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Old 1st Jul 2017, 3:06 am   #221
Radio Wrangler
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Default Re: 807 (maybe) amplifier build. Now EL34

One friend is a radio amateur who's built a number of different HF transceivers by picking bits of circuits from all over the place. Building a multiband transceiver is quite an achievement, and he has built a whole series. Some got published, some he put out as kits when he started a small firm with a friend. However, he avoided PLLs like the plague having had problems with them and no successes. His radios used a bank of crystal oscillators (one per band) mixed with the VFO and filtered, as the local oscillator. He consequently ran ito difficulties of getting people with sparse test equipment to align bandpass filters.

I suggested the PLL approach as a lot easier to adjust and could be set up with just a voltmeter. He described his experiences. He'd built PLLs with oscillators from one source, dividers from another, PSD and loop amps from others. He hadn't realised how bespoke the timeconstants and gain of the loop amp needed to be.

So the next time we met (almost 500 miles each way) I took him through the poles and zeroes business and showed him a recipe approach to PLL loop design.We built one. He made a board for a full all-HF-band PLL summing loop and it worked first time. We had a bit of fun trying to get it to misbehave, and proved to ourselves that it not only worked, it was robust.

I went back North, and he wrote a construction article for Radcom. It was seen as a bit heavy for PW. It won him the RSGB Talbot-Wortley trophy for the best technical article that year. A successful circuit and a gong isn't bad, but I think the biggest return for his effort was entry to a further level in the design game. and the slaying of a dragon.

Thevenin...

This site is sprinkled with warnings about high voltage power supplies and their dangers. But what about high currents? Remember those car batteries with big lead connections on the top joining the cells? People have been severely injured by metal watch straps touching across the four volts between adjacent connections. In the worst cases hands have had to be amputated because the damage was so severe. The current welded the strap in place and the rest is gruesome.

Voltage and current are sort of mirror images of each other. Not the same, but with many similarities. Most of our designs see the signal as a voltage which gets processed. It could equally well have been a current. Good understanding of a circuit sees the signal as a voltage in some places and a current in others.

Thevenin's theorem is one of a pair. A mathematician would see them as duals. There is an exact symmetry between the pair. The other theorem is Norton's. With our bias towards thinking of voltage first, Norton's seems much weirder.

Back to dangerous power supplies. Normal flyback TV EHT is seen as dangerous, but mains derived EHT is seen as VERY dangerous. THe difference being that the latter can easily supply fatal levels of current before the voltage falls much.

However, you can easily make much higher voltages stroking the cat on a dry day, or combing your hair. These processes are routinely survived, provided the cat is the domesticated variety. What saves you is the very large source impedance involved. It causes the voltage to plummet when only a tiny current is taken.

Another manifestation of source impedance is in the good old dry battery. As they aged and the active materials inside were used up, the zinc was still zincy and the carbon was still just as carbony, so their off-load voltage stayed up until things had really gone, but before the off-load voltage fell, the effective series resistance went up. Anything that wanted a bit of current met a falling voltage.

Thevenin's theorem says that any real world voltage source can be modelled by an ideal voltage source, along with a series resistance. AS FAR AS THE THINGS AFTER IT CAN SEE.
What Thevenin doesn't do is model what's going on inside the supply. So a Thevenin equivalent circuit will look like the real thing as far as voltage off load and voltage droop as you load it are concerned. But don't think about the efficiency of the supply or the power dropped in the equivalent series resistor - these may not be representative at all.

This is 'Black Box Theory' and we aren't talking PYE record players. Thevenin's perfect voltage generator and resistor live in a black box. What goes on inside is opaque and needs to be to stop the non-realistic power dissipation misleading the viewer in some cases.

So all Thevenin is, is a way of formally describing how some voltage source droops when you take current.

Norton does exactly the same thing, but his black box contains a perfect current source shunted by a resistor. It looks weirder, but it's convenient sometimes.

