"Why are you applying for this job again? Are you sure you want it?"
A while back I built MoFos. The idea of using the primaries of salvaged power transformers from microwave ovens as chokes came up. I briefly tested one in my MoFos, heard that it worked, and moved on. But I had collected a few of these and I was intrigued with the idea of winding my own choke on the laminations from one, so I cut one open with an angle grinder, hacked together a winding apparatus, and wound a coil. As I had no bobbin, I couldn't get my wound coil to fit back on the E-laminations and so I gave up.
Learning about current-source amplifiers as a match for full range drivers led me to Papa's exploration of the Jean Hiraga Nemesis design, first as the Arch Nemesis and later the SIT Nemesis, which paradoxically and perhaps unnecessarily pairs an output transformer with a FET. Lo and behold therein were specifications for a weird output transformer! One with a 64 ohm primary with an 8 ohm secondary, and a primary rated to 1.3A. How cool! Maybe I could make that!
I did some tiny maths. As Papa said it was a winding ratio of about 2.82:1, so I settled on 264 turns primary against 94 turns secondary, mostly as bass extension to 25Hz required about 0.4H of primary inductance based on a primary impedance of 64 ohms. A 50W capable primary at that impedance could mean 56.6V primary, that meant 3.67A on the secondary, hence 16ga secondaries and 20ga primaries. These windings would just about fit, haha, on the EI-125 laminations I salvaged from two MOTs, which were good by spec to at least 90VA at 60Hz. I then actually got brave and bothered Jack Eliano with this draft design, as he originally built Papa's OPTs and I admire him and have some of his PVA-2n's in a passive preamp, but he shared his final prototype design with me and it had different specs and anything I made with materials at hand would likely have inadequate dynamic range and bass extension. A dark art indeed! Since no one was responding to my E-I laminate purchase inquiries I decided to build my own trafo from my salvaged trash anyway.
I bought one of those cheap manual winding 'machines' & made a mount for it, as well as a mandril and some disc like adapters. I also found a pair of bobbins on ebay that would fit the laminates I had, and settled on a design that would wind half the primary, then a layer of grocery bag, then the secondary (counter-wound to the primary), another layer of grocery bag, then the second half of the primary. I opted to re-use the 16ga enameled wire from the first failed effort, kinks, scrapes and all, and bought a spool of 20ga for the primary. Set it up & started winding.
What a pain! But I got it done & strapped it together with a hose clamp with nothing gapping the E and I laminates. I measured the primary and secondary inductance, and it kinda made sense, maybe: Primary DCR = 2R1, inductance with open secondary = 213mH, inductance with shorted secondary = 0.557mH. Secondary DCR = 0R3, inductance with open primary = 26.7mH, inductance with shorted primary = 0.059mH. Maybe sloppy, but a trafo perhaps.
I didn't know how else to test it, so I dead-bugged the Figure 6 circuit of the Arch Nemesis paper using an IRFP240 pulled from a MoFo and some power resistors I had laying around, and set the gate bias using a little DC buck converter. I settled on 50R for R2, 12R for R3, 1R for R1, and gate bias of about 5V. At 45V of V+ it drew about 0.94A across R1.
To my immense surprise, the darn thing played actual music. Tinny and thin at first, but full and kinda brassy and warm later, actually a surprisingly familiar tone to me from the PVA-2n preamp and MoFo experiences. To my chagrin as I played with the gate bias and V+ the music got thin and then disappeared completely. Pressing on the OPT brought it back temporarily only for it to fade, leading me to wonder if the core was saturating due to inadequate gapping. I dismantled the trafo, put a piece of plastic insulation between the E and I laminates, powered it up and it played music again! Very cool. Can't tell you how pleased I am that my junk salvaged first-time trafo actually made music.
Will have to decide how to permanently gap and mount the laminates as the fading and tinnyness is annoying, and whether I should wind a second core to make a mate for this one. I could even use SITs!
A while back I built MoFos. The idea of using the primaries of salvaged power transformers from microwave ovens as chokes came up. I briefly tested one in my MoFos, heard that it worked, and moved on. But I had collected a few of these and I was intrigued with the idea of winding my own choke on the laminations from one, so I cut one open with an angle grinder, hacked together a winding apparatus, and wound a coil. As I had no bobbin, I couldn't get my wound coil to fit back on the E-laminations and so I gave up.
Learning about current-source amplifiers as a match for full range drivers led me to Papa's exploration of the Jean Hiraga Nemesis design, first as the Arch Nemesis and later the SIT Nemesis, which paradoxically and perhaps unnecessarily pairs an output transformer with a FET. Lo and behold therein were specifications for a weird output transformer! One with a 64 ohm primary with an 8 ohm secondary, and a primary rated to 1.3A. How cool! Maybe I could make that!
