Sorry for my ignorance, but how does 600VDC convert to 250VAC?
Eico HF-81 Transformer Help
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The AC cap is designed to handle the use of ACV which because of its varying nature a more difficult requirement than a cap designed for DCV. The safety cap is also designed to fail in the open position which makes energizing the chassis a non issue, unlike the vintage cap that was in the amp. You can read up on this by searching for safety capacitor.
The 250VAC rating is by UL and/or other safety agencies. It means "approved to use on power lines up to 250V with expected voltage transients" These transient or surge voltage events are frequent and can reach thousands of volts. The highest Y1 rating requires testing at 8000V, Y2 is 5000V. Y3 don't have a surge test, and are not allowed in equipment where safety standards must be met.
So yes, a capacitor marked "250VAC" can be rated higher than one marked "600 VDC"
So yes, a capacitor marked "250VAC" can be rated higher than one marked "600 VDC"
I understand now, thank you. I also plan to change my 2 prong power cord to a 3 prong power cord. How would I go about this in combination with the .033uf 250VAC safety capacitor?
dzkfraser
Super Member
Hi, I just oit an HF-81 at an auction - looks to be in good shape but I haven't opened it up yet - Wondering how you made out with your power issue, did you go with the safety cap or did you opt not to use one because of the new power cord(or did you use one with the new cord?) That whole death cap thing sort of freaks me out, read about them before but that was on a guitar amp, not a hifi
FlaCharlie
Super Member
The only reason it was called a "death cap" was because back then they would just put any higher voltage rated cap in there because there wasn't such a thing as a safety cap. If the cap failed, it failed as a short which put AC voltage on the chassis. The modern safety caps fail open so that can't happen.
Even if you install a 3 wire, grounded, power cord you can still use a safety cap.
Here is a slightly technical explanation than the above of how those values arise, but hopefully not too technical. (I'm deliberately ignoring many complicating factors, including DC offset and pulse conditions.)
AC voltages are specified as a nominal voltage which appears to be the same as a DC voltage. This is an incorrect assumption. A nominal AC voltage actually has an implied RMS (Root-Mean Squared) unit, where RMS describes the equivalent heating capacity of the AC voltage to a DC voltage. In other words, if one ran a DC voltage into a resistive load what AC voltage would be required to provide equivalent heating. The RMS is normally omitted so people forget it's there. That's a mistake which leads to misunderstanding.
Because the AC voltage fluctuates from 0 to a peak value, with the peak being both positive or negative depending upon the waveform half, the RMS value (again, equivalent heating) will be, more or less, similar to an average value, and that value will be lower. Remember, the voltage doesn't spend a lot of time at 0 or at the peak, it's typically somewhere in between. So the AC peak value and the 0 value will move the average value to somewhere in the middle. DC, to contrast, is, by definition, always at its peak value.
In consequence, when the AC mains voltage is specified as 120 VAC the actual peak voltage must be considerably higher to generate a higher average (more or less) value. I'll spare you the calculus, but because of the sinusoidal waveform, and because the two waveform halves are opposite in polarity, the overall swing from the positive peak (one waveform half) to the negative peak (opposite waveform half) must be √2 x nominal mains voltage ≈ 1.4 x nominal mains voltage.
The 1.4 multiplier causes the nominal 120 VAC mains to actually have a peak voltage—again, measuring between the peaks of the two AC waveform halves which are of opposite polarity—of 170 VAC. So, flipping the multiplier around, the DC voltage is going to 0.707 or approximately 71% of the AC peak.
So that's part of why the AC rating is lower In the case of the 600 VDC to 250 VAC conversion, 70.1% of 600 VDC is 424 VAC, which is why 400 VAC is sometimes specified. But further de-rating from 400 VAC to 250 VAC is desirable.
The maximum voltage stress for the dielectric—i.e. the insulating layer between the capacitor's two plates—occurs at the peak value of the applied AC voltage, not its average value, so the capacitor's voltage rating must be lower for AC than for DC, because the AC voltage is really is RMS value and therefore contains the hidden peak of (√2 x the nominal mains voltage). The two voltage types, again, have equivalent heating power, but that doesn't mean the maximum values are identical.
