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#1
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Surface grinding a power transformer.
I have some tube amp power transformers made locally (before the
factory burned down) and also some rather nice Tamradio rectangular cans (used to be Jap 100v primary power trannys until I gutted them). Problem is that the former dont fit in the latter - they miss out by about a millimetre. Thinking back to the days when I messed around with car engines, I wonder what the electrical implications of attacking the side of the lam stack with a cylendar head grinder would be. The Transformers are oversized electrically already (at least 50% over the required power). Any gottchas ?. M |
#2
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" wrote: I have some tube amp power transformers made locally (before the factory burned down) and also some rather nice Tamradio rectangular cans (used to be Jap 100v primary power trannys until I gutted them). Problem is that the former dont fit in the latter - they miss out by about a millimetre. Thinking back to the days when I messed around with car engines, I wonder what the electrical implications of attacking the side of the lam stack with a cylendar head grinder would be. The Transformers are oversized electrically already (at least 50% over the required power). Any gottchas ?. It probably OK to clean them up by taking a mm off all around, but just don't use heat, and don't let them get soaked with grinding liquids, although if oil based it isn't so bad. But you have to clean up ground metal and dry the trannys.... Patrick Turner.. M |
#3
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For the SET amps I build, I had sanded the lams smooth and painted with
epoxy paint - gloss black. Someone who used to own a transformer company told me it was bad, so I called Hammond (I use the 1627SE's). He said there was no problem at all with smoothing out the laminations. The caveats added by Patrick Turner are apt. Dave -- wrote in message oups.com... I have some tube amp power transformers made locally (before the factory burned down) and also some rather nice Tamradio rectangular cans (used to be Jap 100v primary power trannys until I gutted them). Problem is that the former dont fit in the latter - they miss out by about a millimetre. Thinking back to the days when I messed around with car engines, I wonder what the electrical implications of attacking the side of the lam stack with a cylendar head grinder would be. The Transformers are oversized electrically already (at least 50% over the required power). Any gottchas ?. M |
#4
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300B wrote: For the SET amps I build, I had sanded the lams smooth and painted with epoxy paint - gloss black. Someone who used to own a transformer company told me it was bad, so I called Hammond (I use the 1627SE's). He said there was no problem at all with smoothing out the laminations. The caveats added by Patrick Turner are apt. Dave I have heard of ppl restamping the cut out size for the window to make it bigger on standard wasteless patterns. The shearing stresses cause magnetic problems, as would bending laminations. So bending an hammering or shearing are not good. But cold grinding is probably quite OK. When they make C-cores the wind up a spiral of grain oriented steel strip and bond it together with epoxy resin and cure with mild heat, which does not affect the magnetic properties. Then they saw through the 0 shaped core and grind and polish the cut ends so making two C from the 0 which can be clamped together to make very small size of gap in the core, so a C core has an irom µ of maybe 1/3 of the µ of a continuous uncut core, like that used in a toroidal tranny. So its the fine gap that alters the magnetics, not grinding, which actually improves the join, so i can't see why grinding 1% off a bit of the outside off a tranny will change much at all. As regards heat, my experience with grain oriented silicon steel is that it doesn't do it much harm. Last year I found I had accumulated a wheelbarrow full of old trannies with odd windings, or open windings, some audio, some mains, and the job of stripping them down is hard yakka. So I just piled them into an open fire for 1/2 an hour until they turned cherry red, and pulled them out and let them cool over night. The next day I used an small angle grinder to cut the bolts, and cut the copper wire, and the laminations all just fell apart, no longer held tight by varnish and bobbin, also evaporated in the process. A few Kgs of the laminations I gained this way were known to be GOSS with a maximum µ = 5,000. After cooking like I did it didn't change to any great extent, ie, the magnetic properties were the same, and µ remained the same, so I could reuse the material for chokes. This stands to reason because the GOSS I buy has been heated in a furnace for quite some time before it is sold to me. GOSS hase been annealed. When grinding a tranny to fit into a can, one should do the sides one needs to, but after some thought, I wonder, will it work to give a neat and quiet tranny? BUT, and there is always a but, what is the can material? If it is mild steel, there should be a gap between the can and the core, because if not the can may buzz with the external magnetic field put out from the core due to mains flux leakage. In some amps I have built the steel covers over mains trannies have hummed more as clearance reduces, and I have had to resort to drilling holes in the covers, and gluing fibre board to the inside of the steel cover, or making a steel can myself, and potting the tranny by pouring in molten roof pitch heated to about 180C. This last method was able to silence a particularly noisy C-core tranny, not unusual because it is rated for 1.1kW, and despite the Bmax being only 0.85 Tesla. I have also used handmade pots to encase chokes used for choke input PS. So rather than alter the tranny to fit the can, how about making new cans? I do this by cutting a block of wood *accurrately* and with *square angles* to the wanted inside size of the can, and bend some mild steel sheet around the block with clamps, hammer, and blocks of scrap wood to not dent the iron sheet. Then I make a couple of internal fitting lids, and poprivet the lot together I also allow for fixing bolts or angles etc. I seal the inside with some silicone smeared around, and leave a hole say 2" dia big enough to pour in some pitch and have the wires come out, and its done. Wires for pitched trannies have to be sleeved with fibreglass sleeving, since pvc will melt when the hot pitch hits it. There are other potting materials; polyurethane and epoxy is available. But some of this stuff gives off heat when it hardens, and shrinks, so the right stuff for potting is expensive. The simplest way to fit trannies is to use bellends. Patrick Turner. -- wrote in message oups.com... I have some tube amp power transformers made locally (before the factory burned down) and also some rather nice Tamradio rectangular cans (used to be Jap 100v primary power trannys until I gutted them). Problem is that the former dont fit in the latter - they miss out by about a millimetre. Thinking back to the days when I messed around with car engines, I wonder what the electrical implications of attacking the side of the lam stack with a cylendar head grinder would be. The Transformers are oversized electrically already (at least 50% over the required power). Any gottchas ?. M |
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