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Jan 21, 2022 15:07:37 GMT
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Hang on, I've just had a look at the circuit provided and it shows that the cases of the output transistors are their source pin. If this is not isolated from the PCB mounting bracket (which is then connected to the case) then it's no wonder you're having trouble when earthing the case - effectively you are shorting the output to earth! It might be easier to isolate the transistor can from the PCB bracket rather than the PCB bracket from the chassis. Could be as simple as adding a couple of bushes from a TO3 mounting kit (assuming the transistors already have isolating pads underneath them, BoM calls for them but if not, they need fitting too).
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Jan 21, 2022 18:56:38 GMT
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They have insulating thermal transfer pads under them. Infinite resistance between the MOSFET cases and the bracket (double checked this in case they’d broken down). That’s probably where the interference is getting through though, probably capacitative.
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Last Edit: Jan 21, 2022 19:03:07 GMT by Jonny69
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Jan 21, 2022 19:59:57 GMT
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Phew! Good news.
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squonk
Part of things
Posts: 855
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Jan 21, 2022 21:35:13 GMT
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2004 Chevrolet Avalanche Z71 2005 Mercedes CLK320 Cabriolet 1996 Mercedes C180 Elegance Auto Saloon 1996 Rover 620Ti (Dead fuel pump) 1992 Toyota HiLux Surf 1987 Range Rover Vogue (Rusty) 1992 Range Rover Vogue SE (More Rusty) 2006 Chrysler Grand Voyager 2008 Corsa 1.4 Design
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Awesome squonk First issue talks about 3D printers (in 1978!) and advertises a fully-built Atari VCS for a measly £170 (or £860 in today's money!!).
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Jan 24, 2022 12:16:10 GMT
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Decided to put the valve amp in this thread to save clogging the forum up with my crusty old electronics projects. This one I'm fairly sure I built in 1996 during the summer after I left school, while working at GEC-Marconi as a student. I was fairly familiar with valve amp design as I'd followed the Mullard (?) build in EPE magazine in the early 90s and I'd sketched out a number of valve-based tone controls, pre-amps and bandpass filters based on books I'd got out the library. The main thing that had stopped me from ever actually building something was that by the 90s it was almost impossible to get hold of valve transformers (oh, and I was frightened of cobbling things together with 500+ VDC). It didn't take much prompting to buy the Maplin Millennium 4-20 kit when it came out and I figured I could modify it later, though I never did modify it because it worked really well. So it was my main amp for that year. It went to university with me where it started popping fuses. It was out of action for a short time while I replaced the mains transformer, which was still available from Maplin. Valve amps don't like being switched on and off, so in halls where I wasn't paying for electricity, it used to stay switched on 24/7 when I was there and probably used to help keep my draughty room warm. At some point it needed another new transformer but it had been discontinued by that point. I found one online on a very basic enthusiast's site specialising in supporting old audio. That was here: livinginthepast-audioweb.co.uk/millennium/index.htmlAnd it turned out the guy I was talking to about it was the original designer. There were a few issues with the original design. The first was that the transformer was a little under-specced for a stereo pair. There was an uprated design but I can't remember now if I got that one or the standard one. The second was the split-bobbin design of the transformer which made it a little prone to the laminates in the core buzzing. This is what mine had started doing. Third was the fuse was under-rated and it got degraded by the heat in the chassis, which meant they used to blow the fuses regularly. Fourth was a combination of the transformer mounting screws making contact with the inside of the core and a requirement from Maplin regarding the earthing of the top cap, which inadvertently created a shorted turn and could cause the core to buzz. My new transformer had started doing that and that's kind of when I packed it all away to move house and I haven't looked at it since as it has been stashed in the loft. Until today. One end looks decidedly like it got damp: This is annoying: Grim: So, plans. Pop it open and assess the situation (mostly what state my soldering and component placement was like). Check the fuses. Check the condition of the capacitors. Address the transformer issues if possible, then check all the voltages. Sort out the case which is feeling distinctly wibbly wobbly. Paint some bits to make it look a bit nicer. Maybe replace some of the wiring with higher quality cable if needed. Put it back into service.
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Jan 24, 2022 13:45:59 GMT
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Doesn't look too bad inside. Heater wires are a bit wimpy but the rest of it looks fine. Mains suppression at the AC input. 0V is coupled to mains earth with a resistor and cap (if I've read the schematic correctly). Both of those are probably reasons why it doesn't pick up any noise like the MOSFET amp does. Power supply: Amp 1: Amp 2: I unsoldered and removed the mains transformer. It's a bit of a chunk, AA battery for reference: Looking at the pictures I reckon it's the uprated transformer. It's much taller than in the original pics. I have the old one somewhere in the loft so if I can dig that out I'll know for sure. Anyway, with it on the bench I was hoping I might be able to figure a way of injecting it with resin but it looks pretty well sealed up already. I can kind of see what the issue is, the core is made of very thin laminates that slot together like a comb. I'm guessing some of these aren't very well coated in resin and are vibrating. I'll follow the advice given to insulate the screws from the top cap, but I don't think it's going to make a big difference unless it's the cap itself vibrating.
