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Mucking about with valves.

 
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Mucking about with valves.

Post by Dr Wobble » Thu Feb 05, 2015 6:39 am

Reading books and threads ont forum is ok as a way to learn about electronics but you cant beat the hands on approach. I don't do enough experimenting prefering to work from a schematic. This was brought home to me the other day when I was trying to find the ratio of unknown OPT's. after a while I started mucking about cranking the frequency up on the sig gen and noticed that as the frequency went higher, the amplitude decreased. Impedance, reactance and what have you started to make sense, which is nice. Anyhoo...

I was ruminating about this last night and think I know the answer but wanted to be sure. You don't run a valve amp without a load be it speaker or dummy so what about a pre OP stage, tone "stack" etc? Does the anode resistor count as the the load? I reckon it does but before I go mucking about with valves I wanted to consult the men with beards - the oracles in the brown coats. Any other knowledge nuggets from the wise sages of the forum would be, as always, treasured by this neophyte.

Andy.

 
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Re: Mucking about with valves.

Post by Kalee20 » Thu Feb 05, 2015 7:32 am

The amplitudes will change as you alter the frequency, yes, but for an output transformer they should stay in the same ratio nevertheless. Unless, you go to silly frequencies (below 50Hz or above 20kHz).

Running an amplifier without a load can cause a problem if it has a transformer-coupling, because voltages can get high due to inductive effects. Otherwise, no problem. So a pre-amplifier with resistive output won't mind at all. And small-signal amplifiers will be OK too as energy levels are less, and voltage swings will usually be controlled by design (either by inherent ir deliberately-added damping resistance).

 
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Re: Mucking about with valves.

Post by Valvebloke » Thu Feb 05, 2015 9:43 am

I agree. The basic physics, if you like, is that resistors don't store any energy so when you use them as loads they don't have any way of causing damage to other components. But inductors do store energy and that energy is what causes the trouble. When an output transformer has a resistive load on the secondary its primary 'looks' (largely) like a resistor, to AC at least. But when there's no load then it 'looks' like an inductor.

Strictly speaking as long as the stored energy doesn't change (which means the current doesn't change) no harm should be done. So if you can be certain that no signal (music) and no significant noise (e.g. clicks and pops) will go through the amp and also that it won't be turned on or off while you're doing it then it's OK to disconnect the load, say to swap speakers over. It's also the case that if there's negative feedback taken from the secondary then that will help protect the output circuitry in the absence of a load (I recall seeing a claim by Quad that the Quad II could be run safely without a load because of the comprehensive way the feedback was applied). But given that both these scenarios are risky and that output components (valves and transformers in particular) are the most expensive I'd not recommend taking these risks unless you have to.

VB

 
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Re: Mucking about with valves.

Post by Cathovisor » Thu Feb 05, 2015 11:40 am

Dr Wobble wrote:I reckon it does but before I go mucking about with valves I wanted to consult the men with beards - the oracles in the brown coats.

Ahem - in the BBC of old, the engineers wore white coats. Brown coats were strictly for the lower orders... :qq1

(When I joined, you could still draw a white lab coat from Technical Stores. Every now and then I'd don my horn-rimmed NHS glasses and wear it.)

 
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Re: Mucking about with valves.

Post by Dr Wobble » Sun Feb 08, 2015 8:32 am

Thanks for the replies all, very well explained VB. That said what about testing a EF86 say with a de cup cap off its anode ? Any problems there? Would it be ok to test an OP PP stage with a suitable resistor or lightbulb * ? How do forum members muck about with valves ? Not as if a breadboard will do, although Miguel built a HT version.

What do you use when you want a big load in a test setup either for testing an unknown tfmr or OP stage of a transmitter. Showing my ignorance here re transmitters. EG you have a transmitter say 1kw, A) does it need a load, B) if so surely an arial has little impedance or does the atmosphere act as a load? I have a vision of the ionosphere acting like a big resistor. Ehm, too many tabs on my cornflakes in my youth.

Andy.

 
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Re: Mucking about with valves.