But it gets even worse for the poor capacitor.
When a capacitor is used for AC filtering it must be able to deal with higher-voltage spikes and noise on the line. Such spikes and noise—which originate from motors and inductive equipment (including switching power supplies)—may be quite high compared to normal operating conditions. (I'm neglecting the extraordinary conditions of lightning strikes.) So the manufacturer de-rates for the expected spikes and noise and consequently quotes a rating sufficient to ensure the capacitor can withstand normal mains voltages. Hence the de-rating for AC use.
In the case of a safety capacitor the de-rating is substantial to ensure the capacitor will not catastrophically fail in service from a high-voltage AC spike.
Hopefully that explains the issue in a form which can be generally applied to understanding power supplies and filtering. Again, I've eliminated some concepts to keep this short. Well, about as short as it can be.
True.
The capacitor across the mains is intended to make the chassis a soft-ground for AC instead of a hard ground and thus limit the current. The problem comes about when a capacitor shorts out from age or defects and creates the potential for a hot chassis with no current limit. A related problem is a non-polar line cord which has a 50-50 shot of creating a hot chassis. A polarized or grounded plug is required on all equipment to avoid creating a hot chassis if the plug is reversed. When the two-wire plugs were changed to wide/narrow this reversal was prevented, except on older sockets which were ambideathtrous, errr, ambidextrous. Special caps called XY caps are made for noise-decoupling purposes from the AC line. They fail as opens, not as shorts.
Recycleideas
New Member
IMHO, I would just remove the cap. A cap there will only filter out the higher frequency surge and noise. It has very limited effectiveness with real world noise and power line surge. In exchange, if the cap leak you put the equipment user in jeopardy. It definitely need to be remove, it it is an old oil or non-safety film cap. Those cap will leak in time. You can either replace it with a surge protection power bar or put in a proper circuit which use an Varister. BTW. Grounding the chassis with a proper grounded plug are needed safety feature one should put in. It is another safety matter and has nothing to do with this cap however.
I ended up installing a three prong cord and not installing a death cap. I had the same concerns initially because I saw it mostly on guitar amp, but from what I understood they're pretty similar if not the same. I also figured out how to install the power cord thanks to this video:
It is not so black-and-white as you make it out to be. Electrical and power engineering should be the guide here.
Removing Common-Mode and Differential-Mode Noise
It is difficult to know with certainty, i.e. without actual measurements, exactly how effective the capacitor is at removing common-mode and differential-mode noise.
Modern AC power is a mess and is full of hash. Much of the noise arises from motors (HVAC, air conditioner, dishwasher, washer/dryer, microwave, hair dryers, etc.), switching power supplies (computers, wall warts, appliances, etc.), and digital-logic switching (computer control in equipment). This may be in the home or in any home sharing the same transformer at the pole.
Such noise can stimulate parasitic tanks, typically form by the transformer's LC parasitics, to ring. It may also be amplified by a high-bandwidth amplifier, like a vacuum tube. So it is best removed.
The best filters for CM and DM noise are tuned tanks, combined with torroids for common-mode rejection. But the capacitor's reactance drops with frequency so it generally removes the higher-frequency noise. This is why it makes a tolerable common-mode filter which is why it is generally used.
X and Y Capacitors to Remove Noise
The X and Y capacitors are specifically designed for the type of filtering set forth in (1). I can't find my detailed posting about that and don't want to recreate it. Any of the manufacturer papers will describe this. Here's a summary:
The X-type capacitors are designed for line-to-neutral and fail as shorts, which is intended to blow the fuse or trip the circuit breaker. So no shock hazard. Such capacitors must never be used from line-to-chassis.
The Y-type safety capacitors are designed for line-to-ground, and do not leak. The design is to fail as an open, and the capacitor is not leaky. If a Y capacitor fails it ceases to remove noise, but does not create a safety hazard.
The XY-type is designed for either use and fails as an open.
The Y-type safety capacitors are designed for line-to-ground, and do not leak. The design is to fail as an open, and the capacitor is not leaky. If a Y capacitor fails it ceases to remove noise, but does not create a safety hazard.