David
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Old 1st Jul 2017, 5:34 pm   #222
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Default Re: 807 (maybe) amplifier build. Now EL34

This has to be a record for the clearest post on the most complex subject, in the mysterious couple of hours before sunrise in midsummer!
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Old 1st Jul 2017, 5:57 pm   #223
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Default Re: 807 (maybe) amplifier build. Now EL34

Andy,
The designing preamps book is a good one with some easy going theory on feedback and general design. I highly recommend it. You can usually get it on Amazon a lot cheaper than His site.
Some good reading on the site, too.
http://www.valvewizard.co.uk/
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Old 4th Jul 2017, 12:09 pm   #224
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Default Re: 807 (maybe) amplifier build. Now EL34

Thanks David, that clears Thevinin up somewhat. I was actually thinking of Kirchoff when I wrote Thevinin which is another bag of mind bending terms and concepts. I take your point that with application one can get one's head round these Theorem's, I dare I'll do so in the end. I managed to slay the dB dragon this weekend that has been eluding me for some time. I do find this stuff easier to assimilate if I apply it a few times in a practical situation.

Thanks Howard, I'll check that out. I've learned a great deal from The Valve Wizard site, whose content I struggled with at first. After I'd applied it a few times on the bench, most of it sank in.

Back to the amp I swapped over the grids at the weekend which was pointless thinking about it. With a normal LTP with a resistor as the tail you can change which anode has the inverted signal out. With my LTP as set up, with a CCS as the tail and one grid grounded, you can't. The grounded grid triode will always have the non inverted OP, it is this that dictates that PFB will be seen at V1 IP- with the primaries as is.

So, I've done as Howard suggested and swapped the primary over. The amp is now oscillating, which I find quite funny on two counts. Every time I fix one problem another pops up, which results in a wry smile on my part. Secondly I'm trying to fix an oscillator (RF sig gen) that won't oscillate but have an amp that should'nt oscillate, oscillating. Obviously there's some cosmic joker looking over my shoulder laughing at my puny attempt's to bend electrons to my will.

Not to worry, it's good craic and I have an idea why it is mucking me about this time, thanks in part to you all imparting your knowledge.

Andy.
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Old 4th Jul 2017, 10:11 pm   #225
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Default Re: 807 (maybe) amplifier build. Now EL34

There is in fact a very common expression:

If you want an amplifier, make an oscillator.
If you want an oscillator, make an amplifier.



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Old 10th Jul 2017, 2:46 pm   #226
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Default Re: 807 (maybe) amplifier build. Now EL34

After 6 days of fun and games with the amp giving me loads of gyp I put things back as they were apart from the OPT primary being swapped over that is, all is well.
Turns out moving a few grounds and grid resistor ground terminations effect's the stability of the amp considerably.
So amp working like a charm again, stable with NFB applied. Gave it a quick test, and got 0.8% THD at 50w. The amp also now kicks out 162w before onset of clipping, though I don't leave it at that for long as the OPT is rated at 120w.
So I go from the sublime (0.8% THD at 50w for a valve amp) to the ridiculous ( 40v P-P 20khz oscillating hissy fits) all by moving a few grounds 6" or so. Video here - https://youtu.be/S0Qhjzewr-4

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Old 10th Jul 2017, 6:28 pm   #227
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Default Re: 807 (maybe) amplifier build. Now EL34

Seems like a good tie to hit you with Kirchoff!

It's a lot simpler than Thevenin.

Think of a pipe junction, with any number of branches. If you add up all the water flowing into it from all the branches there is a flow that way, then the result must equal the sum of all the outward going flows, or else the pipe is going to start bulging (or else vacuum is going to break out)

Another way of saying it is to count flows into the junction as positive, and flows out as negative (you could do it the other way round, just so long as you are consistent) Then the total flow of all branches added up must be zero.

Kirchoff said the same thing about electricity. The total current into a circuit node has to be zero. You count currents in and out with opposite signs.