Post by Terrykc » Mon Feb 09, 2015 4:44 pm

Dr Wobble wrote:... What do you use when you want a big load in a test setup either for testing an unknown tfmr or OP stage of a transmitter. Showing my ignorance here re transmitters. EG you have a transmitter say 1kw, A) does it need a load, B) if so surely an arial has little impedance or does the atmosphere act as a load? I have a vision of the ionosphere acting like a big resistor ...

A) Yes!

B) Ideally it should have a 'lot' of impedance, meaning a low impedance, rather than a little (by which I assume you mean a high impedance).

If you take a half wave dipole, the conventional feed point at the centre is low impedance. The same is true of ¼λ end fed against a ground plane.

Other aerial types can have wildly varying impedances but the ideal is a low impedance whether by means of a matching device of some kind or simply from physics, as is the case of the dipole.

The reason is quite simple. Let's take your 1kW transmitter. If you assume the load impedance is 50Ω - the typical impedance used for transmission, similar to the 75Ω impedance used for reception - the current flow in the feeder will be 1000/50 = 20A RMS and the voltage will be 1000/20 = 50V RMS. So you will need a hefty lump of coax with a nice thick centre conductor.

Let's say you had an aerial with a feed impedance of 2kΩ. Current is now only 1000/2000 = 500mA RMS but the voltage has risen to 1000/0.5 - a whopping 2kV RMS! What's more, this is equivalent to 5,656V peak-to-peak! So now you need to find a length of coax which is safe up to, say, 10kV, to allow for the increases that will occur whilst the tuning is being adjusted for an optimum match.

So now you want a 50Ω load that will dissipate 1kW. Such devices do exist - obviously - but rather that massive versions of the wire-wound types that you are familiar with, you will probably need an oil-filled, air cooled monster resistor like this:

Image
(RF) 8251T-230. Bird dummy load coaxial resistor. 1000 watts, 50 ohm. 1-5/8" "EIA" QC connector. Oil filled / air cooled. Includes 230º thermal switch.

 
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Re: Mucking about with valves.

Post by Valvebloke » Mon Feb 09, 2015 7:51 pm

Dr Wobble wrote:... what about testing a EF86 say with a de cup cap off its anode ? Any problems there?


The problems testing small signal valves tend not to be the destruction of components by energy which has nowhere to go (as long as you don't carelessly involve the power supply). Instead the issues are at the other extreme. The circuitry is sometimes so low-power that the test gear can load it too heavily. An EF86 stage, for example, can have an output impedance of 100k or more. My HP8903A audio test set has an input impedance which is only 100k. So if I hang the HP8903A as a load onto the EF86 as a source I'll make a potential divider with the two impedances and I'll measure a much smaller signal level than would normally be there.

Would it be ok to test an OP PP stage with a suitable resistor or lightbulb * ?


Suitable resistors are what most people use. But remember that a speaker doesn't look electrically much like a resistor. Its impedance will have a resistive part which can very by a factor of five to ten from the peaks, at the mechanical resonant frequencies of the individual drivers, to the troughs at frequencies in between. The inductive part of the impedance can become dominant at very high frequencies. The capacitative part, especially if the cables are very long, can be sufficient to turn a (badly designed) amp which is nice and stable when it feeds a resistor into a screeching high-frequency oscillator when you hook the speaker up. Or, aggravatingly, the other way round (I could name a name).

Lightbulbs have the problem that their resistance is much higher when hot than when cold. So the answers you get depend on the power you're running at (not good unless you can correct for it). They also blow open-circuit under quite small overloads. Then your load has gone up in a puff of smoke, followed shortly thereafter by your amp (see above). I've used them when I've had to. But not for anything expensive or delicate.

What do you use when you want a big load in a test setup ?


I've got an array of 25W wirewound resistors which hang in a plastic box full of water. They can be plugged up in different ways to give resistances between about 2 ohms and 20+ ohms and they will take a couple of hundred watts or so for a little while.

How do forum members muck about with valves ?


