I know its a stupid newbie question, but looking up the details of my first amp project, I keep on reading the significance of twisting the power cables and shielding them in order to prevent hum. Looking at the schematics, though. I don't really see much to twist since there is only one cable going to the power supply (b+ wire) per channel. So does twisting mean that you have to connect (twist) the power cables for both channels and if so, wouldn't that mean that they would have to be somewhat close together to ensure that?
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Power cables carry 60 Hz so twisting and shielding aren't going to do much for hum.
Or do you mean the DC wires?
Or do you mean the DC wires?
Looking at your diagram on a bigger screen (I have old eyes!), the wires to twist are the ones carrying 6.3VAC. They should also run in the corners of the chassis and not cross over signal wiring.
Thank you. That was very helpful. So do I get it correctly that the 250 V supply is DC whereas the 6.3V part of the Transformer (Hammond 270 DAX) supplies AC and that AC needs extra shielding (for which the twisting helps) whereas DC does not constitute much of an issue ?
It's not shielding per se, it's cancellation of hum fields- and, of course, avoiding accidentally coupling hum to signal circuits.
To answer the gauge question intelligently, the current needs to be specified.
To answer the gauge question intelligently, the current needs to be specified.
"520V C.T. @ 104ma."
The HT current is very low, so almost any wire will supply it safely. HOWEVER, we are dealing with ~500V, which is quite high, so the wire should be rated at 1KV or more.
"6.3V @ 3.5A"
The 3.5A figure is the maximum the transformer is able to supply. You need to add up the current required by all the tubes to see the actual working current, and then check the wire used is OK for this. Single strand wire is much easier to twist and get to stay in place compared with stranded.
The HT current is very low, so almost any wire will supply it safely. HOWEVER, we are dealing with ~500V, which is quite high, so the wire should be rated at 1KV or more.
"6.3V @ 3.5A"
The 3.5A figure is the maximum the transformer is able to supply. You need to add up the current required by all the tubes to see the actual working current, and then check the wire used is OK for this. Single strand wire is much easier to twist and get to stay in place compared with stranded.
Twisting them will increase the mutual coupling between the wires that contain 180 degree signal with respect to one another....
So the proximity of stray field is greatly reduced as opposed to then when the wires are laid out with separation with no twisting.... Whatever small residual magnetic field is present after twisting is best to place as close to GND plane as possible...if you view a FEA with wire along the GND plane you will see the concentration of flux more so into the GND plane and less filed into the surrounding air..
So the proximity of stray field is greatly reduced as opposed to then when the wires are laid out with separation with no twisting.... Whatever small residual magnetic field is present after twisting is best to place as close to GND plane as possible...if you view a FEA with wire along the GND plane you will see the concentration of flux more so into the GND plane and less filed into the surrounding air..
Twisting has little effect on mutual coupling between two adjacent wires, except to the extent that it ensures that they stay close together. However, it is has a great effect on magnetic coupling to other nearby circuit loops, which is why it is a good thing for AC wires.cerrem said:
The wiring layout is based on "Star Ground" ruling over all else.
I believe this leads to more confusion than just about any other Builder problems.
Instead I suggest you think about "circuits". The route around which the current MUST FLOW.
Every input, be it signal, or power, or speaker output (yes it too is an input), is via a circuit and that circuit has a Flow half and a Return half. Both halves MUST exist for the current to pass around the circuit. The Source provides a voltage and the circuit passes the current from the source and eventually back to the source.
Take the Left Signal Input.
In at the Hot terminal, wire to the vol pot top, through the vol pot, out at the vol pot bottom, wire to the input terminal. Assume that the input is "The Source". That description is a circuit. the current flows around the circuit. All the current that comes out of The Source MUST RETURN to The Source. For least interference the circuit should have a low loop area. This requires that the wires and traces and components are all very near each other. The wires are easy: use either screened coax, or screened two core, or a twisted pair. At the components, keep the wires VERY CLOSE to the components. This means no big loops between the RCA socket and the TWO wires. At the vol pot again no big loops.
