I do not notice that anyone mentioned vacuum varnish impregnation (a particular brand of varnish, name not known to me - trusted my transformer manufacturer on this one).
Having seen my share of failed unimpregnated/unvarnished transformers through copper oxidation over the years, that is not an option for me, especially for a (say) $150+ output transformer. It will increase internal capacitance somewhat, but I could never not live with that. Laminations are thus sealed, especially necessary in C-core types. I personally bolt my transformers onto the chassis, then shroud them (separately onto the chassis) for aesthetics. I have occasionally had to mount all 3 transformers close to one another without noticable hum being introduced.
(Please note that some hum introduced when testing with a passive OPT, will be damped by the tubes arrangement when those are conducting.) In my experience power transformers varied widely regarding their stray field. I once found one with the well-known copper foil stray field shorting ring round the core to be worse than another without it. I also once had to reject a power transformer (well-known make) and buy a different one because of the severe induction. (The best position for the former was at a peculiar angle in space.)
Regards.
Having seen my share of failed unimpregnated/unvarnished transformers through copper oxidation over the years, that is not an option for me, especially for a (say) $150+ output transformer. It will increase internal capacitance somewhat, but I could never not live with that. Laminations are thus sealed, especially necessary in C-core types. I personally bolt my transformers onto the chassis, then shroud them (separately onto the chassis) for aesthetics. I have occasionally had to mount all 3 transformers close to one another without noticable hum being introduced.
(Please note that some hum introduced when testing with a passive OPT, will be damped by the tubes arrangement when those are conducting.) In my experience power transformers varied widely regarding their stray field. I once found one with the well-known copper foil stray field shorting ring round the core to be worse than another without it. I also once had to reject a power transformer (well-known make) and buy a different one because of the severe induction. (The best position for the former was at a peculiar angle in space.)
Regards.
In Aug this year I potted a locally available 50W PT to a black square steel can which a lid at bottom to be screwed to the can with commercially available 2-comp. Epoxy resin but the filling is not completely hardened until now with many tiny bubbles in it. I was suspecting the quality of the resin so I mixed the resin and hardener(1:1) on the bench and found it hardened very well. My guess is impregnated varnish to the PT keep the Epoxy from hardening.
If I am correct, which potting material would you suggest?
Many thanks,
Kay
If I am correct, which potting material would you suggest?
Many thanks,
Kay
Hi all,
Mag field decoupling;
Take a look at a common garden variety Fender, like a Bassman. This orientation has the least possible coupling between power and OPT. And, they placed the choke in between the two, another plus. The basic floor plan, for those without access, is that of a T. The upright is made from the power transformer laying on the face of the laminations, with the coil protruding through the chassis. The power transformers (E/I types) core has the short side aimed at the OPT, with the long sides at right angles to the OPT. The OPT is standing up, with it's long core side parallel to the chassis and crossing the upright as a T cross bar.
For E/I transformers, the strongest emissions will come from the center of the long side of the core lamination stack. The short side will be a factor of about 8 times less, with two nulls right at the E/I joint. The coils face will be 8 times less and the joint between coil side and lamination face, another 8 times less strong.
Vacuum potting;
The effects depend upon the Dielectric Constant of the materials used. If the potting material is a polyester resin, Dolph 1105 for instance, the cured DC is about 3.4. If the dielectric barriers have a DC of 2. 2 to 2.8 you will hear a more phase coherent signal, with respect to transients. If the DC of the barriers is 3 to 6 plus, no change to a gradual worsening of transients will be heard. Since this is a D Field / E Field event, you must also be concerned with matching the permitivity of the core material to this Dielectric Constant controlled permitivity in the coil. A mismatch between the two is clearly audible, with a refined speaker system.
With respect to temperature rise, any material, metal or an epoxy plastic of 1/4" thickness, applied to or covering over the transformer coil surfaces, will raise the coil hot spot temperature by 5 deg C, as a rule of thumb. Vacuum potting, without extending beyond the coil surface, with a material like 1105 will provide a 5 degree C lower hot spot temperature, over a duplicate with a dry interior and uncoated surface. Interestingly, every separate layer of metal or plastic is also judged to raise that hot spot temperature by 5 deg C. In a typical transformer, the core is a heat sink for the coil, typically having less watts lost than the primary of the winding. Another thing to consider is that the rise in temperature, under load, is 80% complete in the coil within two hours. It will take another 6 hours for the core and coil to come to a uniform equilibrium and the total temperature rise for the unit.
