RossD wrote:
I noticed sometimes people float heater windings and sometimes ground them with a couple of 100 ohm resistors. The 6sl7 datasheet states a max heater to cathode of 90v, with the small about of bias and a floating heater, could it potentially be exceeded? Float it or ground it?
...
What about the use of a X2 cap and a parallel resistor at the mains ground to chassis?
Everybody get ready, because I'm about to stomp on some toes.
When it comes to grounds, I am of the opinion that most people simply do NOT pay close enough attention. It is really not enough to simply look at a piece of equipment and try to decide on the ground scheme, you really need a "system
grounding philosophy" that applies not only to equipment you build, but everything you use. The thing about the fight between the "direct ground" camp and the "parallel cap/resistor camp" is that the actual
grounding philosophy is never discussed. The cap and resistor separation between chassis and signal grounds was almost an absolute requirement with two wire ac systems to achieve a safe design. Today it, more often that not, leads to funny ground loops and background noise.
So, what do I mean by a "system
grounding philosophy"? This means that the grounds in your system must be managed to both provide safety and to avoid ground loops and currents. This is not as simple as it sounds at first. And it depends on how your equipment is constructed, how it's grounded, and how it's interconnected. It also depends on the type and quality of your power system where the equipment is used. Everyone who has a system should have a ground plan to manage noise and safety. It just makes good sense when connecting multiple disparate pieces of equipment; be they audio equipment, radio gear, or test gear.
Lets look at an example; me. I have a good quality three wire power system with excellent ground path impedance. I have checked ALL my outlets for proper hot/neutral wiring and impedance to ground spike (not a simple task). Given this, I have developed a
grounding philosophy for all my equipment. Here it is:
Matt's Audio System "System Grounding Philosophy"1. All separate pieces of equipment shall use IEC-53 connectors and implement independent 3rd wire safety ground design.
2. Chassis grounds for all pieces of equipment shall be provided by that piece of equipment's safety ground lead. The IEC ground lead shall connect to the main chassis and all exposed metal parts, except for signal/speaker connectors, with a DC impedance not to exceed 2mΩ.
3. System wide signal (and speaker when used) ground reference shall be provided by the system power amplifier.
4. Power amp signal and chassis grounds shall be tied together at only one point, this point shall be within the chassis, and this connection shall have a DC impedance not to exceed 2mΩ.
5. No preamp, equalizer, switch, control, or other equipment shall connect signal ground to chassis safety ground except through a "ground lift switch" which can break ground connection when used in-system.
6. Signal ground reference for equipment other than power amps shall be provided via interconnect shields or separate ground wire connection (e.g. turntables with magnetic cartridges).
7. When multiple signal grounds, via cable shields or ground wires, connect two pieces of equipment, those cables and wires shall be bundled in close proximity to each other to minimize field coupling area.
8. At no time shall signals pass between system components without signal return paths (either shields or conductors in twisted pairs) for each signal connection.
9. Existing two wire commercial components used within the system shall meet these requirements or utilize an isolation transformer to supply AC power.Now this is
my grounding philosophy. I do not mean to suggest that everyone should follow these rules. Everyone should apply rules that are appropriate to their situation. However, I will tell you that my system, regardless of how I configure it, is a silent as death itself with no signal input. And every piece of my equipment has a safety path for fault currents that will not run through anyone touching a chassis or control. This is the way I like it.
So what does this mean to you? The answer is: "it depends". If like me, you have a good quality three wire ground system, you may want to adopt a
philosophy like mine. But maybe not. You will notice that this ground
philosophy tends to drive you to always use either a power transformer or an isolated switching supply in each piece of equipment (except the power amp, however it's a good idea there as well).
As to your question about the x2 cap and a parallel resistor, the chassis ground should always be tied DIRECTLY to the mains ground for safety reasons. If used, the cap and resistor would go between signal ground and chassis ground. But again, this depends on your overall
grounding scheme. As for the 6SL7 heater, I would use two 100Ω resistors to form a center reference to ground.
Now I'll get off of my soap box.
This is just one of those aspects of design that I think tends to get over looked. And bad
grounding can be a nightmare when trying to figure out from where a hum or buzz is coming.
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