I did some tiny maths. As Papa said it was a winding ratio of about 2.82:1, so I settled on 264 turns primary against 94 turns secondary, mostly as bass extension to 25Hz required about 0.4H of primary inductance based on a primary impedance of 64 ohms. A 50W capable primary at that impedance could mean 56.6V primary, that meant 3.67A on the secondary, hence 16ga secondaries and 20ga primaries. These windings would just about fit, haha, on the EI-125 laminations I salvaged from two MOTs, which were good by spec to at least 90VA at 60Hz. I then actually got brave and bothered Jack Eliano with this draft design, as he originally built Papa's OPTs and I admire him and have some of his PVA-2n's in a passive preamp, but he shared his final prototype design with me and it had different specs and anything I made with materials at hand would likely have inadequate dynamic range and bass extension. A dark art indeed! Since no one was responding to my E-I laminate purchase inquiries I decided to build my own trafo from my salvaged trash anyway.
I bought one of those cheap manual winding 'machines' & made a mount for it, as well as a mandril and some disc like adapters. I also found a pair of bobbins on ebay that would fit the laminates I had, and settled on a design that would wind half the primary, then a layer of grocery bag, then the secondary (counter-wound to the primary), another layer of grocery bag, then the second half of the primary. I opted to re-use the 16ga enameled wire from the first failed effort, kinks, scrapes and all, and bought a spool of 20ga for the primary. Set it up & started winding.
What a pain! But I got it done & strapped it together with a hose clamp with nothing gapping the E and I laminates. I measured the primary and secondary inductance, and it kinda made sense, maybe: Primary DCR = 2R1, inductance with open secondary = 213mH, inductance with shorted secondary = 0.557mH. Secondary DCR = 0R3, inductance with open primary = 26.7mH, inductance with shorted primary = 0.059mH. Maybe sloppy, but a trafo perhaps.
I didn't know how else to test it, so I dead-bugged the Figure 6 circuit of the Arch Nemesis paper using an IRFP240 pulled from a MoFo and some power resistors I had laying around, and set the gate bias using a little DC buck converter. I settled on 50R for R2, 12R for R3, 1R for R1, and gate bias of about 5V. At 45V of V+ it drew about 0.94A across R1.
To my immense surprise, the darn thing played actual music. Tinny and thin at first, but full and kinda brassy and warm later, actually a surprisingly familiar tone to me from the PVA-2n preamp and MoFo experiences. To my chagrin as I played with the gate bias and V+ the music got thin and then disappeared completely. Pressing on the OPT brought it back temporarily only for it to fade, leading me to wonder if the core was saturating due to inadequate gapping. I dismantled the trafo, put a piece of plastic insulation between the E and I laminates, powered it up and it played music again! Very cool. Can't tell you how pleased I am that my junk salvaged first-time trafo actually made music.
Will have to decide how to permanently gap and mount the laminates as the fading and tinnyness is annoying, and whether I should wind a second core to make a mate for this one. I could even use SITs!
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Nah but it was a whole foods bag, which is a little chi-chiSuper cool! That’s some fearless amplifier building!
did you cryo treat the grocery bag? Was it made from recycled paper? From an organic shop?
Maybe I'll try some other bags, & make some claims about the sound When you were a kid, did your teachers have you make book covers out of grocery bags for your textbooks? I was that kid.
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Nah but it was a whole foods bag, which is a little chi-chi
When you were a kid, did your teachers have you make book covers out of grocery bags for your textbooks? I was that kid.
I did the same with textbooks!
If I had to guess, you might describe the sound of Whole Foods based trafo as "rich" and "organic".
Sure! Let's see if this link works. If it doesn't, it's the
Dual-Purpose Electric/Manual Coil Winding Machine w/Counter from U.S. Solid
I got mine on amazon as the shipping was free and that made it a tiny bit cheaper than buying direct. Note this thing is described as having two different speeds. I think that is only true with an optional, home made electric drive as the hand crank collides with the shaft on the 1:8 gear if you mount it directly on the 1:1 gear. To get the winding tension I wanted and to minimize winding overlap on a given layer required slow winding with a lot of hand force. I wasn't using the hand crank much at all, but the counter function was super useful.
The Prequel - Fun with Power Tools
Well since y'all enjoyed this, and since I have pictures, here's how you cut one of these microwave-oven transformers open.
I use a vise, an angle grinder, a beefy screwdriver I don't care much about, and a hammer. Use the angle grinder to grind through the side welds holding your MOTs E laminations to the I laminations, and give it a whack. It should pop right off. The primary usually will come out with a little elbow grease, maybe a gentle tap or two. I save these as they legit measure out as air-core chokes with a DCR of about half an ohm and inductances in the 0.85mH to 1.145mH range (n=2). The secondaries are more of a pain to remove, idk if it's the potting compound or the finer wire or what but I usually have to hammer and chisel on these with my not-nice screwdriver to get them out. That's pretty much it, unless you need to reduce the width of the laminations to fit your available bobbins, as I had to. Then it's a finer, not-nice screwdriver, and more tapping with the hammer.