Mucking about is fine for a bit of fun. But when it comes to test and measurement (T&M) then 'mucking about' techniques tend to give 'muck quality' results which can be a bit worthless to be honest. It's easy to waste a lot of time learning a lot of stuff which isn't true. Proper T&M blokes tend towards finicky perfectionism. They wear clean lab coats with biros in the pocket. They make careful notes in neat lab books while they're working (very handy for the coroner if anything should go unexpectedly awry). They may well spend ten times as long building a test jig as they do actually making the measurement, and it'll only have been useful for that one measurement. This approach can seem incredibly tedious. But if it's in your mindset then it can also be incredibly satisfying. And in the end this is what you have to do if you want complex kit to work really well first time.

As a footnote I should say that if you are a genius then you can get remarkably good T&M results cheaply and quickly (and sometimes VERY spectacularly) using test kit knocked up out of bits and pieces found lying around the place. I was lucky enough to meet and to work briefly with Charlie Martin http://www.independent.co.uk/arts-enter ... 5802.html#. Charlie could knock up a serious 2 million volt pulser in a few days and test it using a valve oscilloscope with a Polaroid camera snapping the trace images while the whole scope was floated up at the 2 million volts. The obvious problems were managed (for want of a better word) by hanging the scope from the ceiling of the building on nylon ropes and coiling up its very long mains lead to make an isolating inductor. But Charlie was a genius.

EDIT: It's worth saying that for a scientist he could also be b!00dy funny. See here for example http://link.springer.com/chapter/10.100 ... _25#page-1 or here http://link.springer.com/chapter/10.100 ... _11#page-1

VB

 
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Re: Mucking about with valves.

Post by Herald1360 » Tue Feb 10, 2015 12:42 am

Terrykc wrote:The reason is quite simple. Let's take your 1kW transmitter. If you assume the load impedance is 50Ω - the typical impedance used for transmission, similar to the 75Ω impedance used for reception - the current flow in the feeder will be 1000/50 = 20A RMS and the voltage will be 1000/20 = 50V RMS. So you will need a hefty lump of coax with a nice thick centre conductor.

Have to disagree a bit, here..... Volts for 1kW in 50R is 223Vrms and current is 4.47Arms.

Let's say you had an aerial with a feed impedance of 2kΩ. Current is now only 1000/2000 = 500mA RMS but the voltage has risen to 1000/0.5 - a whopping 2kV RMS! What's more, this is equivalent to 5,656V peak-to-peak! So now you need to find a length of coax which is safe up to, say, 10kV, to allow for the increases that will occur whilst the tuning is being adjusted for an optimum match.

For 2k load, 1kW volts is 1414kVrms and amps is 707mArms.

So now you want a 50Ω load that will dissipate 1kW. Such devices do exist - obviously - but rather that massive versions of the wire-wound types that you are familiar with, you will probably need an oil-filled, air cooled monster resistor like this:

Image
(RF) 8251T-230. Bird dummy load coaxial resistor. 1000 watts, 50 ohm. 1-5/8" "EIA" QC connector. Oil filled / air cooled. Includes 230º thermal switch.


That load has a big connector. The same style also comes (or at any rate came 30 odd years ago) with N or C connectors which are considerably smaller! Size wise, not surprisingly, it's of the same order as a 1kW oilfilled radiator, 'cos that's basically what it is with some attention to the resistor and its tank to get a good VSWR up to UHF.

 
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Re: Mucking about with valves.

Post by Terrykc » Wed Feb 11, 2015 11:24 am

Herald1360 wrote:
Terrykc wrote:... If you assume the load impedance is 50Ω ... the current flow in the feeder will be 1000/50 = 20A RMS and the voltage will be 1000/20 = 50V RMS ...

Have to disagree a bit, here..... Volts for 1kW in 50R is 223Vrms and current is 4.47Arms.

Image

Herald1360 wrote:
Terrykc wrote:Let's say you had an aerial with a feed impedance of 2kΩ. Current is now only 1000/2000 = 500mA RMS but the voltage has risen to 1000/0.5 - a whopping 2kV RMS!


For 2k load, 1kW volts is 1414kVrms and amps is 707mArms.

Image again!