In other words the sum of all currents onto the node must equal the sum of all currents off of it.

If this wasn't so the node would reach silly voltages in an instant (if you consider an ideal node to have no capacitance, but if a real wrld node has some, then treat the capacitance as a connection off of the node.

Handy rules tend to go round in pairs like policemen at night. like Thevenin and Norton.

The one that is twinned with Kirchoff is that if you take a palk round a look on any circuit diagram and wind up back where you started, the sum of all the voltage differences you've passed over comes out as zero.

(Think of M C Esher's staircases as an illusion that looks like it breaks this rule.)

David
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Old 11th Jul 2017, 5:03 am   #228
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Default Re: 807 (maybe) amplifier build. Now EL34

Instead of saying "everything flowing in is positive, everything flowing out is negative and they all add up to nothing", I found it easier to take an educated guess on the directions and say that the total current entering (treating everything as positive) is equal to the total current leaving (again treating everything as positive). If anything turns out negative, it just means you guessed the direction wrong in the first place.

The two are mathematically equivalent, of course, but I just found it easier dealing with mostly positive quantities on opposite sides of the = sign.
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Old 11th Jul 2017, 5:34 am   #229
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Default Re: 807 (maybe) amplifier build. Now EL34

These are two ways of visualising Kirchoff's.

"Total flowing in must equal total flowing out" is the easiest to visualise, and is easy to use in simple cases when you can see what the currents are and know which is which way.

"Everything adds up to zero", as Julie says is mathematically equivalent, but it comes into its own when you aren't sure of which way any of the currents is flowing. The signs will keep you right without having to know in advance. This is necessary when you're using algebra to analyse a network.

David
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Old 11th Jul 2017, 6:05 am   #230
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Default Re: 807 (maybe) amplifier build. Now EL34

I'll admitt to being first bemused by Kirchoff then just angry. It's like a zen koan like "what is the sound of one hand clapping?" Or a Chinese saying "all crows under heaven are black" By that I mean my interpretation was either complex or simple IE "what goes in is what comes out!"


So to a real world situation, if I've read you right; if we have two resistors of 10 ohms joined at one end, with another going out, like a Y and we measure 100mV across the two sticky up ones, the top of the Y , the current at their junction or node is -20mA and we then measure 200mV across the bottom 10 ohm we get + 20mA. Therefore (-20) + (+20) = 0 This is Norton IE current, but the same thing yes?


But what the Dicken's does this tell us, are there real world circuits where the answer adds up to 1.5? And how does this apply to my situation? I assumed that the oscillation was caused by a small resistance in the 6" of bus bar and Miller capacitance was forming a tank circuit. Or are you saying there's a bulge in the pipe caused by the disparity of in and out currents causing the oscillation?


Am not sure I'd be able to measure all the current's/voltages present as some are going to be very small, EG across one inch of wire say. If one was to try and tackle this I can see problems. For instance a voltage amplifying valve stage has an input into G1 and an output from the anode, but what about all those inter electrode capacitance's? You can't measure them so have to use datasheet info times by the gain, this will have some degree of error. Same for the valve base pins. If we say oh, let's say 10puff between pins, but we would be guessing. And then there's the capacitance's and resistance between grounds. Which is where we started.

Andy.
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Old 11th Jul 2017, 7:46 am   #231
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Default Re: 807 (maybe) amplifier build. Now EL34

Hi Andy, the DC case for these laws is quite simple, but gets complex when using AC, but is normally easiest resolved by using impedance (Z), not resistance and reactance(R+jX).
In AC systems it is also time varying and is easiest to resolve when in steady state, not under transient conditions.

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Old 11th Jul 2017, 11:36 am   #232
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Default Re: 807 (maybe) amplifier build. Now EL34

Ref post #226.....

Unwanted feedback can occur if an input circuit and an output circuit share a common path from where their grounds join to the actual 0V point in a system.


