Handbook Amp Project – Part 2 – Mains Electricity and Transformer

DISCLAIMER: The following is NOT to be taken as a definitive procedure for building or repairing ANY electronic device and the author takes NO responsibility for any damage or injury that results from anyone using this guide. It is intended for educational purposes ONLY.

If you have ANY doubt about building or making modifications or repairs to your own equipment then seek advice from relevant qualified persons and/or do your own research first.

Valve amplifiers use and can store high AC and/or DC voltages that can KILL. You have been warned!

Also, I don’t have an expert to proof read my documents, so don’t take everything here as absolute fact, though I try to be as thorough as I can, particularly technical details, as that’s the whole point in understanding something.
However – I WILL make mistakes, although I re-read and check my writing as best as possible.
Do your own research also.
Bear in mind, the Internet is a hot bed of errors in it’s own right, written by people like me mostly, so it is best to check multiple sources of the same info and draw your own conclusions as to what makes logical sense.
You alone, are responsible for your own actions, no one else.
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READ THIS!!!

http://en.wikipedia.org/wiki/Electric_shock

There is a big effect difference between voltage levels that break through the skin conductivity – which happens to be in the 400 to 500V DC range of our valve amp circuits! BE CAREFUL!

Drain the secondary capacitors when the circuit is off and always check the circuit with a multimeter on both AC and DC sides before touching components or working on it – with the mains UNPLUGGED. If the Mains is plugged in, the plug socket and switch contacts are MAINS LIVE still, with the switch in the OFF position.

If you are testing the circuit after a change, make sure all your test leads, probes and croc clips are well insulated at their joints from your fingers, and potential chassis/component contact points (chassis edges can be sharp on leads too). I always connect the croc clips and check Multimeter settings for the reading I wish to take, then double check they are placed correctly for that reading – whether AC or DC – Voltage or Current.

I then plug in the fused mains side lead. I use the mains toggle switch to power the primary – and therefore the secondary windings – with the indicator light connected, so I can SEE power also, as well as checking the on/off position of the switch BEFORE I plug in the mains. I switch OFF and unplug the mains, checking the meter voltage or current level has dropped to 0 (or use a 220k Ohm, 1W drain lead from the capacitors to the chassis ASWELL) before I move any croc clips to a new circuit point for another measurement.

This may seem long-winded to experienced amp techs, but for beginners it helps ensure a safe routine of practice, logical mind set and thought process for safe practice.

For the sake of a few extra seconds, why risk safety by not switching off and draining the circuit before moving things? Anyone can slip or drop something on a live circuit.

A 240V AC shock will certainly help you not make that mistake again if you ever get lazy and forget any of this – if you are lucky enough to get away with it!

My last AC shock left my right arm aching for an hour!

A 500V DC shock may mean you don’t get to worry about any of this ever again…but now you know why from WikiP.

The documented current flow for fatality varies greatly between publications – from as little as 70mA to 200mA – err on the side of safety and DONT get a shock in the first place!

http://hypertextbook.com/facts/2000/JackHsu.shtml

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Mains Electricity

Alternating Current is electricity that flows in one direction in a circuit then reverses its direction and flows in the opposite direction – periodically. This time period is its cycle.

The time period for UK mains is 50Hz or 50 cycles per second, which means the electricity flows alternately in both directions once every 50th of a second – or every 20 milliseconds. (1000ms / 50Hz = 20ms)

This 50Hz frequency is audible to humans and is sometimes heard as a low hum in amps – or the buzzing of a fluorescent light for example. This can be a problem in audio circuitry.

