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Old 9th Jun 2017, 9:00 am   #201
astral highway
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Default Re: 807 (maybe) amplifier build. Now EL34

Hey Andy , hate to say it, you have obviously been working so hard, but my post #149 way backs is about your ground and sprawling wires at that early stage...

How about taking out the big PA valves for now and going back to achieving a really stable environment in all the earlier stages, even if that means radical re-routing of wires and signal paths: does that make sense?
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Old 23rd Jun 2017, 7:52 am   #202
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Default Re: 807 (maybe) amplifier build. Now EL34

Bit of a progress video here - https://www.youtube.com/watch?v=MZTTpBLvRCw . Video quality iffy. trying out new video editing software - Shotcut, which is far better than the hear tearing out turmoil that is Openshot.

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Old 24th Jun 2017, 6:36 pm   #203
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Default Re: 807 (maybe) amplifier build. Now EL34

Not easy to see from the low video quality but isn't the rightmost smaller valve between the EL's, just behind what appears to be a big, black circular panel-meter, redplating ?

It shouldn't be in any circumstances !
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Old 24th Jun 2017, 10:52 pm   #204
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Default Re: 807 (maybe) amplifier build. Now EL34

You are right tri-comp!!! looks like a 6CG7 or maybe a 12B4.

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Old 25th Jun 2017, 5:32 am   #205
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Default Re: 807 (maybe) amplifier build. Now EL34

Yes, sorry dire video quality, I'll post a a better onesoon. Well spotted but no it isn't. The valve is a RCA 12BH7 direct coupled cathode follower, what your seeing is the red print on the valve. Anodes 400v, 12k6 cathode R's, cathodes at 120v, biased @ -14v = 8mA, within it's 3.5w limit.

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Old 26th Jun 2017, 3:20 pm   #206
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Default Re: 807 (maybe) amplifier build. Now EL34

Tried to apply NFB today, but the amp didn't like it, see - https://youtu.be/QxWrAEf4G8U . This is odd as when it was a "prototype" I applied NFB with no bother. I got the same fault as before when it was mis behaving, IE the OPT was "squeeking". I can here the squeek coming from inside the shroud. Time to have a look inside methinks.

Tried various small caps (1000p ) from anode taps - primary of OPT to 43% taps and HT to anodes. No attenuation of fault. I'll get it on the big Tek scope tomorrow which has a higher range and is happier with a higher voltage IP.

One thing I noted whilst monitoring the neg bias of the OP stage was it went from - 35v to around - 100v. No idea if this is indicative.

A.
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Old 26th Jun 2017, 5:32 pm   #207
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Default Re: 807 (maybe) amplifier build. Now EL34

Try swapping the drives from the splitter to the output valves over. You may have had them the other way round when you built your prototype. If they're back to front, you'll get positive feedback instead. Very squealy!!
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Old 26th Jun 2017, 5:41 pm   #208
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Default Re: 807 (maybe) amplifier build. Now EL34

Just had a look at your YTube post, Andy, and left a comment up there. It's looking good, and I imagine you're getting very pleasing results. Is that a big old bin of a speaker in the back of your workshop, smashing out fat bass to that reggae?

You have admirable patience and tenacity and I'm sure you're exactly where you need to be in this build. A few months' time, or whenever, it will all be cased up and ready to entertain. Keep up the good work!
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Old 27th Jun 2017, 4:48 am   #209
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Default Re: 807 (maybe) amplifier build. Now EL34

Thanks for the suggestion Howard, I'll try that. BTW, does anyone know about applying current FB? I've nicked a few ideas off this chaps build - http://www.turneraudio.com.au/100w-monobloc3-2014.html he used current FB in his similar amp.

Thanks for your kind words Al. The "speaker" I'm using is a 100w Celestion C15 15" woofer in a guitar cab, a Celestion 10" speaker cab as mid, and a Fane 140w horn cab, connected to an old Kef X over. this lash up sounds better than my "hifi" speakers. Yeh, reggae sounds especially juicey.

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Old 27th Jun 2017, 7:30 am   #210
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Default Re: 807 (maybe) amplifier build. Now EL34

Hi Andy, usually more or less as per the schematic. A low value NON INDUCTIVE resistor (say about 0.5R or less of resistance wire, not coiled ) and take the feedback across that. This is in the ground return of the opt sec. Capacitor should be adjusted for best results when driving the speaker(s) of choice with a 1KHZ sq wave.