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Jan 24, 2022 18:53:58 GMT
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I came here to post the same, just found that site today while looking for Practical Electronics magazine, they are all there too. Great amps! I built an 18Watt Valve amp from an Amp maker kit about 20years ago, it's on it's third iteration now. Going back over old projects is great fun to see how much you've learnt and improved.
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Jan 25, 2022 10:27:55 GMT
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Yeah I forgot to thank you for this as well. I also navigated back up to the EPE scans - amazing! Picked a random 1993 issue and found the ad in the back page for the speakers I've still got: 200W ones. They've done me well!
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Last Edit: Jan 25, 2022 10:28:32 GMT by Jonny69
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Jan 25, 2022 11:09:46 GMT
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Tested the transformer yesterday after work. Connected up to mains to check the open voltages. +/- 3.1V on the heater lines either side of the centre tap. 380V on the HT which was a bit scary. My Fluke scope at work auto senses the voltage, so going directly from measuring 3V to nearly 400V and not having to adjust the voltage range inbetween measurements always freaks me out a bit. No noise or buzzing, so it must have to be loaded up and have that shorted link through the mounting screw connected. I couldn't hear any loose laminates or hollow bits, so I wondered if it was actually the top cap vibrating and not the core. Going to use some technology to isolate the transformer core and cap from the screws. The original way was to use fibre washers top and bottom but the manufacturing tolerances in the punches in the chassis and top cap makes it almost impossible to stop them making contact. The improved method was to use a skinnier M4 screw instead of the original 2BA ones and to wrap them in electrical tape or heatshrink. The 2022 Jonnymethod is to 3D print some top hat isolation bushes for top and bottom. Screenie from Cura: The holes through the chassis, core and cap are 5.5mm so that should maintain a 0.75mm gap. I might belt and braces with some heatshrink on the exposed middle bit of screw as well.
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Jan 25, 2022 14:03:07 GMT
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Here we go. Some more feet and the top hats. And fitted: They work a treat.
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Jan 26, 2022 12:55:15 GMT
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It lives! Got it running on the bench for a few hours on a sort of burn-in test. All seems fine. I tested the transformer last night with no valves inserted and it seemed noticeably quieter, just a sort of high voltage hum rather than buzzing like I remembered. Heater lines measured just over 6V and 542 VDC on the HT with no load. Decided not to poke about near that too much! Disconnected everything and did the old SIDE checks to verify everything was safe. Today I've connected a dummy load and set it running on the bench with the valves in to check nothing awry happens. Been about 30 minutes and the transformer is still quiet. Forgot how much heat it chucks out! Valve amps don't like running with no load. They'll tolerate a lower load than the speaker, but not a lot higher and you mustn't ever run them open circuit. I've made up some dummy loads which can live on the back when not in use:
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Last Edit: Jan 26, 2022 12:57:17 GMT by Jonny69
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Jan 31, 2022 17:37:38 GMT
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Argh, I had a feeling I wouldn't last long. Decided to build a pre-amp to handle the inputs I want to use. Blaming squonk for this *points finger* I was reading one of the editions of Hobby Electronics in that link where there was a variable EQ based on multiple op amps. One thing led to another and I landed on the Mullard Circuits book and I thought why not make a valve one, based on the fact that I have 4 ceramic valve bases at home doing nothing. Well, that got expensive quite quickly. So far have an original PSU, 4x USSR valves, the log pots, insulated RCA inputs and an input selector switch. Already got resistors and can use either tagboards or stripboard. I'll have to check what caps I've got in the loft. Thinking I may play with the values to boost up some of the inputs relative to the others so the output is consistent across all the inputs.
