Owners of Carver Crimson 275 -Caution/Warning--Potential Increased Risk of Electric Shock
What's going on?: If you own a Carver Crimson 275 amplifier there is a real potential that your amplifier does not have a proper chassis grounding scheme inside. There is a type of grounding system involved, but the third prong of the IEC socket (the actual electrical socket on the inside of the amplifier) is left wide open on my amplifier and and at least one other amp that I have virtually confirmed (not firsthand--only through another owner).
What does this mean?: If one of the hookup wires carrying power/B+ came loose from its soldered weld and touched either the chassis or something conductive to the chassis and the user then touched the chassis/unit with the power on they could get a significant electric shock. There are some seriously high voltages in this tube amp (like many amps of its topology) and the result could be lethal.
Carver Corporation's Response to Status on Grounding: I have notified Frank Malitz of the Carver Corporation about this problem. He responded to me in writing saying the following, exact quotation, nothing more or less:
"the design is under review with engineering and I'll contact you tomorrow." (F. Malitz)
In the interim, users may be at an increased risk of shock and bodily harm. I would encourage you to check with an electronics expert or the Carver Corporation for further guidance, as I'm not qualified to say more than there's no confirmed chassis ground in the unit that I have and at least one other unit.
Finally, unless you are qualified in working with high voltages do not open your unit to check for the ground wire running to the ground prong on the IEC. Please note that doing so with or without a chassis ground should only be done by someone experienced with electronics--i.e. caps must be discharged and care taken to cause an issue. Seek professional help.
I'm just a music lover and hobbyist. I'm merely relaying this information out of concern for the safety of fellow humans. I'm not an expert and perhaps an expert will rule out this identified potential increased risk as non-existent. For now, I can say that it is my opinion that there is a real reason to believe that grounding scheme might be deficient and I will not be using my amp until this is sorted out.
I'll report any response from the Carver Corporation as soon as I hear back. For now, my amp will sit unplugged.
The AC power wiring inside the equipment is double insulated. (NO safety EGC , Equipment Grounding Conductor, is needed) Look on the back of the units. It should say Class ll wiring and or may display a square inside of a square. It costs more to make Class ll equipment, than it does to make equipment that uses the EGC from the AC mains wall outlet. CLASS II power wiring
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This sounds like a hot mess. If the gear is in fact double insulated (which I'm not sure applies JUST to the power supply) then why use a 3 prong IEC connector? Use a 2 prong like other gear which is double insulated uses. And ... why oh why would you connect the chassis ground to signal ground directly? Being a tube power amplifier we have lots of high voltage wiring besides the power supply. In case of a high voltage short to the chassis that high voltage and current gets carried by the delicate ground in the interconnect wiring, which is then a fire hazard, or upstream equipment risk. Really quite a messy set of choices. |
@erik_squires Tube amp or not, if the power switch were to be damaged (such as in shipment) and shorted to the chassis, if wired correctly the main power fuse would blow the instant the unit were plugged into the wall. |
@10229 Two-prong plug devices are usually either double-insulated, or there is a SMPS that has additional isolation that protects the chassis from potential AC faults. I have several CD players like this, but I also have 30 year old preamps that only had 2-prong plugs but had low voltage transformers. @jea48 Grounding schemes have been a matter of debate for many years in the audio world. Some builders believe grounding the chassis to earth causes hum, others do not. I agree that a Class II device with no need for chassis earth ground should have a 2-prong jack. I agree the schematic shows signal and circuit grounds go to chassis, and that the sequence shows fuse, switch, and transformer. It would have been nice to show the jack connection first, rather than just the source AC symbol. Also, RCA jacks and speaker jacks are generally connected to the chassis, would you agree? @erik_squires The photo shows me nothing to believe this is a Class II double-insulted circuit on the primary of the transformer (or anywhere else). It is also fused incorrectly as described previously. The power supply has a primary side at incoming AC voltage which is what is being discussed here. The secondary side of the transformer is the other part of the power supply. If the AC side has a short, for example the power switch as atmasphere has stated, or a broken line (hot) connection, etc. touches the chassis, there should be a low resistance path to ground, but sometimes designers rely on the neutral connection at the service panel to provide that ground. However, properly connected, the fuse will blow. This is true of solid state gear as well, which can have much higher current draw but lower voltages. On the secondary side with the HT voltage going to the tubes and circuitry, if there is a short, it will blow the fuse(s) or in circuits with virtual center tap connections through 100 Ohm resistors to ground, the resistors. Properly designed and fused, the unit is unlikely to damage the interconnects. High-gain signal grounds (preamp, VAS, driver, etc.) are kept separated from high-current grounds (output tubes, output transistors) in most amps to minimize hum or there is some sort of star grounding scheme. Signal grounding schemes are quite varied and hotly debated as well, but they are not safety grounds. There is ample information available that details the need for proper chassis AC safety grounding, but there may be some particular reason why this unit was considered to be safe without it - I just don’t see it based on the schematics. I’m not pro or con Carver, just commenting on the photo and the schematic. Again, it would be very nice if Carver would provide their reasoning which may answer all these concerns. |
