3-wire cords are not necessary and are rarely used in audio equipment. Every piece of tube gear I own is on a 2-wire cord, both my own and commercial designs. I would argue that grounding the chassis to the mains is a poor idea and can lead to many problems.
Having said that, I agree with Hallow that this is not an issue of bias but a power supply issue. Phono preamps are particularly susceptible to noise, because of the high gain and low signal.
Having said that, I agree with Hallow that this is not an issue of bias but a power supply issue. Phono preamps are particularly susceptible to noise, because of the high gain and low signal.
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The metal chassis is painted, when I touch the tube shields the 'magnetic cloud' disappears instantly. The yellow wire (earth) is connected to the chassis but this house doesn't have a ground wire (vintage house). I have other amps and there's no issue. Here's what I did:
- Isolated the PT from chassis. No result.
- Disconnected audio ground from chassis. No result.
- Disconnected power chord earth wire from chassis. No result.
- The transformer has a wire (coming out of the windings) to connect it to ground. Disconnected this from ground. No result.
- Touched with my finger an unpainted area of the chassis. Big result.
Now, with the inputs, outputs, tube shields and circuit surrounded by this capacitive coupled leakage my noise problems are not going to be solved. What should I do? The chassis is quite long, PT is at one extreme the tubes are at the other end. It should be enough distance. The trafo has a thick metal cover as well. Do you think if I replaced it with a potted toroidal transformer my problems are going to be fixed? It is quite strange because this transformer didn't have any issues. It comes from another preamp where everything was OK.
Could be RF oscillation. A classic sign of this is a circuit which is noisy, with squeaks or pops, but it all behaves when you touch it. Your body is adding a lossy RF capacitance to the circuit and the loss is great enough to suppress the oscillation. Look for a stage which changes its DC conditions when you touch the chassis. Add grid stoppers.
Strongly disagree (read up on Class I and Class II safety requirements). There's a reason that it's called a safety ground! It need cause no problems if you do grounding properly- the audio ground and safety ground can be kept separate and connected via a ground breaker.
SY, I'm not an expert on IEC "requirements" or their regulatory applicability in the US, but I promise you I have owned hundreds, perhaps thousands of different audio components from every major manufacturer that came with 2-wire cords and worked fine. So I assume they are both legal and safe in their lawyer's minds 
Anyway, my comments were not about safety. If concerned with electrocution from the primary side, you can always put a GFI plug on a 2-wire cord, like they do on hair driers.
Anyway, my comments were not about safety. If concerned with electrocution from the primary side, you can always put a GFI plug on a 2-wire cord, like they do on hair driers.
2-wire cords are only allowed if there is DOUBLE isolation guaranteed AFAIK (at least in Europe).
That means i.e. a plastic case inside metal chassis.
The mains carrying parts would then have to be isolated themselves again (i.e. by being in some non-conducting housing themselves).
This is usually done in your standard CD player that has a metal case (for optics and feel) and comes with a 2-wire cable. An icon similar to a square inside a square shows that the equipment is compliant with that rule.
I don't know about the US requirements too much either but in Europe you need definitely a 3-wire cord and ground the chassis on the third wire (direct connection, no "ground brakers" etc. on that side) if you are using a metal chassis and have HV carrying components directly in there.
Martin
That means i.e. a plastic case inside metal chassis.
The mains carrying parts would then have to be isolated themselves again (i.e. by being in some non-conducting housing themselves).
This is usually done in your standard CD player that has a metal case (for optics and feel) and comes with a 2-wire cable. An icon similar to a square inside a square shows that the equipment is compliant with that rule.
I don't know about the US requirements too much either but in Europe you need definitely a 3-wire cord and ground the chassis on the third wire (direct connection, no "ground brakers" etc. on that side) if you are using a metal chassis and have HV carrying components directly in there.
Martin
Well, almost there. Just a little of hash noise now. MUCH, much better. I did two things: add a grid stopper at the grid of the CF and remove the ceramic cap across the PT primary. Since the blue ceramic cap across the switch was leaky I couldn't trust her twin sister anymore. Chassis is no longer 'radioactive'. Hey, thank you guys.
Yeah, that's true. But this wasn't the case. XY blue caps they were. I have the bad habit of recycling stuff...maybe they were just bad or maybe I overheated them with my soldering iron. One was leaky, the other I don't know. Sometimes the innocent pay for the sins of others. Anyway, if my problems with the chassis were because of a faulty cap or because of RF oscillation as DF96 described.... I'll probably never know.
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