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Turns out I didn't have 4 ceramic bases, I only have 2 small ones and they seemed to belong with an abandoned project: That was going to be a 2-3 W mono valve amp powered by 4x AA batteries, maybe to power my radio conversion. There's an OEP audio transformer from RS and the date is 2012. The two valves are a Russian equivalent of an EL84 (output stage) valve and what looks like a Mullard EF86 (input stage) valve. In the box is a 555 timer running at something like 50hz through a power transistor, which goes backwards through a 4.5V mains transformer to make a dirty 300 Volts AC for the high tension side. That goes through a bridge rectifier and that seems to be where the circuit stops. There's a flywheel diode on the input of the transformer. I seem to remember it made in excess of 700 Volts DC open circuit after rectification and made a proper high voltage frazzling sound when running, which made me QUITE nervous. I remember what happened. I tried to be clever with the 0V / GND and blew something when I switched it on with the new 0V wiring. Without a scope, I couldn't easily work out if I'd blown the 555 timer or the power transistor. I'd had a batch of cheap Chinese eBay 555s where not all of them worked and diagnosing the problem at home without actually being able to see what was going on was impossible. Got it in the lab now, so I'll add it to the list...
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I checked that little 6V-300V PSU board. I started by taking the transformer off and tracing the circuit back to remind myself what it did. So it's a 555 astable running as close to 50hz with 50% duty cycle as I could get it. That goes into a PNP transistor, through a diode (to protect the circuit) and into the 4.5V side of a small mains transformer. Transformer seemed to be showing continuity, 555 seemed to be working, PNP and diode seemed to be working, so it was odd to see nothing coming out the transformer. Turned out to be a broken connection on the stripboard. Then much HV buzzing ensued, as previously. I put a flyback diode across the transformer input and that quietened things down but I couldn't get a clean signal out of it. At least this brought the spiky voltage down enough to be able to get the scope on it so I could have a look. This was what was showing going in: And this was what was being kicked out the other end: Not great! Time to start playing with capacitors. This is all a bit of a dead end though. The circuit description in the Mullard book has some specifications for the mains transformer. 60mA on the HT at 300V and 1A for the heaters. That's 18W on the HT and 6W for the heaters. The 4.5V transformer is only 3VA, which isn't enough for that and a total of 24W is a bit much for 4 AA batteries. Pulling 4+ amps from my 2100mAh rechargeables probably won't work!
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Last Edit: Mar 3, 2022 12:31:12 GMT by Jonny69
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Pre-amp progress: things have escalated. I was busy choosing components on eBay and I suddenly inherited a stash of what turned out to be 1960s-1970s capacitors which, with an initial skim, look like they include the values I need. I was originally going to build this with modern components on stripboard, for a sort of neo-original layout but with better component tolerances. Now I think I'm going to be building it the old way on tagboard with old components. Before that happened, I had a think about power supply. Building from scratch isn't a cheap thing to do because you have to factor in the transformer and a high-voltage smoothing capacitor, which can both be expensive components. I was browsing eBay and happened to spot a complete-looking original PSU removed from some bit of kit or other. It had the right diode rectifier valve in it, so I knew it was in the right ballpark in terms of power and voltage. Came from a seller who seems to strip old audio equipment and flog it on. It had almost no description and vague pictures which meant I couldn't see a couple of bits: I'm pretty sure the seller knew exactly what it was, so it probably meant it either didn't work or the seller had pinched something off it and was selling the rest on. I took a risk anyway, since it had the transformer, a Mullard EZ80 rectifier valve, a dual smoothing capacitor dated 1961 and it was all mounted in a little sub-chassis which is good for earthing and shielding. It looked in remarkably good condition when it arrived. It took a bit of head scratching to work it out. A couple of things confused me: 1) the mains in went into a 2-pin header with options for 235V or 215V, but there was no direct route through for the neutral and 2) the outputs from the EZ80 went to and from a 9-pin B9A-size header which made it difficult to follow. I decided the best line of attack was to de-solder the transformer and work backwards. I knew which ones were for HT and heater because they went direct, I knew which were for 235V or 215V AC input live, so I then knew which one should be neutral. I checked continuity between various points to determine the earthing. This turned out to be completely conventional so I hooked up a temporary earthing point. Once I'd checked the resistance between various terminals on the transformer, I was happy to connect mains directly to it. Open voltage was 440V AC on the HT line and just under 7V AC on the heater lines. I was then happy to connect up the heater wiring to see if the valve lit, which it did, then connect the HT to see if the smoothing cap was still ok and see where the voltage was going to: Everything working and 300V DC on the HT once heated up. The bit that was confusing me was how the neutral was supposed to get through. I could see it coupled to earth through a capacitor. It went through another 2-pin header but didn't seem to connect back. I figured it might have gone through a fuse or something, but then I noticed the connector in the mains input header was actually connecting live and neutral directly together. It was just in the wrong place. When I moved it, the circuit made sense and it all worked when I hooked it up. So that's the PSU for the pre-amp sorted. The waxed paper capacitors are in perfect condition so they can stay. I'll have to add a drain resistor so it dumps the HT when switched off, because at the moment it's still holding 50V DC after 10 minutes. I'll also need to have a think how I'm going to couple 0V/GND on to it, because it's currently on the chassis and connecting 0V to earth is Ghostbusters don't cross the streams bad. I may actually just isolate it inside the main chassis, then it won't be a problem.