Modern dielectric materials are rated by thermal means. Typical materials used are referred to as class A, B, F, C & H. These classes are passed out by safety agencies, based upon a very rigorous test of voltage withstand, between twisted pairs of coil wire, heated in a vacuum, with a representative supply of all of the materials to be used with the wire, in a coil construction. As an example, for class B, the materials must withstand 130deg C, as an insulation system and are expected to have 300k hours mtbf. If you look at the materials you will find that they all will carry a thermal rating from NEMA of class F, 155 deg C. The safety agencies automatically derate these materials for there own insulation category purposes.
Actually using an exterior can, filled with anything but a thermally conductive potting compound will raise the hot spot temperature by between 10 and 20 deg C, after thermal equilibrium has been reached. On the plus side, there is almost no other way to acheive medical standards for surface creepage of voltage, for patient connected equipment. For audio purposes, it is unwarranted, except for SY's sexy looks comments.....
Just some more trivia to add to your transformer decisions.
Bud
Mag field decoupling;
Take a look at a common garden variety Fender, like a Bassman. This orientation has the least possible coupling between power and OPT. And, they placed the choke in between the two, another plus. The basic floor plan, for those without access, is that of a T. The upright is made from the power transformer laying on the face of the laminations, with the coil protruding through the chassis. The power transformers (E/I types) core has the short side aimed at the OPT, with the long sides at right angles to the OPT. The OPT is standing up, with it's long core side parallel to the chassis and crossing the upright as a T cross bar.
For E/I transformers, the strongest emissions will come from the center of the long side of the core lamination stack. The short side will be a factor of about 8 times less, with two nulls right at the E/I joint. The coils face will be 8 times less and the joint between coil side and lamination face, another 8 times less strong.
Vacuum potting;
The effects depend upon the Dielectric Constant of the materials used. If the potting material is a polyester resin, Dolph 1105 for instance, the cured DC is about 3.4. If the dielectric barriers have a DC of 2. 2 to 2.8 you will hear a more phase coherent signal, with respect to transients. If the DC of the barriers is 3 to 6 plus, no change to a gradual worsening of transients will be heard. Since this is a D Field / E Field event, you must also be concerned with matching the permitivity of the core material to this Dielectric Constant controlled permitivity in the coil. A mismatch between the two is clearly audible, with a refined speaker system.
With respect to temperature rise, any material, metal or an epoxy plastic of 1/4" thickness, applied to or covering over the transformer coil surfaces, will raise the coil hot spot temperature by 5 deg C, as a rule of thumb. Vacuum potting, without extending beyond the coil surface, with a material like 1105 will provide a 5 degree C lower hot spot temperature, over a duplicate with a dry interior and uncoated surface. Interestingly, every separate layer of metal or plastic is also judged to raise that hot spot temperature by 5 deg C. In a typical transformer, the core is a heat sink for the coil, typically having less watts lost than the primary of the winding. Another thing to consider is that the rise in temperature, under load, is 80% complete in the coil within two hours. It will take another 6 hours for the core and coil to come to a uniform equilibrium and the total temperature rise for the unit.
Modern dielectric materials are rated by thermal means. Typical materials used are referred to as class A, B, F, C & H. These classes are passed out by safety agencies, based upon a very rigorous test of voltage withstand, between twisted pairs of coil wire, heated in a vacuum, with a representative supply of all of the materials to be used with the wire, in a coil construction. As an example, for class B, the materials must withstand 130deg C, as an insulation system and are expected to have 300k hours mtbf. If you look at the materials you will find that they all will carry a thermal rating from NEMA of class F, 155 deg C. The safety agencies automatically derate these materials for there own insulation category purposes.
Actually using an exterior can, filled with anything but a thermally conductive potting compound will raise the hot spot temperature by between 10 and 20 deg C, after thermal equilibrium has been reached. On the plus side, there is almost no other way to acheive medical standards for surface creepage of voltage, for patient connected equipment. For audio purposes, it is unwarranted, except for SY's sexy looks comments.....
Just some more trivia to add to your transformer decisions.
Bud
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