Now go through EVERY input and work out the Flow wire/route and the Return wire/route. Minimise loop area in EVERY circuit.
If you have a dual polarity power supply you would have three wires: +ve flow, -ve flow and PSU Zero Volts Return. Couple all three together as a twisted triplet. Minimise loops at the ends of this triplet. This probably applies to your CT wire. Is it the two 260V wires that are the Flow routes?
Finally go back to your schematic and compare to your new close coupled circuits. Is any connection missing?
This could be a reference wire where the input is referenced to the output. Does a tiny current flow in the reference wire? How would that current return to it's source?
Route the reference wire along the route that the return current passes.
I believe this leads to more confusion than just about any other Builder problems.
Instead I suggest you think about "circuits". The route around which the current MUST FLOW.
Every input, be it signal, or power, or speaker output (yes it too is an input), is via a circuit and that circuit has a Flow half and a Return half. Both halves MUST exist for the current to pass around the circuit. The Source provides a voltage and the circuit passes the current from the source and eventually back to the source.
Take the Left Signal Input.
In at the Hot terminal, wire to the vol pot top, through the vol pot, out at the vol pot bottom, wire to the input terminal. Assume that the input is "The Source". That description is a circuit. the current flows around the circuit. All the current that comes out of The Source MUST RETURN to The Source. For least interference the circuit should have a low loop area. This requires that the wires and traces and components are all very near each other. The wires are easy: use either screened coax, or screened two core, or a twisted pair. At the components, keep the wires VERY CLOSE to the components. This means no big loops between the RCA socket and the TWO wires. At the vol pot again no big loops.
Now go through EVERY input and work out the Flow wire/route and the Return wire/route. Minimise loop area in EVERY circuit.
If you have a dual polarity power supply you would have three wires: +ve flow, -ve flow and PSU Zero Volts Return. Couple all three together as a twisted triplet. Minimise loops at the ends of this triplet. This probably applies to your CT wire. Is it the two 260V wires that are the Flow routes?
Finally go back to your schematic and compare to your new close coupled circuits. Is any connection missing?
This could be a reference wire where the input is referenced to the output. Does a tiny current flow in the reference wire? How would that current return to it's source?
Route the reference wire along the route that the return current passes.
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It would appear that The "power supply"must be a two phase valve rectified arrangement since it is outputting 250VDC from a 260-0-260 transformer.
There is no 500volt wire to be concerned about.
There is no 500volt wire to be concerned about.
Can someone educate me on this point? Thank you for a simple 'yes' or a more elaborate answer, if applicable.
Its important to remember the possible shock risk from the two ends of the 260-0-260 <<
Ie its important to be aware of the maximum potential within any chassis.
Not just to ground. Including +/- Rails on DC.
So the maximum potential in equipment is the maximum potential between any two points that could be measured. (not referenced to ground)
I twist all AC power cables (within chassis)..each to their own I guess.
Regards
M. Gregg
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It was the recommendation to only use 1kv rated wire that I thought was an overreaction when there is only a B+ of 250v.
Whats the insulation value between the AC on the output of the power Tx? (if I twist them together)
Whats the back EMF from the output Tx?
You are correct that 1KV is a bit over kill..however its interesting. I use 600v..PTFE which is still small in cross section.
NB its interesting to note that not all PTFE cable has a good working voltage..
As with all things you could use 250V insulation and it would withstand it for years..same with current ratings.
Reason is the insulation when put together is double the thickness.
To ground its within spec..
The temp rating would be the current issue..or VD
A problem with PTFE is it deforms easily so don't pull ty-raps tight..in this case PVC is better.
I still prefer PTFE..Old habbits etc.
Regards
M. Gregg
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Well the insulation on the Power Transformer secondary is supplied by the manufacturer in this case so I would hope it is up to the task
😀
With Hammond it needs to be non stick you could fry an egg on the power Tx's
The insulation wrap ripens and goes brown after a few hours use..a bit like leaves in autumn..😀
Regards
M. Gregg
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