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93fxdl
Posted a lot
Enter your message here...
Posts: 1,991
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Feb 10, 2022 12:13:17 GMT
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Electronics are a dark art to me, I sort of understand resistors and diodes but anything more is witchcraft, But many years ago around about 86 ish, two brothers I knew built a full car diagnostics machine from a maplin kit. They ended up selling it to a garage, where about the only thing which got used was the timing light Ttfn Glenn
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Feb 11, 2022 17:36:24 GMT
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Just realised I didn't post a finished pic of the MOSFET amplifier. This is it in situ: I still need to sort the earthing but from the previous posts I'm pretty sure I know what to do. Preamp progress. The case arrived - a cast aluminium Hammond 1550J. It wasn't cheap but it's exactly the look I was looking for and it'll polish up. I'd done a cardboard mockup of where everything was going to go, so transferred everything over to the real case to check, then drilled the holes. Here's a mock fit to see what it looked like: I wasn't happy with how the valves sat proud like that. There was too much risk of a valve being knocked or pulled out and a small finger (or mine) accidentally touching a leg which could be up to 300V DC. I decided to recess them which would mean no access to the valve legs until it's actually fully pulled out the base. I really wanted to use rubber isolators but the M3 ones are too big diameter to clear the bases. Instead I made some stand-offs to sink the bases down in the case and cut down some screws to suit: That sinks them 8mm below the surface. Looks much better and is far safer: On the front panel is the on/off switch, bass and treble control and input selector switch for four inputs. For now I've opted for no volume control because both the amps have their own volume controls. I'm going to adjust the gain for the input from my TV (which is much lower than everything else for some reason) using the feedback loop, so everything should come out the end at the same level. The four screws in the top secure the PSU, which is isolated from the main case. On the back, you can see the four inputs and single output, IEC power socket (with mains filter) and the rectifier valve and capacitor sticking out the back: So you can see it's not actually very big; it's 275mm wide by 175mm deep and 67mm tall. I wanted a retro look, hence using button head Phillips screws and not my usual stainless hex heads. I may change these to countersunk for a slightly cleaner look. I'm not completely happy with the vintage switch I've used. It makes a fantastic clunk when it switches; I'll leave it for now, but I may swap it for a matching lit rocker switch like my other amps have. See what it looks like once the knobs are on, which are going to be the same as on the other amps. 45mm on the selector and 23mm on the bass and treble.
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Feb 14, 2022 22:19:44 GMT
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Got an hour after work and wired the power supply into the preamp: See where I’ve added a tagboard interface to connect the power to the main circuit. Two rows for the 6V heaters, two rows for the HT and the rest for 0V. I also coupled the 0V to earth like the Maplin valve amp has. I mentioned the power supply chassis was 0V/GND. I’ve got it mounted on insulated posts inside the case. To couple it, it’s got the same 0.22uF capacitor and 100 Ohm resistor in parallel that the valve amp has, connecting the two parts: Hopefully, with the filtered IEC mains socket, it’ll soak up any rubbish finding its way in from the 230V mains. I’d quite like to try this on the MOSFET amp as well, because if it works it’ll save me quite a lot of stripping down and reassembly.
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Feb 16, 2022 21:38:06 GMT
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Not had a huge amount of time on this, but got the power supply mounted up in its final place and started wiring it up. I started wiring components onto one of the tagboards but realised I was going to rapidly get confused because I’d changed a couple of components for my modified circuit. So I decided to change the circuit diagram with Tipex and photocopy it, which basically gives me a fresh circuit diagram to work to on paper without loads of shiny white paint bits making it look distracting. I wired in the heaters and stopped there to think about component placement where it won’t pick up hum from the heater AC (which is at relatively high current compared to the rest of the circuit. In my valve amp, the 6V heater wires are +/- 3.15V AC with 0V in the middle and the wires are twisted together to prevent too much electromagnetic field. The theory is a) there’s no straight run of wire putting a field into another and b) when one wire is +ve the other is -ve so any induced fields cancel each other out. On this PSU it’s around 7V AC on one side and the other is tied to 0V. I don’t want to twist these together because by doing that it’ll make the 7V AC induce a field in the 0V and I’m pretty certain it’ll make it hum. So I’ve run them separately and away from each other. I’m thinking carefully now about how I keep the inputs sufficient distance away from those wires so it doesn’t pick up any noise.
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