UK mains voltage is 240V RMS (root mean square) and a sine wave because the electricity is produced by a rotating wheel of coils in a magnetic field at a power station, or some similar device – windmill etc.

http://en.wikipedia.org/wiki/Power_station

If you want to know why we use AC mains and not DC, it is mainly for efficiency of power transfer over long distance, and was born from the War of the Currents historically:

http://en.wikipedia.org/wiki/War_of_Currents

There is an intrinsic relationship between the circle (rotation) and a sine wave over time.

http://en.wikipedia.org/wiki/Sine_wave

400px-ComplexSinInATimeAxe.gif

 

This means that RMS AC actually has a peak voltage level to its sinusoidal waveform that is higher than the RMS value by about 1.414 times (square root of 2), which means the 240V UK voltage peaks at about 339.5V but for a short period of time of the cycle. RMS values of AC voltage, current (and so power), are used as they are a statistically accurate equivalent to the same DC voltage and current values for electrical work, or energy, expended in a given circuit.

http://en.wikipedia.org/wiki/Root_mean_square

For a derivation of the maths proof of average AC power compared to a DC circuit, see here:

http://www.doctronics.co.uk/rms.htm

A sine wave is the simplest mathematical AC waveform and “purest tone”, so a building block for all others. It looks like one of these:


Mains Electricity Transformer

The first main component of interest in an amp circuit is the mains transformer which has the primary side connected to the mains Live and Neutral wires, via the appropriate fuses (one in the plug to protect the lead (usually a too high value of 13A!), and one in the IEC socket for the equipment). The mains Earth wire is connected to the metal chassis as a safety feature to prevent a shock hazard by tripping a household circuit breaker and/or blowing a fuse, should either the neutral or live wire connect to the chassis due to breakage or other fault condition – such as a human shorting it to Earth!
An individual’s electrical resistance can vary a lot depending on skin condition – sweat etc. – from 1000 to 100,000 Ohms.

http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance

From Ohm’s Law, you can see the 100 times difference in current flow these numbers would make between getting a real “shocker” that wakes you up pronto, or being electrocuted depending on circumstance and what part of the body the current flows through, and how much, such as across the heart. This is why electricians put the left hand in the back pocket when testing a circuit with a probe, so if they slip and finger touch, any current will go down the right arm to the feet (hopefully), then Earth, and not across the chest via the left arm. (Let’s hope their head isn’t touching a cable tray either…)

Every premises should have an RCD box, these days, not just an old fuse wire box. Test it regularly with its trip switch. It should turn off all electric circuits in the house, or you can have individual circuit breakers for circuits that feed things like just the shower or cooker. These should have individual switches so you don’t have to turn off the whole house to do work etc.

RCDs prevent death or injury mostly, by acting sufficiently quickly (<30ms) on an imbalance of current across either the Live or Neutral line to each other, or Earth.

If you have ever seen someone cut through a live cable with a pair of pliers, you may have heard a big bang and seen a big flash, as the current melts two holes in the pliers before the fuse wire blew.

(The stunned look on the face of the person who did it is amusing too after – assuming they didn’t get hurt from having non insulated handle pliers! I have seen this insulation split with the instantaneous expansion of the pliers due to the heat!).

  • A classic actually – an ex BT manager, not a tech, I worked with for a time back in 1996, did it once when we were stripping old phone cable from a building. Just as I saw him, and the words were coming out my mouth – “NO! Not that one…….Bang! Hahaha…It knocked out the power to the rest of the floor where some people were still working, but no one minded…It was a BT building after all…Geoff looked a bit white for a while after.

RCD’s should prevent that “bang” happening, as they should stop the voltage – therefore the current – rising to values high enough to melt metal in the first place (13 Amps for example).

http://en.wikipedia.org/wiki/Residual-current_device

Schematically, a transformer may be shown similar to:


Or for the Ampmaker.com PP18 circuit:


Note the possible differences in fuse values – 250mA to 1000mA, and secondary coil voltages – 230V to 275V AC depending on amp design.
The secondary coil is usually a Step Up type for use in valve amps, as higher voltage is usually required so there is a higher output voltage swing potential for the power tube section that connects to the output transformer that feeds the speakers.
The mains transformer for this Project is shown here – NOTE the often overlooked and taken for granted, 50Hz stated usage frequency – more on the importance of that later:

http://www.ampmaker.com/store/0-190-275V-20W-power-transformer.html


This transformer’s primary side is made up of two 120V sections that have to be solder linked together for 240V operation -120V for Europe or US.