Well, I can see what I did wrong there but can't work out why!

Having checked the time of the original post, I can't blame it on a visit to the pub, either!

 
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Re: Mucking about with valves.

Post by Herald1360 » Thu Feb 12, 2015 12:57 am

You've divided 1000watts by 50ohms to get amps but that sum actually gives you amps^2. The actual current is therefore sqrt20 or 4.47. Dividing watts by amps to get volts is fine, but only if you have the right amps to start with.

Same problem with second 2k resistance calcs.

W = V*I = (V^2)/R = (I^2)*R

3:44pm..... about the right time for a quick email after a liquid lunch :cca

 
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Re: Mucking about with valves.

Post by Terrykc » Thu Feb 12, 2015 11:33 am

Herald1360 wrote:You've divided 1000watts by 50ohms to get amps but that sum actually gives you amps^2. The actual current is therefore sqrt20 or 4.47. Dividing watts by amps to get volts is fine, but only if you have the right amps to start with.

Yes, I was well aware of that! As I said ...

Terrykc wrote:Well, I can see what I did wrong there ...

... but I've no idea how I came to make such a stupid mistake!

Terrykc wrote:... but can't work out why!

Having checked the time of the original post, I can't blame it on a visit to the pub, either!

Herald1360 wrote:3:44pm..... about the right time for a quick email after a liquid lunch :cca

Once upon a time, possibly (probably?) but it is very rare to find me in a pub so early these days ...

 
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Re: Mucking about with valves.

Post by Dr Wobble » Thu Feb 12, 2015 4:51 pm

Thanks for the replies. I still dont get impedance though, try as I might. How can a Z of 10k be less than 10meg ? It doesnt sound right. How can a DMM have less of a load with a higher impedance.

Re mucking about with electrons, today I was testing a PSU. I have mains AC coming in through an IEC socket, via a switch( both neg and poss , RF supprssor cap across mains, then a fuse. I have the PSU in a metal box which is properly earthed and I do mean properly.
Anyhoo, I powered up a little PSU - usual bridge rectifier, big cap then took a reading. Having finished I switched off. I tried to diischarge the cap for safety, its at 300v DC, but it wouldn't diischarge. Checked the earth, checked the mains socket earth- all ok. Odd.

I'm using a toroid transformer which is has an earth to the metal bolt through its centre and to some of its centre taps. The winding I was using has no centre tap though.. Should I have connected the negative of the PSU to earth? This is the only thing I can think of. Probably get shouted at here, but best to ask.

Andy.

 
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Re: Mucking about with valves.

Post by Refugee » Thu Feb 12, 2015 5:16 pm

The capacitor will take about 10 minutes to discharge with just a DMM connected.
A 100K resistor will discharge it 100 times more quickly as it will suck 100 times as much power from the capacitor.

 
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Re: Mucking about with valves.

Post by Refugee » Thu Feb 12, 2015 6:13 pm

A guess based on the fact that the loading of a DMM made little difference.
The currant or power are much of a muchness with this basic experiment.

 
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Re: Mucking about with valves.

Post by Dr Wobble » Thu Feb 12, 2015 7:30 pm

The DMM had nothing to do with it. I tried to discharge a 150u cap using a 60 ohm resistor on a length of wire one end clipped to earth, this usually discharges the cap instantly, not this time. I,ve done this lot's of times with a similar set up. As I said, odd. Works fine with the negative terminal of the cap shorted the the positive.

Heres a bad schematic to show what I'm going on about.Thats a crude resistor and earth symbol BTW.

Andy.
Attachments
Discharge 5.JPG

 
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Re: Mucking about with valves.

Post by Terrykc » Thu Feb 12, 2015 7:59 pm

Could this be the missing link, do you think ...?

Missing_Link.jpg

 
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Re: Mucking about with valves.

Post by Herald1360 » Fri Feb 13, 2015 1:42 am

Dr Wobble wrote:Thanks for the replies. I still dont get impedance though, try as I might. How can a Z of 10k be less than 10meg ? It doesnt sound right. How can a DMM have less of a load with a higher impedance?

