How A Valve Guitar Amp Works.

[Muzza]

That's a cool looking amp, perhaps that could be a good design template for the PBG guitar amp.

If you put it next to a Blues Junior, it's almost exactly the same height, width and depth. It just doesn't have the sloping front face - all sides are parallel.

[Muzza]

If anybody wants to build an amp and you have no electrical experience, I would suggest you get assistance from somebody who does. There are voltages in excess of 350V dc in some of these amps, some as high as 600v I believe. (Doc, correct me if that's an exaggeration.)

I'm not sure why anybody would bother with a 12 volt / 240 volt initial input transformer, unless you want to use it for busking.

The danger of these amps isn't the 240v AC primary winding voltage, ('tho that IS lethal...) it's the high DC voltage in the circuit wiring that catches people out. Even if you had a 12 volt transformer feeding the amp, the circuit voltages would still be in the 300-400 volt range.

[DrNomis_44]

If you put it next to a Blues Junior, it's almost exactly the same height, width and depth. It just doesn't have the sloping front face - all sides are parallel.

I think I remember hearing a Blues Junior Amp at some stage, they're great little amps, wouldn't mind owning, or building one.

[DrNomis_44]

If anybody wants to build an amp and you have no electrical experience, I would suggest you get assistance from somebody who does. There are voltages in excess of 350V dc in some of these amps, some as high as 600v I believe. (Doc, correct me if that's an exaggeration.)

I'm not sure why anybody would bother with a 12 volt / 240 volt initial input transformer, unless you want to use it for busking.

The danger of these amps isn't the 240v AC primary winding voltage, ('tho that IS lethal...) it's the high DC voltage in the circuit wiring that catches people out. Even if you had a 12 volt transformer feeding the amp, the circuit voltages would still be in the 300-400 volt range.

You're actually correct, the voltages in a Valve amp can easily be anywhere from about 350 V DC to 600V DC, I have even seen one design for a 150 Watt Valve amp that had voltages as high as 750V DC which is scary to think about, and I totally agree with your safety concerns there, there is a very thin margin for errors when building a Valve amp, and the errors could prove fatal if you're not familiar with the procedures you need to follow when working on a Valve amp, I definitely would advocate getting help from someone more experienced if you're a first-time Valve amp builder.

As an example, before my Marshall MA100C amp was fixed I remember measuring voltages as high as about 600 V DC in it, the guy who fixed it managed to bring the voltage down to about 450V DC, still high enough to be very lethal though.


Our Australian 240V AC mains power that we get from the 3-pin wall sockets is actually not 240V AC, that is what's call the Root Mean Squared Voltage, in reality it is something more like about 678.9V AC, and considering that most 3-pin wall sockets are capable of supplying anything up to about 15 Amps, this makes it even more lethal.

[Muzza]

I have to correct your last point Doc. 240V rms is, in layman's terms, the 'average' of the AC sine wave. The 'peak' voltage of 240V rms is about 340 volts. (339 point something).

(To clarify, as well as rms value of the sine wave, there is also something called the 'average' value. I used the term 'average' in the above explanation so non electrical people have a better chance of comprehending these concepts.)

Even if you accidentally touch one phase of a 3 phase system, you're still 'only' going to get a 240 volt (rms) wake up call.

If if you're ever unlucky enough (or stoopid) to touch 2 different phases of a 415 volt 3 phase system, you're gonna cop a peak voltage of about 586 volts. I don't know where you got the value of 678 volts from.


Just to explain a little further to those who are still awake and still following this, Australian mains voltage is 240v rms @50hz.
This means the 'average' (sic) voltage is 240 volts, but the actual voltage is like a wave. In one 'cycle' of this wave it starts at zero volts, in 5 milliseconds (mS), it rises to +340volts (ish) 5mS later drops to zero volts again then rises to -340volts (ish) and back to zero. This whole cycle takes 20mS and happens 50 times every second, which is where the 50Hz comes from.
If you get a 240volt 'kickinthepants', you're actually copping 340volts one hundred times each second. The good news is, your also getting zero volts one hundred times every second. (Gives you a break)

[DrNomis_44]

I have to correct your last point Doc. 240V rms is, in layman's terms, the 'average' of the AC sine wave. The 'peak' voltage of 240V rms is about 340 volts. (339 point something).

