I feel your pain and share your concern. The two 120 volt primaries were intended to have 120 volts between them, not 240. The varnish can only do so much. How much can it take? And does a lot of mechanical stress (magnetostriction) make things worse? I guess building amp and subjecting it to ghetto bass for hours would be one way to find out. Far different use conditions than playing hi-fi quietly at home.I believe the 4kV hi-pot test is done between primary and secondary? I assume there is some kind of insulation tape between primary and secondary windings. What (hypothetically) bothers me is the risk for insulation breakdown between the primaries at large voltage swings. The primaries are usually bifilar wound, right? I believe you when you say your amps with interleaved toroids work fine, but this is still causing a bit of headache. Maybe it´s just me being paranoid here, if pri-pri insulation breakdown was a common thing noone would sell mains transformers with dual primary windings.
If you have unwound one of these you will not find any tape between primary and secondary, or that is my recollection from last time i did it.In order to comply with safety regulations, primary and secondary windings need a safe insulation barrier (thus more than varnish, typically a PETP film) between them. Depending on the regulations, two primaries might also need additional insulation means
Why would I want to build a 1 KW vacuum tube amp when THIS (cabinet still not finished) is the guitar amp I use most often. It makes a blazing 4 watts which is quite loud even in a 2000 square foot basement? I'm still questioning myself about that one as I work on it.Building an 100+ W all tube bass amp is one of the projects I have in the pipeline, for this I have already wound an output transformer (1,8k:4-8R) on a double C core from an old Siemens power supply. An ambitious project and a rather stupid one too, I already have at least three bass amps and the one I use the most is about the same size as that output transformer, weights a lot less and delivers 200W thanks to SMPS and class D technology. Might as well use my homemade OPT for a tube subwoofer amp instead...
In fact, I have, and there was an insulating layer present between the primary and secondaries. It could be that different regulations apply according to the country, but in Belgium another insulation layer is always required between mains and exposed parts. I think that according to EU regulations, the requirement has even become more stringent, requiring minimal thickness of insulating materials for example, but I am now out of the circuit, and I don't follow these developments closelyIf you have unwound one of these you will not find any tape between primary and secondary, or that is my recollection from last time i did it.
1000 maybe, but just 100+ isn’t impractical at all. It still a lot smaller/lighter than a speaker cabinet. The 200 watters started out as “the bass amp” but after listening to one for five minutes convinced me not to have it relegated to amateur-hour instrument amp duty. I ended up with a more practical 136 watts - but used standard off the shelf output trafo. The power trafo was home made (From a $15 donor core, end bells that were lying around, and maybe 5-10 bucks worth of magnet wire). All cheap off-voltage tubes - easy when winding your own power trafo. When trying to save money on transformers often the place to START is with the power trafo - solutions there are much simpler than the output trafo, even for large or very large amplifiers.Building an 100+ W all tube bass amp is one of the projects I have in the pipeline, for this I have already wound an output transformer (1,8k:4-8R) on a double C core from an old Siemens power supply. An ambitious project and a rather stupid one too
Would Tubelab consider such a thing? Well there are a pair of 600 ohm SE OPT's from Toroidy waiting for my cathode follower experiments, and a few big 650 volt SiC Jfets to try in place of the tubes. I'm thinking of building a two stage UNSET board with solid state parts which I have called FETSET. See this thread:Since we´ve been talking subwoofer tube amps lately, this one tickled my imagination a bit. 80R p-p is too low for a plate coupled amp but perhaps a sweep tube PP cathode follower or a Circlotron would work? Think "OTL assisted by a 3:1 transformer for slightly better impedance matching". OTOH, limit ourselves to tubes? Perhaps a pair of big mosfets with Schade-style local feedback and/or CFB from the 9,5V windings would work wonders here?