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Old 27th May 2020, 3:30 pm   #1
retailer
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Default Starting a SE amp project

After removing the filament transformer from an old emission type valve tester (Eico 625) for a project I was left with a metal case that looked just right for a small single ended stereo amplifier chassis, I'm thinking SE 6AQ5's and 6AU6's as I have quite a few of each of these, the mains transformer is no issue I have a good collection so should be able to find one suitable, the output transformers are another matter. I have a small collection of small radiogram style SE output transformers and even a pair that would do the job 5K to 16 ohms, but on testing the frequency response I find it starts to fall off at around 120Hz or so and by 80Hz there is not much there, I'd like something better. Going through my collection of salvaged mains transformers I found a pair that look to be able to be rewound, size wise about 50W so should be ok for around 4W single ended. Two good things about them is they are identical and they have bobbins which will make the winding easier.

I strip one down, the laminations come out easily which surprises me as usually the first 2 or 3 are damaged to the point where they are unusable, unwinding the secondary and counting the turns works out the turns per volt is under 5, to me this seems rather low for the size of the transformer, but possibly the laminations are good quality and maybe even grain oriented after all steel is cheaper than copper. Once unwound I find the bobbin is actually two bobbins one inside the other no doubt to provide a good deal of isolation between mains and secondary, it is a bit of a blow to my plans, I have no bobbin, I'll just have to wind the transformer on a tube, doable but not as convenient as a bobbin. The laminations are .014" thick which I think is not too bad, the core area is 1.13 squ inches. Most of the laminations have varnish on them, some more than others, I dunk all of them in a plastic tub full of thinners, I'll give it 3 or 4 days and hopefully the varnish will float off, it's no fun inhaling the thinners while scrubbing each lamination with a rag to get all of the varnish off, every time I do this I think of my uni days and organic chem practical, the demonstrators kept hammering into us - the fume hood is there for you to use if you can smell it it's damaging your health. Now I have the laminations I can start on the design process
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Old 27th May 2020, 7:44 pm   #2
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Default Re: Starting a SE amp project

Watching with interest.

I had a similar issue with the bobbins when I did the two small PP transformers in that I could not re-use the bobbins, so I made two. Guess I am lucky in that I have a small garage with machine tools in there. I milled some brass to be the size of the former plus 1/2mm, then I used some stiff clear plastic from packaging and cut it into strips the width of the winding area. These strips were then formed around the brass former and glued with PVCu Conduit solvent cement as used on plastic drain pipes. When set I formed the cheeks of the bobbins by using some single sided copper clad board etching the copper off it. The filing to fit was the most tedious job to do for me but in lock down not a lot else going on. The cheeks were then araldite'd in place and left for a day to fully set. You may have seen the result on my other post, not brilliant but was better then nothing for me.

Have fun.

Adrian
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Old 28th May 2020, 1:23 am   #3
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Default Re: Starting a SE amp project

There is an excellent article in Radiotronics that describes exactly that, and was designed by none other than Fritz Langford-Smith. Radiotronics magazine was the trade journal of Superadiotron valves.
The suggested article is from March 1955, and works perfectly well. I use a stereo version for my PC speakers. Radiotronics can be downloaded from Davids superb www.americanradiohistory.com.
I actually obtained mine from Frank Pocnet many years ago.
I attach the circuit for your perusal, but I can send you the whole article if you wish

Joe
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Old 28th May 2020, 8:39 am   #4
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Default Re: Starting a SE amp project

Quote:
Originally Posted by retailer View Post
every time I do this I think of my uni days and organic chem practical, the demonstrators kept hammering into us - the fume hood is there for you to use if you can smell it it's damaging your health. Now I have the laminations I can start on the design process
Interesting project. Re solvents, remember that with some, they do harm at levels at which you cannot smell them and there is always the problem of "de-sensitisation" occuring as you work with any solvent. You've been happily working for some time, then someone enters the room and comments on the horrendous stench!

B
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Old 28th May 2020, 2:13 pm   #5
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Default Re: Starting a SE amp project

Working with what I term the universal transformer equation from RDHB on an Excel spreadsheet I work out the turns per volt at 50Hz for 1 Tesla and then again at 1.5 Tesla at 1.5 Tesla the turns per volt work out to approx 4, perhaps the core material is a better quality. I redo the transformer equation cal at 30Hz for the frequency and use a flux density of .55 Tesla, not only will the core will be carrying a small dc standing current there will also be some thin non magnetic material (paper?) separating the stacked E's and I's. I'm not sure if this reduction in flux density is right but I guess I'll know once it's done and built. The equation yields 20 turns per volt so the primary turns will be 3000, split into 3 primary at 1000T each and 4 secondary at 42T each, for 5K:16ohms if I need 8 ohms then I'll parallel two of the 42T secondaries. I use an Excel worksheet to add the winding heights for all of the layers etc inc insulation and determine it should all fit but only just, I think a bobbin may have helped here as the margin would only be around 1.5mm per side (thickness of the bobbin) against the 3mm margin per side without a bobbin. I plan out the start and end positions of all of the windings plus the connections between them and draw up winding plan that I can refer to while winding. The only thing that concerns me is that the primary winding may a bit on the high side as far as DC resistance goes.