Used not too successfully in the PCL83 version of the Pye BB

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Old 27th Jun 2017, 8:20 am   #211
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Default Re: 807 (maybe) amplifier build. Now EL34

Hi Andy,

You've just run into two classical problems:

Valve amplifiers with quite low amounts of negative feedback because several factors in their design tend to cause more phase shift than can be easily handled. The output transformer is best known and probably the least tractable cause, but there are others. Leak's Point One design got down to about 1% distortion open-loop and feedback reduced this to the 0.1% of its name. This implies he achieved a stable and manufacturable design with about 20dB of feedback. For a valve amplifier, that's a lot. Arthur Radford was a designer of transformers, and he designed some special ones for his amplifiers, but Arthur Bailey contribured a phase splitter that was effectively cascoded to both outputs. This greatly reduced another contributor of phase lag. All these things combine.

You can put an open-loop amplifier together quite easily. Nick a circuit section from here, borrow one from there, play about with a few standard sections for the rest. It's how a lot of electronic circuits, and all sorts of other things are designed though 'inspired by' usually replaces the 'nicked' word.

What you can't do is nick someone else's feedback design. You have to evaluate the gain and phase characteristics across the audio band and up to silly-looking frequencies and then work out how much feedback you can safely employ. It won't be as much as you'd like, so you see if your feedback network can compensate for it a bit. It can, but only a bit... a bit smaller than you'd like. So you revisit the main amplifier design. Where can you reduce phase lag? Every little helps. Anode load resistors fall in value, bias currents go up. Cathode followers may sprout. Fancy interleaved transformer winding schemes appear. And with all of this you can eventually dare risk 10-20dB. Of course, this reduces the gain of the finished amplifier, so you needed to start with this built into the open loop design as extra gain to be chucked away when the loop is closed.... but the amplifier still needs to do the low lag thing even with this elevated gain.

So the open loop amplifier needs to be designed with feedback in mind.

I once did a silly valve amplifier. Lots of feedback, trivial distortion numbers, ridiculous bandwidth. Output transformers were right out. It used banks of PL519s in a totem-pole configuration and the earlier stages looked more like a fast oscilloscope Y amplifier. It worked. It worked very well. What you heard was what was put into it. All 'Valve sound' was engineered out. The power consumption was terrible, the heat from the thing was too much. It served as a comparison with some other designs I was trying, then it came apart and the bits recycled into other things.

To understand feedback and stability, the easiest way is to learn the maths. Like those junior school sums about 'If it takes seven men three days to dig a trench 3ft wide and 5 ft deep 300yards long.....' Would you rather have to do the sum or have to dig the trench? Treat maths as a labour saving device which actually works. If you see it as unpleasant, the alternatives are worse.

You said you'd heard that the phase shift had to be less than 180 degrees. That's sort of right. It's just incomplete, though. It's less than 180 degrees over the entire frequency range where the gain around the loop is greater than unity. So you don't test at a spot frequency, you have to go to much higher frequencies than audio. Measure the gain and phase until you get so high the gain falls below 1.

THEN you run into the nasty that the things you could add to run the gain down before the phase shift builds up will themselves create extra phase shift. You get sucked into a nasy race. You need a plan to have any chance of winning.

David
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Old 27th Jun 2017, 4:10 pm   #212
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Default Re: 807 (maybe) amplifier build. Now EL34

I never got my head all the way round this.

I could work out the complex transfer function of a passive network with no feedback in terms of the ratio of two big polynomials in (jw), and plot it as a Bode plot, or Nyquist plot, or pole-zero plot, or whatever. If it had some (complex) gain I could build that in too I guess as a multiplier to the numerator of that big fraction. I think I understand about interpreting a Nyquist plot to try not to 'envelop' the (1,0) location, but to what does that plot pertain - the gain from input, right round the loop back to the point where the feedback would be re-injected, but with the loop broken at that point?

I think my confusion is that I don't know how I would write those polynomials down if the loop were closed.
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Old 27th Jun 2017, 5:19 pm   #213
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Default Re: 807 (maybe) amplifier build. Now EL34

OK, that's a nice starting point.

You're interested in the gain/phase around the loop as if the loop had been broken somewhere (it turns out broken anywhere is the same).

You don't have to worry about the closed-loop gain. If you get the open loop right and have the feedback factor you want, then the closed-loop looks after itself.

Using an opamp as an example. If it has a gain of 10000 at 1kHz and your feedback network is intended to give a closed loop gain of 10, non-inverting, then the feedback resistors attenuate by a factor of 9 (it's the non inverting circuit, remember) so the overall open loop gain would be 10000 *(1/9) which is 1111.11.. if my mental arithmetic is working, at that frequency.

Doing it with the big complex fraction is part of learning the method is valid, but thereafter everyone uses the short cuts.

