New AC plug wiring for ST-70

If I installed a new AC plug (the type with one wider blade), which side (fuse or switch) would that wider blade get wired to? Would it even matter?

Thanks.

Bob Latino wrote:Hi,

On a two wire cord, such as on a Dynaco tube amp, it really doesn't matter. The white neutral wire inside the AC outlet is supposed to go to the "wider blade" on the outlet itself. Some say that this wire should go to the switch and the black wire side should go to the fuse - BUT - I have connected those power cords up either way and it makes no difference in the sound of the amp or the noise level of the amp. On the original Dynaco amps the plug on the AC cord was not polarized (with one wide blade and one smaller blade) and Dynaco gave no instructions about what wire goes where when you installed the power cord.

Bob

Hi Bob,
Thanks for the fast response!!

Bob Latino wrote:Hi,
On a two wire cord, such as on a Dynaco tube amp, it really doesn't matter. The white neutral wire inside the AC outlet is supposed to go to the "wider blade" on the outlet itself. Some say that this wire should go to the switch and the black wire side should go to the fuse - BUT - I have connected those power cords up either way and it makes no difference in the sound of the amp or the noise level of the amp. On the original Dynaco amps the plug on the AC cord was not polarized (with one wide blade and one smaller blade) and Dynaco gave no instructions about what wire goes where when you installed the power cord.
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Which is often an advantage, because one orientation of the line cord may result in lower hum than the other.

It will make no sonic difference to the amp (because the transformer primary has no "polarity" and is isolated from chassis) but I think convention dictates the black ("hot") wire of the polarized power cord should first go to Fuse, then to switch, then to transformer. The other primary from the transformer is now considered neutral and is connected to the "white/neutral" conductor from the power cord. If you use a 3 prong grounding plug, the "safety" green ground wire should be fastened to the chassis as close to the entry point as possible. The safety ground should not be attached to any screw that could loosen, such as any screw passing through anything plastic which could compress and loosen. If you go with a 2 prong polarized plug, the "narrow" tang is considered "black/hot". I would strongly recommend against using a non polarized plug. You can consider this the first upgrade to your ST-70.
Personally, I think the benefit of having a safety ground outweighs the risk of developing ground loop noise. One way to minimize ground loop potential is to make sure all components are plugged into the same circuit, keeping all plug polarities the same. Another thing you can do to minimize hum riding into the amp over the ground conductor of the input's interconnect cable is to connect the ground side of each input jack to chassis through a 0.01uf ceramic capacitor to a chassis mounted solder tag. Any high frequency AC will be drawn to the chassis rather than the circuit. The input jack's normal ground conductor to the driver board ground input remains the same, the capacitor is an addition.
PS-Worst Case Scenario of not switching the hot wire and not having safety ground would be assuming the amplifier is turned "off" (because neutral wire is switched open) and short develops between hot wire and chassis. If no safety ground is present, chassis will be energized with no low impedance path back to circuit panel to trip breaker. Amplifier will be "off", yet chassis will be hot. Chassis fuse will be useless as it is not in circuit. All you need for a shock is to contact the "hot" chassis and a nearby ground.
See attached photo examples of input ground capacitors to solder tags and green ground wire to solder tag at power switch. Also attached is photo of how I put an IEC "tail" on the back of the amp instead of a 2 wire power cord. Middle photo shows how I fixed a grommet to the power cord to keep it from pulling out. I used a short piece of high tensile wire twisted around the waist of the grommet till it gripped the power cord firmly. Excess wire was clipped flush with internal grommet.
Rich
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heyraz wrote:It will make no sonic difference to the amp (because the transformer primary has no "polarity" and is isolated from chassis) but I think convention dictates the black ("hot") wire of the polarized power cord should first go to Fuse, then to switch, then to transformer. The other primary from the transformer is now considered neutral and is connected to the "white/neutral" conductor from the power cord.  If you use a 3 prong grounding plug, the "safety" green ground wire should be fastened to the chassis as close to the entry point as possible.  The safety ground should not be attached to any screw that could loosen, such as any screw passing through anything plastic which could compress and loosen.  If you go with a 2 prong polarized plug, the "narrow" tang is considered "black/hot".  I would strongly recommend against using a non polarized plug. You can consider this the first upgrade to your ST-70.
Personally, I think the benefit of having a safety ground outweighs the risk of developing ground loop noise.  One way to minimize ground loop potential is to make sure all components are plugged into the same circuit, keeping all plug polarities the same.  Another thing you can do to minimize hum riding into the amp over the ground conductor of the input's interconnect cable is to connect the ground side of each input jack to chassis through a 0.01uf ceramic capacitor to a chassis mounted solder tag.  Any high frequency AC will be drawn to the chassis rather than the circuit. The input jack's normal ground conductor to the driver board ground input remains the same, the capacitor is an addition.
PS-Worst Case Scenario of not switching the hot wire and not having safety ground would be assuming the amplifier is turned "off" (because neutral wire is switched open) and short develops between hot wire and chassis.  If no safety ground is present, chassis will be energized with no low impedance path back to circuit panel to trip breaker. Amplifier will be "off", yet chassis will be hot. Chassis fuse will be useless as it is not in circuit.  All you need for a shock is to contact the "hot" chassis and a nearby ground.  
See attached photo examples of input ground capacitors to solder tags and green ground wire to solder tag at power switch. Also attached is photo of how I put an IEC "tail"  on the back of the amp instead of a 2 wire power cord.  Middle photo shows how I fixed a grommet to the power cord to keep it from pulling out.  I used a short piece of high tensile wire twisted around the waist of the grommet till it gripped the power cord firmly.  Excess wire was clipped flush with internal grommet.
Rich