Andy.


Impedance is a measure of how hard it is for a potential difference (measured in volts) to push current (measured in amps) through a load (measured in ohms).

In common language, load also means the power dissipated in the component concerned. So there's the apparent contradiction that a higher load (greater power) is taken from a source by a lower load resistance. :bbd

10k (10000) is less than 1meg (1000000) so it is a lower impedance (Z), but the DMM with a higher impedance than a moving coil meter will load the circuit it is connected to, less.

Sometimes it's all a bit Alice in Wonderland, after all in English flammable and inflammable (two apparent opposites) mean pretty much the same as each other. You just have to "know".......

Fun, isn't it?

 
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Re: Mucking about with valves.

Post by Dr Wobble » Fri Feb 13, 2015 8:10 am

I'm pretty clear on the difference between current and power, power = V x I , IE W. However, RMS and what have you I'm a bit shaky on. See below*

Thanks all. I see what your saying about the cap discharging, IE no circuit. I'd kinda got it in my head that a charge of electrons flowed to ground regardless. I think I got mixed up with mains AC, which AFAIK has the neutral referenced to earth, so a circuit.

Thanks Chris,fun indeed! I sort of understood Z a little better when I read your post. now I'm not so sure. I will keep re reading it in order to hammer it home. Can you explain please why a DMM with a Z of 10M loads a circuit less than an analogue meter?

*This is the problem I find with learning about electronics from the internet , through reading and a little experience; IE no one directly to ask in the same room. I know a lot more now than I did five years ago but I have a few holes in my knowledge, with some things only half learned and a few subjects like impedance and dB's make my head hurt no matter how hard I try to understand. I'll get there..... eventually. I really appreciate you all helping.

Andy.

PS Terry, how did you do the graphics ont schematic.? I have S plan,which is easy to use, but as a demo it wont export. What program do you use.? Groan, more things to learn.

 
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Re: Mucking about with valves.

Post by niall » Fri Feb 13, 2015 11:45 am

Maybe we need to clear up the terminology a bit.

Impedance is the AC equivalent of resistance. If a load is purely resistive, i.e. has negligible capacitance or inductance, it will have the same impedance and resistance, like a well designed dummy load.

Where a load has capacitance, inductance or both, it will have a different impedance ("AC resistance") at different frequencies.
There are simple formulae for working out the impedance of a capacitor or inductor at any given frequency.
Capacitor impedance reduces (resistance falls) as frequency increases. Inductor impedance increases with higher frequency.

When a circuit has both inductance and capacitance it will have a resonance point at a particular frequency. Depending on the circuit configuration this will be either an impedance "hump" or a "dip".

Aerials appear as either inductive or capacitive depending on their effective length relative to the measurement frequency, thus they have a resonant frequency.

Voltmeters have an apparent resistance across the terminals so connecting a meter to a circuit is the same as connecting a resistor of that value across the measurement points. How that resistor compares to the impedance of the circuit at the measurement points defines how much effect it has on the circuit conditions. DMMs have a much higher resistance than analogue meters so the effect is generally much less.
A complication here is that service manual measurements may have been taken with an analogue meter with a resistance which does significantly affect the circuit, and the voltages given in the manual are those you would expect to see with the specified meter in circuit. Using a meter with a different resistance will give different answers in this case.

 
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Re: Mucking about with valves.

Post by Kalee20 » Fri Feb 13, 2015 2:36 pm

Herald1360 wrote:
Dr Wobble wrote:Thanks for the replies. I still dont get impedance though, try as I might. How can a Z of 10k be less than 10meg ? It doesnt sound right. How can a DMM have less of a load with a higher impedance?

Andy.


Impedance is a measure of how hard it is for a potential difference (measured in volts) to push current (measured in amps) through a load (measured in ohms).

In common language, load also means the power dissipated in the component concerned. So there's the apparent contradiction that a higher load (greater power) is taken from a source by a lower load resistance. :bbd

10k (10000) is less than 1meg (1000000) so it is a lower impedance (Z), but the DMM with a higher impedance than a moving coil meter will load the circuit it is connected to, less..