The XY-type is designed for either use and fails as an open.
I refer you to the manufacturer's datasheets for additional information.
Grounding is no Panacea
Grounding the chassis may create serious noise problems.
(1) It may create ground loops when equipment is plugged together using shielded cables.
(2) It connects the chassis to the house wiring which is a giant radio antenna.
(3) If any computerized equipment (audio or video) in the house is dumping digital noise to electrical ground this enters the amplifier and many solid-state or tube amplifiers have the bandwidth to amplify it.
(2) It connects the chassis to the house wiring which is a giant radio antenna.
(3) If any computerized equipment (audio or video) in the house is dumping digital noise to electrical ground this enters the amplifier and many solid-state or tube amplifiers have the bandwidth to amplify it.
A Surge-Protector or Varistor (MOV) Does NOT Remove Noise, Constitutes a Fire Hazard
A surge-protector does not remove noise. It typically only contains a varistor as "protection", which it is not; the MOV varistor is a fire hazard.
I'll post my previous comments on that.
A few words of caution which I learned the hard way: the MOV (Metal Oxide Varistor) may silently fail, offering no protection, and a failed MOV can start a fire and burn down your house. It is not a robust device, in fact it is sacrificial like a fuse, and it silently fails without warning or indication, unlike a fuse.
The underlying problem is inherent in the MOV construction: grains of metal oxide packed together, much like a sugar cube. The grains function as diodes, which is hardly surprising since the first dry diodes were fabricated from conductive metal oxides demonstrating rectification effects at the junction. (Many metal oxides exhibit such behavior, which is why a crystal radio may be constructed using a rusty high-carbon razor blade as a diode to rectify the AM signal.) Each of the oxide diodes has a forward-voltage drop across it, and the aggregate thus has a breakdown voltage to conduct all the way across the device, much like a zener diode.
The MOV has so many grains that the breakdown (conduction point) voltage can be high, typically arranged to be about sixty volts to hundreds of volts. Because the grains are jumbled together in no particular order the device will bi-directionally conduct. (A carbon-composite resistor has similar behavior, which is why it has peculiar effects, like a voltage-dependent coefficient of resistance. Why I always urge replacement with metal film. Bu that's an interesting side discussion which can be found elsewhere on AK, not the main point here.)
Each time an MOV suppresses a surge the oxide grains are heated and are slightly damaged, with the intra-grain junction resistance drops. This creates lower-resistance pathways through the jumble of grains. Which creates more heating, and thus more damage. Thermal runaway eventually occurs. The lower-resistance, and thus higher-current, pathways then merge and the MOV silently shorts out, i.e. become a dead short across the AC line , typically hot to neutral or hot to ground, and it thus becomes exceedingly hot before the breaker in the socket strip trips. The manufacturer datasheets warn that a reduction in resistance of 10% is considered to be a failure. TEN PERCENT. Who regularly checks all of the MOVs in the house to verify proper function? Is this like a fire-alarm or CO detector which must be checked twice a year?
Some manufacturers increase the inter-grain resistance to make the MOV into a high-value resistor so when it eventually fails it does not burst into flame. This reduces the ability of the MOV to suppress transients.
Some years ago I had an MOV fail—to the best of my knowledge I had no suffered a lightning strike or similar overvoltage condition—and saw the smoke emerging from the strip. I will not use an MOV as it likely voids one's fire insurance. I regard them as dangerous and unsafe, and removed them from my socket strips. I have opened socket strips and seen MOVs discolored from the heat.
This is a well-known problem. Yet the FTC has not banned them. Go figure.
Manufacturers reduce the risk of MOV fires by adding current-limiting fuses, thermal cutouts, and even encase the MOV in cement to limit temperature rise. I believe it was Monster which specifically advertised that it encased all MOVs in a cement block. All of this ignores the fact that the MOV is continually failing and may not offer any protection when a significant over-voltage condition occurs. Such events are, of course, rare, but the point is protection; when the event occurs the MOV will likely not offer protection. The longer the MOV is in service, the less likely it is to properly function.