Resurrecting on old thread as I've been researching this like crazy and so far all the advice points to @heyraz.

I'm cleaning up my M125 amps by doing a complete gut and rewire- one of the recommendations made to me is to change the switch to use the HOT lead vs the neutral as it is currently wired. From the comments above - this seems left over from two prong.

So I'm going to rewire as hot to fuse to switch to transformer.

I hope this is OK.

Thanks as always.

Back in the day, the two-pronged plug on the power cord of the ST-70 was not polarized and it certainly wasn't grounded. The fuse and switch were wired on each leg of the two conductor AC line. This is the way it was done 60 years ago when all electrical outlets in American homes had only 2 wires and nobody was seriously concerned about safety.

Today, there is no reason to perpetuate this kind of thinking. Almost every home has grounded, 3 wire electrical outlets and modern wiring convention dictates that both the fuse and the power switch should be wired to the hot side only.

I always rewire my vintage tube amps using an IEC-compliant power cord where the chassis is grounded to the mains. Despite opinions to the contrary, proper grounding and the use of an EMI filter (safety) capacitor actually prevent noise created by ground loops. I've never had noise issues and I'm always confident that my grounded test equipment can be operated safely (e.g. my oscilloscope and function generator).

I also rewire the power supply so that the fuse and power switch are both on the hot side of the AC line, with the fuse being first. The common side of the AC line goes directly to the primary of the power transformer and the ground wire is soldered to the main chassis grounding point of the amp.

Any tip on howto mount IEC inlets ? Tools used , IEC mounts etc.

For something like the Dynakit ST-70 I don't like to punch new holes into the chassis. Instead, I just install a 3 meter, 3 wire power cord, with a 3 prong plug.

Ok.
Installing an IEC intake takes some grinding, but is worth the effort.

I agree that it's usually worthwhile to install the IEC connector into the chassis and I do that in most of my restorations that include many modifications as well (the attached photo shows one installed into a heavily modified Magnavox 9302 chassis). But, I elected not to modify the chassis at all in my ST-70 restoration because I wanted to keep the ST-70 fairly "stock" with only a few safety mods. Nothing to radically change its appearance.

If you want to cut a chassis to accommodate the IEC connector, all that is required is a drill to start the hole, a nibbler to enlarge and shape the opening, and a file to finish cleaning and sizing the opening. It doesn't take a lot of time or work. Because I do so many of them, I use a square hole punch specifically designed for this purpose, but this would be an unnecessary expense if you're only using it a few times.