To be clear, I was wearing long sleeves, long pants, work gloves, and safety glasses over my normal glasses when I did this. I am a bit wary of angle grinders and I would wear a face shield if I had one. Also this process shoots red sparks everywhere and leaves a fine black dust downwind of the blade, so I wouldn't do it in say your woodshop.
Well since y'all enjoyed this, and since I have pictures, here's how you cut one of these microwave-oven transformers open.
I use a vise, an angle grinder, a beefy screwdriver I don't care much about, and a hammer. Use the angle grinder to grind through the side welds holding your MOTs E laminations to the I laminations, and give it a whack. It should pop right off. The primary usually will come out with a little elbow grease, maybe a gentle tap or two. I save these as they legit measure out as air-core chokes with a DCR of about half an ohm and inductances in the 0.85mH to 1.145mH range (n=2). The secondaries are more of a pain to remove, idk if it's the potting compound or the finer wire or what but I usually have to hammer and chisel on these with my not-nice screwdriver to get them out. That's pretty much it, unless you need to reduce the width of the laminations to fit your available bobbins, as I had to. Then it's a finer, not-nice screwdriver, and more tapping with the hammer.
To be clear, I was wearing long sleeves, long pants, work gloves, and safety glasses over my normal glasses when I did this. I am a bit wary of angle grinders and I would wear a face shield if I had one. Also this process shoots red sparks everywhere and leaves a fine black dust downwind of the blade, so I wouldn't do it in say your woodshop.
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The Sequel - Finishing Up
Today I thought I'd try a full-range horn loaded speaker and a different gapping material, after first sanding off the residual potting compound from the E and I mating surfaces. The trafo got two pieces of grocery bag paper (Whole Foods only! Very important!!), cut to expose the channels where I'd ground out the welds. Also I sized the I-laminations that originally came with this trafo's E-laminations as I'd been using the other trafo's correctly sized ones out of convenience.
I'd been puzzling how to refasten the E and I's. There are some bolt holes but only on the laminations from one trafo, so fabricating end plates to bolt them together would only work for one and seemed a hassle. I also wasn't into the hose clamp aesthetic so I tried a bead of JB-Weld along the seams. Electromagnetism being cool and all holds the I's to the E's while there is energy through the coils, so I had the thing playing while I laid the beads and for a while afterwards as the epoxy cured. Hopefully this will hold.
So how does it sound? Well here's a video for ya of a little Van Morrison. These FE126En's are shouty and the trafo doesn't help matters, but it's still cool as heck to me that this works at all.
***
I can't decide yet whether to make this one's brother or it's cousin following Jack's turns ratio. I only have one other bobbin and may not have enough wire left for even one, so we'll see.
Today I thought I'd try a full-range horn loaded speaker and a different gapping material, after first sanding off the residual potting compound from the E and I mating surfaces. The trafo got two pieces of grocery bag paper (Whole Foods only! Very important!!), cut to expose the channels where I'd ground out the welds. Also I sized the I-laminations that originally came with this trafo's E-laminations as I'd been using the other trafo's correctly sized ones out of convenience.
I'd been puzzling how to refasten the E and I's. There are some bolt holes but only on the laminations from one trafo, so fabricating end plates to bolt them together would only work for one and seemed a hassle. I also wasn't into the hose clamp aesthetic so I tried a bead of JB-Weld along the seams. Electromagnetism being cool and all holds the I's to the E's while there is energy through the coils, so I had the thing playing while I laid the beads and for a while afterwards as the epoxy cured. Hopefully this will hold.
So how does it sound? Well here's a video for ya of a little Van Morrison. These FE126En's are shouty and the trafo doesn't help matters, but it's still cool as heck to me that this works at all.
***
I can't decide yet whether to make this one's brother or it's cousin following Jack's turns ratio. I only have one other bobbin and may not have enough wire left for even one, so we'll see.
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Aaah, the infamous Grocery Bag Output Transformer...! I´ve made some too, using recycled cores and paper from grocery bags, with some epoxy glue on each paper strip to keep the windings in place. Works like a charm but I must admit it was a bit nerve-racking to apply 570V to one of those transformers when I experimented with with the russian GU-72 transmitter tube...
Haha, yeah nothing higher than 45V so far for me. I used masking tape after each winding layer to hold things down as I wound. I was concerned painting epoxy would be messy and make the winding depth uneven, and also there was some comment somewhere about possibly increasing interwinding capacitance? As it was I almost ran out of room in the winding window for all these layers so less taping would have been good.