(To clarify, as well as rms value of the sine wave, there is also something called the 'average' value. I used the term 'average' in the above explanation so non electrical people have a better chance of comprehending these concepts.)

Even if you accidentally touch one phase of a 3 phase system, you're still 'only' going to get a 240 volt wake up call.

If if you're ever unlucky enough (or stoopid) to touch 2 different phases of a 415 volt 3 phase system, you're gonna cop a peak voltage of about 586 volts. I don't know where you got the value of 678 volts from.

Cheers for that, I obviously got my maths wrong so thanks for clarifying things for us.

Here's how I got my value, I started by dividing 240V AC by .707 to get the peak voltage, then multiplied the resulting figure by two to get the peak-to-peak voltage, maybe I took the wrong approach, maths was never my strongest subject at school.

[fender3x]

cool amp Muzza, so you built that from scratch ?

1+ Cool amp!

[DrNomis_44]

I'll see if I can add some more info on Valve Guitar amps in the morning, maybe what we could do to continue the discussion is to have a look at one of the many classic designs of Valve Guitar amps, the Fender Champ, since it is a relatively simple circuit, I'll see if I can do a simple explanation of how the circuit works, together with a hand-drawn schematic and block diagram, I'll try and make it easy enough to understand, might also include some info on it's history too, so stay tuned.

[FrankenWashie]

If you get a 240volt 'kickinthepants', you're actually copping 340volts one hundred times each second. The good news is, your also getting zero volts one hundred times every second. (Gives you a break)

I'm not so sure you're going to benefit greatly from that break Muzz! Seriously though, great stuff from both of you. its all great and valuable information, especially the safety aspects.

[DrNomis_44]

I'm not so sure you're going to benefit greatly from that break Muzz! Seriously though, great stuff from both of you. its all great and valuable information, especially the safety aspects.

Cheers, personally the safety aspects of working on Valve Guitar amps cannot be stressed enough, which is what I was originally getting at, I've had some pretty close calls when working on Valve circuitry in the past, and know from first-hand experience just how dangerous it can be if you're not careful, but as long as you follow certain procedures you can minimize the safety-risks involved.

Whenever I work on any piece of electronic equipment that's mains-powered, I follow this procedure to ensure my personal safety:


1, First thing I do is turn off and completely disconnect the piece of electronic equipment from the 240V AC mains outlet, I usually do this by pulling the 3-Pin mains plug out of the wall socket after switching the wall socket and the piece of equipment off, if it uses an IEC "Kettle-Cord" I completely disconnect that from the equipment I'm working on.

2, I use my Digital Multimeter set to read DC Volts on it's highest DC Volts range to check for any residual voltage in the circuit, I attach the black multimeter probe to a suitable grounding-point in the piece of equipment, and I put my left hand either behind my back or in the back pocket of the shorts or trousers I'm wearing, and I hold the red multimeter probe in my right hand while I probe for voltages, this ensures that I don't get any dangerous shocks going across my chest, if I do accidentally happen to get a shock.

3, If I do get any significantly high voltage readings, I use a resistor of a suitable value and a couple of alligator-clip leads to discharge any power-supply filter caps to ground while monitoring the voltages with my multimeter.

4, Once I am satisfied that there are no dangerous voltages present in the equipment I'm working on, I consider it safe to work on, then I commence working on it.

5, Once I have finished doing the work I Double, and Triple-check that I have done everything correctly before I plug the equipment into the mains wall socket prior to testing the equipment for correct operation.