The replacement protection device, which is far safer, is the solid-state Transient-Voltage-Suppression (TVS) diode also known as the "transil" or by the older name of "thyrector". See: https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode
As the TVS is a silicon device, functioning much like a zener by conducting at a certain voltage, it does not have the same issue with gradual degradation. The TVS cannot handle much current, but it handles more than the MOV, so the surge it must suppress must be of short duration and within its capabilities. This typically is the case.
Here are a few links I had previously located and posted to demonstrate why the MOV is dangerous and should never be used:
The burning issue of metal oxide varistors and how to stop thermal runaway
https://www.rs-online.com/designspa...ide-varistors-and-how-to-stop-thermal-runaway
After prolonged use they can degrade and the material that makes up the disc can start a leakage current due to internal damage after a surge event, this will create a higher leakage current over time and before you know it we have a thermal runaway event.
This can result in a fire, a quick look online at MOV Fires in a search engine will show the extent to which MOV failure has led to house fires and the finger normally gets pointed at the surge protection socket strip. These socket strips can be over loaded and that is a fire risk but the MOV design if fitted badly can be a cause of fires also.
...
But the above raises an important point, MOVs are very popular in appliances and equipment whether that appliance device is fixed or portable.
Looking inside many different appliances I find MOVs soldered or strapped across the main terminals or connections without a fuse or thermal disconnection device of any kind.
All MOVs have the same reaction to over stressed events and those that do not have a protective device associated with them are a fire risk.
SURGE SUPPRESSOR FIRES by Jim Pharr, Fire Marshall, Gaston County, North Carolina
http://www.esdjournal.com/techpapr/Pharr/INVESTIGATING SURGE SUPPRESSOR FIRES.doc
Recent fires involving multiple outlet devices toted as surge suppressors raised attention at the Gaston County Fire Marshal's office primarily when one such fire occurred in a fire station. Investigation of a fire that started behind a desk in an office revealed the ignition source was a surge suppressor. Immediately prior to the fire workers were attempting to connect an emergency generator to the firehouse's electrical system to provide an alternative electrical source. Workers noted problems with the system and later, after the fire, discovered the generator was shipped from the factory with a loose neutral (a condition that causes voltage differences between legs of a 120 volt electrical system). Fire fighters noted fluctuations in their radio and other electronics thus started to disconnect all electronics from the system. In the office area they discovered a small fire burning behind the desk. A portable fire extinguisher was used to suppress the flames and an investigation was initiated.
Within that firehouse, three separate surge suppressors were recovered and examined. Each had failed, the one caught on fire, another suppressor ceased working, while the third continued working but later was found to have failed internally. These findings, coupled with suspicion of suppressor involvement in other fires, prompted in-depth examination of possible reasons.
https://www.rs-online.com/designspa...ide-varistors-and-how-to-stop-thermal-runaway
After prolonged use they can degrade and the material that makes up the disc can start a leakage current due to internal damage after a surge event, this will create a higher leakage current over time and before you know it we have a thermal runaway event.
This can result in a fire, a quick look online at MOV Fires in a search engine will show the extent to which MOV failure has led to house fires and the finger normally gets pointed at the surge protection socket strip. These socket strips can be over loaded and that is a fire risk but the MOV design if fitted badly can be a cause of fires also.
...
But the above raises an important point, MOVs are very popular in appliances and equipment whether that appliance device is fixed or portable.
Looking inside many different appliances I find MOVs soldered or strapped across the main terminals or connections without a fuse or thermal disconnection device of any kind.
All MOVs have the same reaction to over stressed events and those that do not have a protective device associated with them are a fire risk.
SURGE SUPPRESSOR FIRES by Jim Pharr, Fire Marshall, Gaston County, North Carolina
http://www.esdjournal.com/techpapr/Pharr/INVESTIGATING SURGE SUPPRESSOR FIRES.doc
Recent fires involving multiple outlet devices toted as surge suppressors raised attention at the Gaston County Fire Marshal's office primarily when one such fire occurred in a fire station. Investigation of a fire that started behind a desk in an office revealed the ignition source was a surge suppressor. Immediately prior to the fire workers were attempting to connect an emergency generator to the firehouse's electrical system to provide an alternative electrical source. Workers noted problems with the system and later, after the fire, discovered the generator was shipped from the factory with a loose neutral (a condition that causes voltage differences between legs of a 120 volt electrical system). Fire fighters noted fluctuations in their radio and other electronics thus started to disconnect all electronics from the system. In the office area they discovered a small fire burning behind the desk. A portable fire extinguisher was used to suppress the flames and an investigation was initiated.
