I wonder if you've already had discouraging advice from a commercial transformer company and now you are here looking for some alternative source? I can't direct you to anything I know of but it's not going to come from a tube amp hobbiest bit of kit. You have such different load and power requirements. To get 10M secondary with a 1:10 turns ratio you will have a primary of 100K. Your load will pull .2W... Nothing using such high voltage in the AF range would have those specs. I think you should get advice from a commercial builder for a specialized one-off tranny if your physics lab experiment is that important. Good luck.
Thanks a lot for all this useful information. I just contacted Lundahl for advice. My first try was a transformer from a microwave oven. It principally works, but the frequency is limited to ~300Hz. For a quick try, I gonna order one of these output transformers mentioned earlier and operate them over the secondary....let's see (luckily most of our samples have more than 100MOhm impedance as is, therefore, a very light load). As I have something to share, I will post it here 🙂
You might have some success investigating some oldskool TV flyback transformers that normally operate in the 15kHz range and can handle that voltage. The step up is quite high though and you won't need 150v to drive it to only 1.5kV. I don't know how one would handle 500Hz. Don't know what the input Z is for one.
You might also be able to salvage a flyback transformer from an old CRT type computer monitor for free or $5.
You will be lucky if they accept to make just one custom transformer for you. They even refused to change the air-gap on pair of their transformers I bought saying that they don't do this kind of custom service. Also Lundahls are not famous for being well behaved at high frequency and in most cases they need compensation network to get a target response.
Alex (50AE in this forum) makes whatever you want...within reasonable limits.
Be careful....I've done this before.
Many amplifiers, most perhaps, do not like the LCR load of a transformer used in this way and tend to resonate strongly with the right (or wrong) load.
Output capacitors don't make the issue better (or worse), DC coupled need to be very very good.
Many amplifiers, most perhaps, do not like the LCR load of a transformer used in this way and tend to resonate strongly with the right (or wrong) load.
Output capacitors don't make the issue better (or worse), DC coupled need to be very very good.
Sorry but I find your suggestion quite unsafe.
It might work "at home" in a non critical job , but 1000% overload, just as a "working" voltage is crazy dangerous.
If secondary voltage will be 1500V RMS it should pass a HiPot test of 5 to 10 kV.
Remember 1500-3000V is used for mere 120-240V primaries 😱
My suggestion for original poster: do NOT repeat NOT search for advice in Amateur Forums, even less a non Physics or High Voltage dedicated one.
Even less for a Forum known to suggest nonsense such as using 12V power transformers as OTs and similar quality advice.
Search for a Pofessional winder and have a suitable transformer built to needed spec, it is very doable. .
Agreed
JMF usually talks sense.
For a hi pot on domestic 230V primary isolation, its usual to test at 1.5kV, some manufacturers will use 2kV (especially for 3phase stuff: the derivation is from twice working voltage plus 1kV)
For a 1500V output, isolation would want to be tested at a minimum of (1.5×2) +1 = 4kV.
Megger Insulation resistance at 1000V then flash Hi pot at 5kV, retest insulation resistance at 1kV
I'd feel much happier about that.
JMF usually talks sense.
For a hi pot on domestic 230V primary isolation, its usual to test at 1.5kV, some manufacturers will use 2kV (especially for 3phase stuff: the derivation is from twice working voltage plus 1kV)
For a 1500V output, isolation would want to be tested at a minimum of (1.5×2) +1 = 4kV.
Megger Insulation resistance at 1000V then flash Hi pot at 5kV, retest insulation resistance at 1kV
I'd feel much happier about that.
There are some from Antek that do have the required secondary voltage rating. The 8T800 would do it with it’s secondaries in series. Multiple smaller units with their secondaries in series might work, but probably not as safe as one intended to operate at the full voltage. Power toroids like these usually work pretty well up to at least 10 kHZ, and often you get better depending on your loading. Higher impedance is better (the pole due to leakage reactance goes up in frequency). Neon sign transformers are another possibility, but I have my doubts about frequency response on those.
30kHz requires ferrite cored transformer probably. Something like a fly-back transformer but with only a 1:10 turns ratio. At 30kHz the load will be mostly capacitive and thus much less than 10M (just 100pF at 30kHz is about 50kohms, orders of magnitude lower than the load you think it is!). I would guess its unlikely there's something out there that's a good match to the requirements, a bespoke transformer is probably needed.
The self-capacitance of the windings will probably dominate any impedance calculations - that 10M load is effectively open-circuit at ac... You can't expect to see anything like as high as 100k input impedance as you might naively expect with 1:10 ratio and 10M load.
You may need all of the 300mA output current of that amp at the higher frequencies. This sort of design gets complicated as you can see.
A split-bobbin allows higher voltages to be reached with thin wire insulation (1.5kV is too high for standard magnet wire) - if you have N bobbin sections, each only sees 1/N times the overall winding voltage. It also reduces capacitance due to the larger gaps between sections.
And you have to worry about saturation at the lower end of the frequency range, so you'll need to know the ferrite parameters and be happy calculating with magnetics.
So in summary you probably something with maybe a 50VA rating, 1:10 ratio, split bobbin secondary, ferrite core, free from saturation down to 100Hz or so would be wise, frequency response flattish to above 30kHz.
Worth seeing if any ferrite manufacturers have transformer design tools online?