Thinking of the bobbin, I recall I wound a small output transformer for the optical compressor I built a while back, I used a salvaged transformer from a partially dismantled bit of Telefunken gear, the transformer had a bobbin made up of 6 pieces of flat tag board type material (phenolic?), the pieces interlocked like a childs puzzle toy, I though it was quite clever. I still have 3 more of the same transformers, so I gave them a good look and after few hours work I came up with 6 hand cut pieces (plus a few failures) that lock together quite well, I used a pcb board from an abandoned old project and scraped off the tracks, the component holes remain but that does not affect it. It isn't something I would want to do all the time as filing the bits to shape is time consuming a few strokes of the file and try the fit - a few strokes more and try again etc, etc, a few strokes too many and it won't hold together, I think it is time for me to get a 3D printer. I go back to my Excel worksheet and plug in the new value for the winding width using the new made bobbin and the fill is reduced from 82% to 75% I feel much better as I always seem to go over the calculated %fill. With the new info from Excel I draw up another winding plan and cut slots into the bobbin cheeks for the terminating wires, I did have a pic of the complete bobbin but it some how vanished while being transferred to my pc, it turned out rather well almost perfectly square, sometimes you can get lucky .

Joe - thanks for the circuit it is the sort of thing I had in mind.
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Old 29th May 2020, 11:16 am   #6
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Default Re: Starting a SE amp project

I make a timber 'core' that fits into the bobbin so it can be mounted in my winding machine, to make sure it runs true I set it up in the lathe to drill the hole for the mandrel used to mount the bobbin in the winder - in the past I have had bobbins that wobble and if the wobble is too great then it can be easy for the wire to catch on the corner of the bobbin cheek, it usually isn't noticed until the winder stops, which means unwinding and starting again.

I start with the first secondary at 45 turns, I want the winding to start on one side and finish on the other. On my home made winder I can set the wire traverse step distance in 0.02mm increments with a thumb wheel switch, I set the step distance so that 45 turns takes up the full width of the bobbin. The primary is next at 1000 turns, I want the start and finish both on the same side so I need to have an even number of layers, if the wire was close wound then the full 1000 turns would take 5.1 layers so I set the step distance so the turns are spaced very slightly apart rather than side by side, it works out well I finish 6 layers at 995 turns and add the extra turns by hand. The rest of the windings go on easily with no issues as do the final layers of insulation and the leadout wires. The laminations have been soaking in a closed container full of acteone, just about all of the varnish is soft enough to be wiped off with a rag, a few need scraping with a brass chisel. With the secondary set for 16 ohms and a 5k 'load' on the primary I connect the secondary to my signal gen and a scope on the primary, not exactly how it will be connected in the proposed amp but it gives me an idea on the frequency response, the -1dB points are at 30Hz and 25Khz, at 20Hz the response is at -3dB, I was hoping for a -1dB point of 20Hz, but I guess 30Hz is not all bad.

Next step is a bread board setup of one channel, for testing.
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Old 31st May 2020, 8:26 am   #7
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Default Re: Starting a SE amp project

Nice work, look forward to seeing the rest of the build.

Andy.
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Old 9th Jun 2020, 2:19 pm   #8
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Default Re: Starting a SE amp project

A rough breadboard test of my circuit showed some promise, so I went ahead with the construction - the chassis is the carry case for an Eico 625 emission type valve tester, a bit grubby here and there but most of it decent condition, 100% of the hardware components used in the amp have been salvaged from other gear. I drew up and printed a wiring plan with Corel Draw, I believe this helps eliminate possible errors at the time of wiring/soldering things up. Only working on one channel for now, once that is sorted I wind the second transformer.

The first test with a dummy load and signal gen was a total fail, no output at all, disappointing considering my breadboard test went ok, I had to give it away for the day as dinner time had arrived, I planned to give it some thought over night - but it was on my mind and so I spent the best part of 3 hours in the evening trying to track down the fault/error with no joy, I did track it down to the 6AU6 preamp and eventually the 3 deg workshop temp got the better of me and gave up for the night. The majority of the components are mounted on a tag board, I thought the fault must be with my tag board and even though I was careful to install the underside links in the correct place I figured I must have made a mistake somewhere so first thing the next day I started to remove the tag board, I turned the amp over to get at the screws securing the tag board and noticed a white film on the top of the 6AU6, it was a new valve still in it's original box when initially installed, I started, pulling it from the socket, I heard the sound of crunching glass, I must have at some time dropped it on the valve, probably at the time I connected the dummy load. Even though I have quite a few 6AU6's I was more than annoyed with my self for being so clumsy.
With the tag rewired, re-secured and another 6AU6 installed I fired up the amp to find I had hum on the output, another disappointment - hum was almost non existent in my breadboard test setup the only real difference in the two was that I used a monblock power supply I had lying around - one of a pair I bought a few years back, so looks like I need to do some work most probably on the power supply.
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Old 11th Jun 2020, 7:44 am   #9
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Default Re: Starting a SE amp project

Looking good, ehmm, had that one, circuit works fine with wires everywhere and the amp looking like an osprey's nest, put everything together neatly, problems. Character building. Also left a 12BH7 in a valve tester overnight, came in, in the morning, damn thing was white, the pip had got knocked off somehow, much weeping and gnashing of teeth.