The easy way looks terrible to start with. You look for the roots of the big fraction of the open loop gain. Some roots force the value to zero, some force it to infinity. BUT you don't have to factorise the fraction, you can quickly learn to spot the roots just looking at the circuit. Once you know where the roots are you have nothing worse than a few pythagorases and arctangents to calculate.

David
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Old 27th Jun 2017, 9:19 pm   #214
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Default Re: 807 (maybe) amplifier build. Now EL34

I could sketch a pole-zero plot for something simple (2nd order lo-pass or something) and use it to sketch the Bode plot (mag and phase) for the transfer function through it. But now suppose such a network, like this amplifier we're meant to be discussing in this thread (sorry D.A.!) had a feedback path from its output back to its input, with some more jw's in it. We could make an easy (or perhaps better to say 'tractable'!) example of stuff in this path by making it, i don't know, a 2nd order bandpass with magic (pure multiplication) gain of G, say. With the loop broken, that branch doesn't do anything, but with it closed, all that mag and pha complication 'does things'. Supposing the loop re-connects with itself at point A at the overall input, would I break it there and then work out the transfer function from A back round to A (if you see what I mean)? And how do I write down the overall transfer function Vo/Vi when the loop is closed?

Sorry to be dense, but I'm stuck on this. I used to do a lot of this, but (as you can see) never with feedback!
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Old 27th Jun 2017, 11:13 pm   #215
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Default Re: 807 (maybe) amplifier build. Now EL34

Don't worry about a loop being closed. You can get all the info needed for calculating its stability margins from open loop analysis.

When the loop is closed the classic feedback equation comes into play.

Let's say the gain of the amplifier without feedback is G where G is an equation with poles and zeroes as required. And the feedback arrangement has a transfer function of B (traditionally the letter Beta)

Closed loop gain = G/(1+BG)

If G becomes very large, then the 1 in the denominator becomes trivial compared to G, and so it approximates to:

Closed loop gain = G/(BG)

Which simplifies to Closed loop gain =1/B Which is the approximation result used for opamps.

But here we're interested in the case there G still has some effect, so we must use the true gotm

CLG = G/(1+B)

G and B can be simple numbers or full blooded equations but whatever they are, they combine this way.

You can consider bringing in the feedback gradually, moving smoothly up to its full intended level. The poles and zeroes on its s-plane move in accordance with some simple rules (look up root-locus techniques) and lets you estiate behaviousr closed-loop.

David
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Old 28th Jun 2017, 12:17 am   #216
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Default Re: 807 (maybe) amplifier build. Now EL34

Thanks very much! This

https://en.wikipedia.org/wiki/Closed...nsfer_function

turned out to be useful in showing me how to derive your equation, once you gave me a start.

I had a weird education (acoustics) made by bolting all sorts of things together from a wide range of disciplines, with a fair bit of depth but massive holes all over the place. It's nice to plug the odd one now and again, and nicer still now it doesn't matter.
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Old 29th Jun 2017, 4:35 am   #217
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Default Re: 807 (maybe) amplifier build. Now EL34

All is well. had a play yesterday by starting out swapping over the OP's off the cathode followers. AHHHG! Hideous OP sine (video on way) What the? So I sat down with a rough schematic adding the sine phase at each stage. After some thought I sussed, as Howard suggested it comes down to the phase splitter. One grid is grounded, it is this grid's relationship, or rather the same triodes anode to the grounded OPT secondary that is causing the grief.

To save difficult unsoldering, for test purposes I swapped the OPT sec taps around, IE black 0v became signal out positive, 8 ohm tap became 0v/ground. Cutting a long story short applied NFB no bother. What is more encouraging is that there is is no instability at all with NFB applied.

There's more to do though. I noticed that THD wasn't greatly reduced for quite a bit of a reduction in power OP. THD is less than 1% with open loop gain at normal listening levels rising to 2% THD at 105w OP, not bad. I realise these are only rough figures. Frequency response is pretty flat from 100hz to 12khz and squarewave reproduction is good.

Stability is actually very good with NFB applied. I'm afraid the maths approach is beyond me at present, try as I might I can't understand it David. I'd need to take 0 or A level maths before getting to grips with those equations. Talking to someone about this, they said finding the transfer function of an amplifier is a can of worms in itself.

Andy.
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Old 29th Jun 2017, 6:33 am   #218
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Default Re: 807 (maybe) amplifier build. Now EL34

Hi Andy,

If you'd applied what would conventionally be referred to as 10dB of feedback, for the same drive voltage at the input, the output voltage from your amp would fall to 1/3.16 of what it was before. the power would fall to one tenth! the distortion would fall to 1/3.16 in theory due to the feedback, but would fall further due to the reduction in output level.