Within that firehouse, three separate surge suppressors were recovered and examined. Each had failed, the one caught on fire, another suppressor ceased working, while the third continued working but later was found to have failed internally. These findings, coupled with suspicion of suppressor involvement in other fires, prompted in-depth examination of possible reasons.
The latter article (the .doc file) contains photos of failed MOVs and analysis. Impressive.
Recycleideas
New Member
What noise suppression circuit would you recommend?
If you don't have a background in electrical or power engineering, and don't have access to someone properly experienced who can assist you, I suggest you don't build devices which may create safety hazards. Assuming you have the technical skills, I still take no responsibility for the outcome of home-built power devices.
Having written that, for off the shelf solutions, I have used the EMI/RFI balanced LC filters in sealed cans manufactured by Cornell-Dublier, Corcom, Delta, etc. Many companies manufacture such devices. Inexpensive and easy to drop in. Make sure whatever you use is rated by UL, CE, etc. Do not purchase non-name brands from China, as many have faked certifications and do not contain the specified components.
Safety capacitors can be used if one uses the proper type (X or Y) for line-to-line, and line-to-ground.
To address the high-voltage spikes a TVS diode will nicely do.
Use fuses or circuit breakers reasonably sized to ensure a fast blow if an overload or short condition occurs. Putting in a 10 A slow-blow or circuit breaker pretty much ensures your mains breaker for that line will fire before the local protective device. Fuses have a curve, and are generally tolerant about overloads. It takes twice the rated current to blow within a few seconds. Ponder that sobering fact. Fuses exist to prevent fires, not to protect equipment from damage.
I suggest always using a GFI. All of my outlets are GFIs and I use them with a socket strip if I'm doing work outside my home. That's not for power suppression, that's for safety.
dzkfraser
Super Member
Nice vid - pretty straightforward - don't think there is a polarity switch on the Eico, did you replace the cap with a Y(or X/Y) or just leave it out after replacing the cord.
dzkfraser
Super Member
dzkfraser
Super Member
Starting to look over this amp - the previous owner had an on/off rocker switch inline on the power cord, Uhg. I figure I'll replace it with a simple chassis mount SPST toggle, The only problem being where to mount it. From what I can glean, the original construction had the on/off wired into one of the selector switches. I'll have to look into this
FlaCharlie
Super Member
That's actually a pretty decent solution, IMO. You could also switch it using a power strip.
The original switch is part of the Treble control and finding a suitable replacement might not be possible. I've seen examples where the issue was "solved" by drilling a hole in the front panel and installing a toggle switch. Unfortunately, this drastically reduces the value of the amp.
Another possible solution is to rewire the existing Tape slide switch which, I believe, is an EQ for reel to reel inputs. So, assuming you don't need that function, you could simply bypass its original function (wire it to use only one of the settings) and rewire the switch as a power switch.
dzkfraser
Super Member
I wouldn't drill the front panel, no way, the amp case is mesh grill on top, sides and back. The previous owner had mounted a fan onto the back with L brackets screwed through the mesh holes. Thinking of cutting a small aree of the grill on the back and mounting a removable plate with the switch mounted to it. That way the switch can be removed asily if the case needs to be opened. There doesn't look to be any rom on tthe chassis base. I'll look into the slide switch
FlaCharlie
Super Member
Since these amps have come to attract a lot of collector interest I'd advise against cutting anything, even the mesh grill, as it will hurt the value of the amp. Rewiring the slide switch does no cosmetic damage and it's a mod that can be easily reversed. Or, you could use the existing L brackets to mount a panel with a switch and run the wires through the mesh grill. No need to cut anything.
I completely left out the xy cap as well, but I though about adding it.
I thought about doing that if the original power switch on my Eico wears out, do you really think the tape switch can work as a power switch?