First off, what sort of hum, 50 or 100hz. Looking at your build, noticed a few things. Htr wiring looks ok, no loops, however your live wires to the sw on the front, I twist mine. Second your earth wire is a bit long, should be a short as poss, also is it sharing the chassis ground connection? It shouldn't.

On the left hand side, brown wire, looks like OPT pri, it's looped, like a coiled snake, cut it short as poss, same goes for other wiring,you have a bit of a lazy loop on the ground from big cap board to LHS tag board: as short and neat as possible is the way. Next, check for ground loops, the IP ground should be isolated from the chassis, there should only be one ground to chassis, and that should be as close as poss to your reservoir cap negative, all gnd's go back to this, in this order. 1st, OPT/HT gnd, OPT secondary gnd (if used), then progressively smaller current stages till lastly you have your IP stage gnd. It looks like you've done this, but can't see a chassis gnd.

Try shorting the IP to ground, do you still have hum?

Hope that helps, Andy,.
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Old 12th Jun 2020, 3:14 pm   #10
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Default Re: Starting a SE amp project

The coiled snake wire is connected to the anode, I intend to play around with neg feedback once I have the amp working ok, I have the components on the tag board but not connected in circuit yet.
Hum is there with the input shorted, I was able to spend a little bit of time on the amp the last few days, and poking around with the scope found hum that looks like half wave unfiltered dc of around 300mV on the screen of the output valve, I clipped a 33uF cap between the output valve screen and ground and the hum disappeared, strange though, the power supply I used to test the breadboard circuit is at least 50 or 60 yrs old has a small choke and still has the original 24uF Ducon grey plastic covered filter caps, I've had it at least 15 yrs and had never fired it up till last week, I ran the voltage up slowly with a variac over the course of a few days. The power supply I built has 2 x 47uF (modern made in China) filter caps plus choke, so looks like I may have power supply issues, I'll get the other output transformer wound and second channel added before I tackle the power supply, adding a second channel will affect the power supply.
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Old 13th Jun 2020, 7:46 am   #11
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Default Re: Starting a SE amp project

Screen supply's are usually de-coupled, same as negative bias supplys which often aren't. Power supply's are tricky for SE amps, you need very low ripple, I'd be tempted to use far bigger caps but your valve rectified I think. Are your other stages de-coupled?

For this amp I'd be tempted to use SS rectification either CLC filter with large caps or regulated. A simple regulated supply is easy to do with a mosfet as the series pass element with a zener reference. The stiffer your supply, the better.

Andy.
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Old 14th Jun 2020, 2:00 am   #12
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Default Re: Starting a SE amp project

I wanted to keep the amp all valve, it is more of an experiment really in preparation for a few more ambitious valve amps down the track.

I was able to spend an hour or so on Saturday, the power supply is 6V4 with a LCL filter, I pulled the filter caps and found the second one which should have been 47uF 500V checked out at 87uF with an ESR of 140 ohms on my universal component tester, replaced the cap and all hum has gone. I was able to finish winding the second transformer so I'll get on wiring up the second channel.

I must admit the capacitors have been sitting in my spares drawer for over 5 yrs so while it was not expected it not a surprise. I have a single to 3 phase VFD on the lathe the VFD, this warning is in the first few pages of the user manual -

"It is better not to store the inverter for long time. Long time storage of the inverter will lead to the deterioration of electrolytic capacity. If it needs to be stored for a long time make sure to power it up one time within a year and the power-up time should be at least above five hours. When powered up the voltage must be increased slowly with a voltage regulator to the rated voltage value."
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Old 14th Jun 2020, 6:58 am   #13
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Good to hear the hum has gone. It's a good idea to test capacitors or any component come to that before use. I tested a handful of NOS 47u/20v caps the other day, look to be at least 50 years old, most tested ok for leakage, capacitance etc after reforming but a few were duff.

Compared to that I had some 100u 250v caps, less than a year old that were mostly rubbish, and a jar full of two year cap that had mostly all leaked physically, it really is a bagatelle.

Andy.
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Old 1st Jul 2020, 3:18 am   #14
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Default Re: Starting a SE amp project

After thinking about it for a while, I remembered this amp from when I was a kid. It works extremely well!!! So I thought as you were looking for a project, or ideas, I scanned this after digging through my stack of magazines. I hope it inspires you, or at least gives you some ideas.

Best of luck,

Joe
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Old 7th Jul 2020, 4:21 am   #15
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Default Re: Starting a SE amp project

Thanks Joe what I came up with as the final circuit is very close to that, it is essentially finished but I've had to put it aside to do some amp repairs.
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