So an amplifier designed to have a negative feedback loop is designed with the appropriate excess of gain. run without the feedback connected it will have stupid sensitivity. With the feedback operating, the gain falls and the sensitivity comes down to what was planned.

Getting the higher open-loop gain means pushing the input and phase splitter stages for more gain. Which means higher resistance anode loads, higher Gm valves etc. These things tend to mean more phase shift which ruins stability, and so very careful design is needed to get the wanted gain while minimising phase lag, and not letting the measures which boost the gain make more distortion.

So an amplifier which will do the right job with feedback operating will show too much gain and too great sensitivity when run without it.

Designing a valve amp which works moderately well open loop is pretty easy. Compared to transistors, valves will let you get away with murder. Designing a valve amplifier which gets all the benefit it can from a closed loop is a serious undertaking. So most people building their own stick to a reference design like the Mullard ones which don't push things too far and will work stably with fairly mundane transformer designs. Many companies' amplifiers are nothing more than light variations on these. Some companies, Leak, Quad, Radford pushed things further, pretty much to the limit, needing fancy output transformer design to make it work.

It can be taken a lot further, but you have to ditch the output transformer and ditch the familiar architecture. I did it once out of curiosity. The amplifier was huge, power hungry and of a complexity where I might as well have been using transistors. It could have appealed to some people who judge on cost and looks... reassuringly extreme on both counts.

In traditional education, the maths involved in feedback analysis isn't taught at O or A level, though the foundations are. It is a major component in the maths taught as part of an engineering degree. Most people find that scary, and engineering degrees have a general reputation of being all mathematical and hard. However, outside the traditional education system, in the real world, it is quite do-able if you want and are interested. It isn't even hard. Some bits like algebra need a fair bit of practice until you get familiar with them and you start to see patterns. Other bits involve weird concepts like imaginary numbers for frequency and you have to suspend your disbelief until you see them doing useful things to designs in our universe!

I've seen people who dropped out of sciences without O levels get their heads around this stuff. It rewards them with a toolbox which can do all sorts of things and the feeling when seemingly disparate things snap into place is wonderful. If you have the curiosity, and are exposed to these things you'll absorb it slowly and it will sneak up on you. It's a lot easier to teach these things to someone who hasn't been told that it's supposed to be hard, which tells you something

David
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Old 29th Jun 2017, 4:36 pm   #219
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Default Re: 807 (maybe) amplifier build. Now EL34

Andy,
Sounds like progress. You could also just swap the primary anode leads of the o/p tx over which would keep your secondaries correct.
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Old 30th Jun 2017, 8:46 pm   #220
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"So an amplifier designed to have a negative feedback loop is designed with the appropriate excess of gain....." I did that David on the front end,the ECC83 I set up as per the Philips datasheet for high gain and low THD, IE 400v HT, 220K anode R, 1k2 cathode R. From memory I get 4v RMS for 100mv IP.

The PS is a LTP cathode running off a CCS off a -80v rail, with + 350v HT though I kept the anode R's small in value, IE matched 36k (2 x 18k). This gives me a good voltage swing, 60v RMS at the cathode follower's into 47k OP grid resistors at 10mA per side. This drives the OP stage with no clipping for an OP of 144w . Not sure what that is in dB, but plenty of excess gain to spare for NFB.

I have tried to teach myself FB and associated maths, and will again at some point try to get my head round it and other similar challenging subjects, but am starting at a very low baseline of knowledge. I'm struggling with the "basics" and can't find an "in". It's knowing where to start, what basics to learn, getting a strong base to build on, before I try to get to grips with the complex maths associated with feedback. To be honest though I've been putting it off. Advice welcome.

Ohm's law, no problem. 1/(2piRC), ok there, but struggle with Thevenin. From what I've gleaned I need to understand Nyquist's thereom. One of the problem's I have is finding the right reading material. Without exception reference reading, The Art of Electronics for example, goes too fast and assumes a certain level of knowledge.

I'll stick at it, I've found a pretty good ish Utube teacher here - https://www.youtube.com/channel/UCdh...bzxebigAlA2rDg and some of these Indian lectures are good too - https://www.youtube.com/user/nptelhrd . What they lack is practical applications of that material. It bugs me that there are no answers for instance to the AOE set questions at the end of the chapters.

Good suggestion Howard, unfortunately the OPT leads are cut and won't reach for a swap over. I've ended up switching over the PS triodes round.

More soon, thanks again for your interest and suggestions, Andy.
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