If you have a common ground impedance of 10milliohms, say, (not at all unlikely) then an output circuit current of 1A will cause a feedback voltage of 10mV to the input circuit. Add in enough gain from input to output and a few unpredictable phase shifts and anything can happen. One of the anythings (in real life the most likely one) is that the circuit now hoots.


One way to avoid this is to return every single ground in the circuit independently to the famous "Star Point". This may be overkill for audio if you can quantify things a bit and just keep sensitive circuits away from power ones, and at higher frequencies stray inductance/capacitance in longish ground connections may be a worse problem than just tying everything down to a ground plane or chassis. You really do need to think about where any ground currents are going on their way back to the PSU.
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Old 13th Jul 2017, 5:29 am   #233
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Default Re: 807 (maybe) amplifier build. Now EL34

Thanks both. Theory aside amp now working like a good un. Frequency response is flat from 15hz to 6khz where it rolls off gently to a 3dB point at 20hz. THD is 0.3% at 50w rising to 0.8% at 112w. I have plenty of gain left; with a 1.3v RMS IP I have 136w before clipping. So can lose a few more DB on NFB, but think I'll stick there for now as any improvement in THD is negligable, unhearable and I'll probably run into instability again.

I'm going to struggle measuring THD lower than this as my Marconi TF2331 is at it's limit. My signal source is also a bit high over 1v.

So, all that's left to do is attach another protection device that will turn off the amp if the OP stage EL34's Ik goes over 100mA. I've breadboarded a circuit, just have to test. I also have to put a clipping indicator LED driver on the same PCB together with a VU driver. am in the process of trying out a few circuits. See att. I tried out the ESP one, but didn't like it. The first attached works well.

A.
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Old 12th Sep 2017, 2:55 pm   #234
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Default Re: 807 (maybe) amplifier build. Now EL34

Bit of an update video here - https://youtu.be/qHmO9qxaRNo I'm still slogging away at it, winding another mains transformer. I've found a cheap source of big toroid's on ebay, think it was 30 for a 700VA, which is a tad bigger than the other one I wound, IE 625VA. I went over the playing field yesterday to measure out the HT winding's, which are 2 x 243.3ft, but ran out of copper wire. I have some on order from Brocott's. Then it's a case of twisting the two together to form one bi-filar wire, bung it on a foot long "bobbin" then wind it on. With that done the other secs are a doodle.

I've also built a VU meter driver. It uses a TL074 opamp, one as precision rectifier, another as a voltage amp and another as buffer. I didn't use the last bit. The only modification I did was add a 100k preset on the IP, as the circuit as is, loaded the OP off the ECC83 too much. I've also had to add an attenuator SW because with an OP off the ECC83 of 6v RMS, this means the meter tops out , IE FSD, at about 35W. this is just a 6K8 R, with a SPST SW.

I also added a peak/clipping indicator, which turns an LED on at 5.9v RMS, glowing brightly at 6v RMS.

Lastly I've built a protection circuit that fires a triac at around 100mA cathode current. Each EL34 has a wire running off the cathode to the protection PCB. This is just a peak detector in essence. It took a lot of experimentation to get it right. As I said at 1v the triac fires, which supplies 12v to a relay, with cuts the negative mains to the mains tfmr. There is also an ultra bright diode on the front panel to indicate a fault.

These circuits are all on one PCB, with the PSU, which is a half wave rectified +/- 15v, stabilised by two 5W zeners.

If anyone's interested, I'll attach the circuits.

Andy.
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Old 12th Sep 2017, 3:59 pm   #235
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Default Re: 807 (maybe) amplifier build. Now EL34

Hi Andy,
I have been following your project with great interest, and although I have no need for such a beast I would be interested to see some more of your peripheral circuits if you would be prepared to post them.

