Hi guys,
I am building a tube preamp using either 6n2p-ev or ECC85 (not decided yet) in SRPP, so B+ current is very small, let's say, 75-100mA should be enough for two tubes, and heater currents are either ~340mA (6n2p-ev) or 435mA (ECC85), so let's round up to 1A for both. Plus, I'd like to use a tube rectifier EZ81 (in half-wave mode), it adds 1A more to the heater windings.
Here is the list of the transformers I am choosing from (a compromise between the quality, price and availability in EU) https://sklep.toroidy.pl/public/assets/Mains_for_tubes_ENG.pdf
The model 80/001 with 275V (0,12A) and 3,15 - 0 - 3,15V (7A) or even 80/002 with two heater windings fits perfect, but my concern is a size of a transformer, as it should fit into a limited space of the integrated amp (I'm replacing its low-quality build-in preamp) i.e. it's diameter might be <= 90mm. If going down to their 60/002 model (which is 86mm in diameter), it'll give only 3,15 - 0 - 3,15V (2,5A) (let's ignore its lower B+ atm).
So, my question is if currents in this list are already given provided a 50% power overhead or not? Does anybody have an experience with regards? I mean, according to all recommendation across the internet I need something capable to give at least 2A x 2 for heaters, but maybe these transformers are already good enough providing the real specs?
Or one can say no, only 80VA models, then I'll try to fit it, maybe, cutting off for these 5mm here or there =)
I am building a tube preamp using either 6n2p-ev or ECC85 (not decided yet) in SRPP, so B+ current is very small, let's say, 75-100mA should be enough for two tubes, and heater currents are either ~340mA (6n2p-ev) or 435mA (ECC85), so let's round up to 1A for both. Plus, I'd like to use a tube rectifier EZ81 (in half-wave mode), it adds 1A more to the heater windings.
Here is the list of the transformers I am choosing from (a compromise between the quality, price and availability in EU) https://sklep.toroidy.pl/public/assets/Mains_for_tubes_ENG.pdf
The model 80/001 with 275V (0,12A) and 3,15 - 0 - 3,15V (7A) or even 80/002 with two heater windings fits perfect, but my concern is a size of a transformer, as it should fit into a limited space of the integrated amp (I'm replacing its low-quality build-in preamp) i.e. it's diameter might be <= 90mm. If going down to their 60/002 model (which is 86mm in diameter), it'll give only 3,15 - 0 - 3,15V (2,5A) (let's ignore its lower B+ atm).
So, my question is if currents in this list are already given provided a 50% power overhead or not? Does anybody have an experience with regards? I mean, according to all recommendation across the internet I need something capable to give at least 2A x 2 for heaters, but maybe these transformers are already good enough providing the real specs?
Or one can say no, only 80VA models, then I'll try to fit it, maybe, cutting off for these 5mm here or there =)
75-100mA??
For example the plate current of 6N2P-EV is no more than 2 mA. ECC85 take some 5...7 mA.
From that list of Toroidy, even the smallest TSTA15/001 is sufficient for your project.
Yeah, but its 1.1A heater will be barely enough for two tubes, iff its "real world" rated power, not even considering to add a rectifier with its extra required 1A heater. As I said, let's ignore the B+ discussion for the moment.
Half-wave is not good for transformer utilization, especially a toroid. If you must have a tube rectifier, use a hybrid bridge with two SS diodes and EZ80 or 81. 2.5A or 3A heater will be plenty. (heater voltage will still be slightly above 6.3V)
You'll only be using about 20 VA; why think about 80 VA transformers?
You'll only be using about 20 VA; why think about 80 VA transformers?
It's not that I must, it's more that I'd like to try =) Now I am wondering, why is half-wave not good "especially" for a toroid? Well, it's not a big deal to add two diodes, but that's limiting now for only transformers with a middle point.
As I said, because everywhere I find a tip that the power of the transformer must be at least two times more the required consumption. Two srpp tube heaters give up to 1A current plus a rectifier's 1A means 2A doubled => 4A @ 6.3V secondary windings, if considering a recommended headroom (just my thinking out loud).
And this is exactly my question - if anybody has an experience with toroidy, how are they in this regards of having 50% power headroom? Are they already include this idea in their specs or not?
Please, understand me correctly, on the paper it's always all good, but in reality it's not, so I am searching for some real world experience. Otherwise I can do basic math on my own ;-)
Maybe TSL100/001?
EI84 core, 2x260V 0.12A, 2x3.15V 3A, 6.3V 2A, 5V 2A.
https://www.tme.eu/de/details/tsl100_001/transformatoren-mit-befestigung/indel/tsl-100-001/
EI84 core, 2x260V 0.12A, 2x3.15V 3A, 6.3V 2A, 5V 2A.
https://www.tme.eu/de/details/tsl100_001/transformatoren-mit-befestigung/indel/tsl-100-001/
Unfortunately, I didn't note any of those comments, but by just searching even here: https://www.diyaudio.com/community/threads/transformer-problem.48420/post-544719
Upd. just accidentely found this claim, which, I think, is a good point from other side as well https://acupwr.com/blogs/news/about-that-two-times-the-wattage-rule
idk
A transformer output voltage is rated at its rated current; it will be higher at lighter load. Small transformer might be 5% high at half load.
Transformer temperature is proportional to load current squared, so heating is 25% of maximum at half load. That's all the windings added together. And temperature is really the limiting factor for transformer reliability.
Half-wave rectification results in higher power supply ripple. at line frequency, so would need about 4X more filtering (double the ripple at half the frequency). Not good for a preamp. The net DC current makes an asymmetric AC current, and the higher peak current results in more heating in the winding and core for the same DC current. Maybe not enough to matter with 10 mA current, but the RMS (heating) current in the winding is cut in half with a full-wave bridge. A center-tapped winding can also be used for full-wave, but needs about 30% more transformer VA than a bridge for the same DC current. Again, not a problem at 10 mA.
Transformer temperature is proportional to load current squared, so heating is 25% of maximum at half load. That's all the windings added together. And temperature is really the limiting factor for transformer reliability.
Half-wave rectification results in higher power supply ripple. at line frequency, so would need about 4X more filtering (double the ripple at half the frequency). Not good for a preamp. The net DC current makes an asymmetric AC current, and the higher peak current results in more heating in the winding and core for the same DC current. Maybe not enough to matter with 10 mA current, but the RMS (heating) current in the winding is cut in half with a full-wave bridge. A center-tapped winding can also be used for full-wave, but needs about 30% more transformer VA than a bridge for the same DC current. Again, not a problem at 10 mA.
Thanks a lot for such a nice explanation. I think, I'll start with diodes and toroid relying on the VA specs provided (and, as correctly pointed out, on a low current consumption), and then just out of curiosity try to use a tube rectifier and at least take some measurements.
Well, as almost always, trying by doing seems to be the best answer =)