That's put it all rather well. Lower resistance or impedance means current is impeded less, so you get more current - generally more power - greater loading.

It doesn't always work like that, unfortunately. Sometimes you really want a particular current, and adjust the voltage to make it right. Extra resistance or impedance means you have to use a higher voltage to get your chosen current, in which case the higher the resistance, the greater the power; the greater the load.

It all shows that current, voltage, power, resistance have their own identities.

Herald1360 wrote:Sometimes it's all a bit Alice in Wonderland, after all in English flammable and inflammable (two apparent opposites) mean pretty much the same as each other.


You could say that they are apparently very similar, though. Flammable derives from flame; inflammable derives from inflame ie to make very flamey. So flammable = it burns; inflammable = it burns easily. The opposite is non-flammable.

 
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Re: Mucking about with valves.

Post by Cathovisor » Fri Feb 13, 2015 3:26 pm

Kalee20 wrote:You could say that they are apparently very similar, though. Flammable derives from flame; inflammable derives from inflame ie to make very flamey. So flammable = it burns; inflammable = it burns easily. The opposite is non-flammable.

Yet in- is frequently used a negation to an adjective, e.g. insensitive, inadequate, incoherent, inexplicable... ah, the joy of the mongrel language that is English!

From the New SOED (1996 ed.)

With reference to fire freq. repl. in official use by flammable, to avoid possible misunderstanding as 'not flammable’

 
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Re: Mucking about with valves.

Post by Herald1360 » Sat Feb 14, 2015 12:37 am

Dr Wobble wrote:
PS Terry, how did you do the graphics ont schematic.? I have S plan,which is easy to use, but as a demo it wont export. What program do you use.? Groan, more things to learn.


Don't know about what Terry did, but a simple trick I use for modding other people's posted schematics is to display the picture at a reasonable size on screen, do a print screen (alt+prt scrn in Windows) then paste into Paint. After that you can do what you like with it in Paint, save it as a jpg and post it again.

 
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Re: Mucking about with valves.

Post by Dr Wobble » Sat Feb 14, 2015 6:37 am

Posted yesterday- disappeared. Thanks, I,ve got impedance now, or at least that bit. Still it bit hazy on impedance relation between stages/circuits though. Still the fog has lifted. Thanks for your patience and help.

Re graphics, I know how to print screen, put it into paint but that's where I'm b*ggered. For instance how do you include text and draw groovy arrows like Terry? My shapes never stay still and have a mind of their own. Drawing two circles the same diameter for instance elludes me.

Cheers, Andy

 
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Re: Mucking about with valves.

Post by Terrykc » Sat Feb 14, 2015 3:38 pm

Herald1360 wrote:
Dr Wobble wrote:
PS Terry, how did you do the graphics ont schematic.? I have S plan,which is easy to use, but as a demo it wont export. What program do you use.? Groan, more things to learn.


Don't know about what Terry did, but a simple trick I use for modding other people's posted schematics is to display the picture at a reasonable size on screen, do a print screen (alt+prt scrn in Windows) then paste into Paint. After that you can do what you like with it in Paint, save it as a jpg and post it again.

That is exactly what I used to do when I was using Xp but W7 now includes a Snipping Tool which is even easier as you select exactly which part of the screen you want to copy.

I still use Paint to edit or draw my own schematics, though.

As I've said before, I always save my work using .png format for lossless compression unless the original has a lot of jpeg artefacts - my own originals don't, of course!

 
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Re: Mucking about with valves.

Post by Terrykc » Sat Feb 14, 2015 4:14 pm

Dr Wobble wrote:Re graphics, I know how to print screen, put it into paint but that's where I'm b*ggered. For instance how do you include text and draw groovy arrows like Terry? My shapes never stay still and have a mind of their own. Drawing two circles the same diameter for instance elludes me ...

Perhaps you are selecting your shape and then dragging it around? Press <Escape> to deselect it!

For a Text box, click the A in the Toolbar ...

Arrows and identical circles ...?
Paint-help.png
Arrows and drawing identical circles ...

Does that help?

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