Keep up the good work.
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Old 13th Sep 2017, 6:06 am   #236
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Default Re: 807 (maybe) amplifier build. Now EL34

Aye, it weighs a bl**dy ton, but to be honest it's not that loud. I managed to crank it up to 10 as it were, without killing the guinea pigs who share my workshop and it was grand. I do need to put earplugs in for signwave testing though. This was with my lashed together test speaker, which is probably inefficiant.

I forgot to say I have been using the amp to listen to music and it sounds well. However it's a bit bass heavy, which is easily solved. It's tricky to appreciate any difference though, to other amps, as it's a mono amp at present, so I stuck to old Beatle's stuff, a bit of Donovan before he went all trippy and Charlie Mingus sounds especially good as does Pentangle - double bass heaven.

Lastly for a while, I had to reduce NFB a tad, increasing the NFB R from 4 to 5k. This means NFB is about 1/50th. I'm still not completely au fait with the math's and have no idea how to apply the result to reduce oscillation. I intend to build a simple circuit using an opamp to drive a single ended EL84 in an effort to nail the math's etc at some point soon.

Finally after all the changes THD is 0.04% @ 1/2W, 0.08% @ 12.5W, and 0.18 @ 50 and 112W using 1khz. F response is flat from 50hz to 6khz where it starts to roll off, down by 3dB @ 20khz. Square wave looks good at 1khz with a bit of rounding of the leading edges, but at 10khz it looks more like Grandad's blunt old #3 saw used for cutting breeze blocks. Meh, it'll do.

Thank's for your interest Eddie. Actually, the peripheral circuit's have been the most interesting and fun bit on this amp, I've learned a lot building them. I nicked most of them, but modified them somewhat, spending hours bread boarding circuits, good craic. I'll do a complete schematic when i get a mo and post.

A.
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Old 13th Sep 2017, 6:57 am   #237
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Default Re: 807 (maybe) amplifier build. Now EL34

Most loudspeakers are extremely inefficient. A few are merely very inefficient, and the rest are amazingly inefficient.

Perhaps the easiest illustration of this is that a single trombone going flat out produces around half a watt of acoustic power.

If all the power of your amplifier got turned into sound, we'd have to arrange helicopter drops of food and tents to the survivors in Lincolnshire.

David
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Old 13th Sep 2017, 9:22 am   #238
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Default Re: 807 (maybe) amplifier build. Now EL34

And if we all lived in a little tin flute as does David,
There would be nowt sound at all.
Cmon Andy!!! thumbs up to you!!
Well done!! Tahh rahh.

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Old 13th Sep 2017, 12:24 pm   #239
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Default Re: 807 (maybe) amplifier build. Now EL34

Quote:
Originally Posted by Diabolical Artificer View Post

Lastly for a while, I had to reduce NFB a tad, increasing the NFB R from 4 to 5k. This means NFB is about 1/50th. I'm still not completely au fait with the math's and have no idea how to apply the result to reduce oscillation. I intend to build a simple circuit using an opamp to drive a single ended EL84 in an effort to nail the math's etc at some point soon.

A.


Do plots of the open loop gain and phase from the point where the NFB is applied to the point where the input to the NFB signal is taken from. (Without your circuit I can't say exactly, but that would usually be from the cathode circuit of a front end valve to the loudspeaker output.)


Once you have that, you can fairly easily see where (in the frequency domain) problems will arise if too much simple feedback is applied. It'll be somewhere near where the open loop phase shift reaches 180 degrees.


After that, the fun starts if you want to apply more feedback at lower frequencies and back it off as the phase shift increases. You need to cook up a suitable R-C network in the feedback path to do this.......
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Old 13th Sep 2017, 12:25 pm   #240
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Default Re: 807 (maybe) amplifier build. Now EL34

I used to go to Deep Purple, Who, and ELP concerts, so I need a bit of sound power nowadays

The thing in my living room was soak tested at 200W/channel for a full day back when I built it, and the transmission line speakers go low enough to keep any pipe-organist happy.

David

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