units listed for $20-$50, and of course, lighter.

1.5kva and 2kva units ought to power your whole band. These will weigh something like 80-100 pounds, so a wheeled cart ought to work for them.