Above – Mains voltage reading 242V.
The secondary side is split into 3 sections of 2 windings, for 190V or 275V options; and the lower voltage, higher current (3 amp) section for the 6.3V AC valve heaters.


Above – secondary 275V winding reading 294V unloaded.

Above – sec 190V winding reading 201V unloaded. Note the insulation tape to cover metal were the 4mm banana type test leads join, and a sleeve on the black croc clip.
The red probe is a spring loaded hook type so grips on.
As you see, this voltage can vary between actual transformers stated values due to windings and iron former characteristics. This voltage will drop under load also. 10% difference in measured voltages was an acceptable norm for the Ashton build. This would give a 30V swing on a 300V reading so a loaded value from around 270V upwards probably (the voltage would not increase to 330V under load obviously!).
Any more or less than 10% at this point A+ in the circuit (the 220k resistor) may indicate an error in the later amp build or a component fault.
Using the 220k drain lead as a load at this point does not show any drop in voltage across the secondary as the current draw (1mA) is negligible. I don’t expect to see anything here until the four 1N4007 rectifier diodes are added, that should have a usual 0.6V silicon NP junction bias forward voltage across each one. That would be 2 diode voltage drops of about 1.2V for each positive and negative half wave cycle. Even then, probably not noticeable.
I’ll look at that in the next Post – the Rectification section.
A Multimeter can be put in series with the primary and/or secondary to measure the current drawn by the two sections to try to understand what suitable size fuse to put in the primary circuit. When under no secondary load, the primary current flowing is small and known as the magnetizing or exciting current:

http://www.butlerwinding.com/store.asp?pid=28350


Above – Primary current (12mA) with no secondary load.
When the secondary is under full load this primary current will be much higher also – remember the basic theory, for a 100% efficient transformer, power in = power out. So VI.primary = VI.secondary.
The PP18 schematic states a primary 1A fuse as suitable, but the Ashton requires only a 250mA primary fuse, partly as it runs at a lower secondary voltage (230V) also anyway – but this is still 20 times higher than the 12mA, no load, magnetizing current I am reading here. I am getting an idea of the difference in current draw between load and no load conditions. The PP18 circuit may be near 4 times higher again at 1000mA maximum primary current under a full secondary load.
OK, back to that 50Hz usage on the label.
As a transformer has Inductance:

http://en.wikipedia.org/wiki/Inductance

and so, Impedance:
http://en.wikipedia.org/wiki/Electrical_impedance

to AC, the current draw cannot be determined even remotely accurately by just measuring the DC resistance of the primary coils (16 + 16 Ohms) of a mains rated transformer, like you might do for a really rough idea of speaker coil impedance (whether closer to 4 Ohms than 8 Ohms maybe), as this gives a reading which is meaninglessly high for the mains transformer potential current draw:
From Ohms Law: V = IR
240V/32 Ohms = 7.5 Amps – this would probably burn out the coils if correct, or 240V DC applied! – so note the massive difference in the behaviour of AC and DC voltages for the same DC resistance when it is turned into a coil and/or wrapped round a magnet.
The real tell for this principle is if you measure the DC resistance of an Output Transformer’s 4, 8, or 16 Ohm windings, as they may show as a Meter short, yet they have a definite AC “resistance” or Impedance (Z), probably measured at the usual industry test frequency of 1kHz. This Impedance increases for higher frequency and decreases for lower. This is partly due to the Skin Effect:

http://en.wikipedia.org/wiki/Skin_effect

(as well as the faster changing magnetic fields induced in the transformer) caused by Eddy Currents in the wire, or the way electrons crowd toward the surface of a wire more, (so not travelling down the centre as well), the higher the frequencies become. This effectively means the cross sectional area of the wire being decreased, so less current path, so overall resistance effectively increases also.
Another point of interest to be aware of is that the phase of a transformer is determined by the direction the secondary wire is wound around the core. It can only be the same as the primary or 180 degrees out. This has no bearing on the DC side as this is determined by the positioning or direction you set the rectifier diodes as to which diode points becomes your circuit DC positive and negative. You can check this phase on a dual beam scope (or a DDM via the web links below), using a 10 X voltage probe for Mains voltages (my scope only measures 20V max on screen, though the maximum safe input is 500V DC + peak AC) if you can’t find the dot notation phase markers on the transformer (see web links).
NB: This is an important concept for the output transformer section if using Negative Feedback link from the OT (like the Maggie amp), as you may damage a tube if you connect to the wrong side of the transformer and turn on. You may get squeal or high pitched sound/resonance from the valve but the damage may already be done. See the Power Section Post when it’s done.

Other important Mains usage points

Also in this Mains section – just to mention some other important points re general valve amp usage and builds: –
There are two main types of pilot lights available for On and/or Standby switch indication – make sure you put the right bulb types across the right part of the circuit:

(1) 6.3V AC coil bulbs or 5V DC LED types (used with diode D103 in the Champ 12 circuit below)



to go across the secondary heater windings;

http://www.ampmaker.com/store/Bulb-for-indicator.html

(2) 220V AC neon types that can go across the primary or secondary.

http://www.ampmaker.com/store/Neon-indicator.html

This neon light can also be used across a rectifier bridge (I just tested it across the 4 rectifier diodes on the 275V winding = 388V unregulated DC and it works (whether this shortens life in the long run I don’t know):

275VNeon
http://en.wikipedia.org/wiki/Neon_lamp

It therefore goes across the standby switch board side positive terminal and the board side negative:

Now you know that when the On switch turns on the mains current to the primary it also allows the secondary heater coils to warm the valve plates before use, then the Standby turns on the power from the High Tension secondary to the circuit board via the rectifier diodes, to power the DC sections of the whole amp. Allow some time (1 minute or so) for the valve plates to warm up and emit electrons before playing, so the valve is less stressed. This time period to warm up is the silence you may have heard up until the mains hum starts coming through the speakers on amps with no Standby switch (e.g. my Marshall, Vox and Champ).

Correct use of the Standby switch can prolong tube life and prevent overheating while keeping the amp “ready to go” once warm – during a live set break for example. Turning amps on and off frequently with the mains switch is not good practice because of thermal shock to the valves from rapid heating and cooling, and current surges. Try not to roughly move or jar an amp when turned on if possible, as the valve filaments (like a bulb) will break easily – wait until it has cooled down before moving it.

4/1/14 – Standby switch Mythbust from Blencowe:
http://www.valvewizard.co.uk/standby.html
“In a properly designed amp, a standby switch is nothing more than an expensive, oversized mute switch.”

Having said all that, it still amazes me how robust this technology is. I got a 1957 Murphy radio in a boot sale, with the mains lead cut off, and this thing was really tidy inside the chassis – a bit dusty is all – but some of the valves looked black and dead. I did the right thing and checked over a few of the components to get a general idea of condition, most of all checking a low resistance chassis Earth connection was intact, so if there WAS a problem, the RCDs would trip.

I half mains tested it using a 60W light bulb in series with it for initial turn on, so only about half mains voltage went to it at first. This limits damage in the case of a problem (any wax and old paper type capacitors exploding and making a mess etc.). It showed that the heater filaments for each valve were working too. Happy that it wasn’t smoking anywhere, with a healthy hum from the speakers, I then tried it at full mains.

It worked first time. The only thing wrong with it is was the 2 station dial, 6.3V bulbs were dark, with 1 blown! Amazing – it’s 7 years older than me!

You may be able to get replacement 6.3V, 100mA bulbs in cycle shops, as they were used for rear lights apparently, before LEDs…

Further Reading

http://mysite.du.edu/~jcalvert/tech/transfor.htm

http://www.butlerwinding.com/store.asp?pid=28350

http://www.allaboutcircuits.com/videos/39.html