Can't tell you how pleased I am that my junk salvaged first-time trafo actually made music. 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. 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. To my chagrin as I played with the gate bias and V+ the music got thin and then disappeared completely. 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 immense surprise, the darn thing played actual music. At 45V of V+ it drew about 0.94A across R1. I settled on 50R for R2, 12R for R3, 1R for R1, and gate bias of about 5V. 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. Secondary DCR = 0R3, inductance with open primary = 26.7mH, inductance with shorted primary = 0.059mH. 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. What a pain! But I got it done & strapped it together with a hose clamp with nothing gapping the E and I laminates. 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. 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 bought one of those cheap manual winding 'machines' & made a mount for it, as well as a mandril and some disc like adapters. 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 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. 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. ![]() ![]() 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. 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. 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. 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. As I had no bobbin, I couldn't get my wound coil to fit back on the E-laminations and so I gave up. 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. ![]() I briefly tested one in my MoFos, heard that it worked, and moved on. The idea of using the primaries of salvaged power transformers from microwave ovens as chokes came up. ![]() "Why are you applying for this job again? Are you sure you want it?"Ī while back I built MoFos.
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