If I remember correctly FC-20S has max. 18H inductance.
It's not too much for #45 tube.
Hi Victor, That's great news about the choice of ISO FC-20S. I've just got a pair of these new FC-20S myself and I really like the new specs on the ISO(Tango). More than just the high bandwidth of 90kHz, the roll off at high-freqs in electrical terms is like a 2nd order linear phase, and smooth in phase domain(response) as well. This transformer will have minimal ringing at all and preserve the shape of the signal even though there's amplitude attenuation ie so music notes will have just lower sound but same tone.
Not to detract from your SET 45 project, ever thought of bringing a direct coupled - parallel single ended 300B? or some other high power triode 211/GM70/845 in future... with OEM transformers(to keep cost down) and option for high performance output transformers? A lot to ask... haha
Any consideration to the 6SN7 instead of 12AU7? Lower distortion and more linear. Linlai makes an outstanding 6SN7 that pairs very well with my EL45 tubes… both very expensive tubes, but new production and better than any NOS I’ve listened to by far. As a bonus look great with the EL45 tubes! (There is also an abundant choice of other, reasonably priced, very good new production as well as a fair stock of NOS)
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Sorry typo I meant a parallel single ended triode using 300B etc. I got the 211/845 from you a few weeks back.. nice kit
Victor, I like where you are going with this.
I don't know what choke and mains transformer you have in mind but if my search is correct FC-20S 27,000 Japanese Yen = $186.69 USD (I assume per unit, not per pair), doesn't seem too expensive to offer an upgraded copper top option for those who don't mind spending a little extra over the base ~ $2000 kit, no?
Sure would look nice and I'm perfectly ok with building my own wood base for the copper top ... just saying
Thank you Jac from Emission's valuable suggestion
Thank you MP9'comment...
ISO FC-20S is a 5k 80mA transformer. 80mA is relatively high current for this small core, at 5k load. Inductance is 17H only, which is fine for 2A3, but not for 45. This makes the low frequency behavior not so exciting at 5k. You can see the impedance of 5k, becomes 4k already at 100Hz. At the Ri of 1400…1800 Ohms of the tube, the output voltage will not drop so drastically, but it will. And from there it starts to drop.
This transformer fits better to 2A3 than to 45. With 2A3 you need less inductance, because we talk 2k5 only, and moreover the tube is only 700 Ohms Ri.
Lundahl LL1663-50mA is 5k as well, but it has 34H, so it will work better. It will give the same drop at 50Hz, as ISO FC-20S has at 100Hz. ISO has also transformers with more inductance, like the FC-40-5S, but this is a 40Watt type, and it’s much too oversized.
A solution would be, ISO builds an FC-20S with 50mA instead of 80mA. That will bring up the inductance up to 27H, and impedance curve improves a lot. All this needs is another air gap.
it is not a easy job to work on this project..
It requires your help.. to make it prefect and afforable
You know there is no prefect solution...It also involes a lot of trade off (price and sound quality)
At this time, Hashimoto H-20-7U and Lundah LL1663 will work ok with this project
Thank you MP9'comment...
ISO FC-20S is a 5k 80mA transformer. 80mA is relatively high current for this small core, at 5k load. Inductance is 17H only, which is fine for 2A3, but not for 45. This makes the low frequency behavior not so exciting at 5k. You can see the impedance of 5k, becomes 4k already at 100Hz. At the Ri of 1400…1800 Ohms of the tube, the output voltage will not drop so drastically, but it will. And from there it starts to drop.
This transformer fits better to 2A3 than to 45. With 2A3 you need less inductance, because we talk 2k5 only, and moreover the tube is only 700 Ohms Ri.
Lundahl LL1663-50mA is 5k as well, but it has 34H, so it will work better. It will give the same drop at 50Hz, as ISO FC-20S has at 100Hz. ISO has also transformers with more inductance, like the FC-40-5S, but this is a 40Watt type, and it’s much too oversized.
A solution would be, ISO builds an FC-20S with 50mA instead of 80mA. That will bring up the inductance up to 27H, and impedance curve improves a lot. All this needs is another air gap.
it is not a easy job to work on this project..
It requires your help.. to make it prefect and afforable
You know there is no prefect solution...It also involes a lot of trade off (price and sound quality)
At this time, Hashimoto H-20-7U and Lundah LL1663 will work ok with this project
Here is the update for the OUTput Transfomer
ISO FC-20S -> WE need IOS to lower the current
H-20-7U -> will not modfy the opt
Either ISO or H-20-7U is imprefect for 45 tube application unless they can modfy the opt.
I taked to Lundahl Last night
it seems that LL3720 is a good match. Mr Lundahl will make a 5K version for this project.
https://www.lundahltransformers.com/wp-content/uploads/datasheets/3720.pdf
From Lundahl
We are developing a new output transformer, LL3720 (6k : 8 ohms) specifically for single end application.By applying some tricks when winding the coils, the capacitive coupling between primary and secondary sections are minimized, which results in a better high frequency response. The transformer is suitable for triodes with plate resistance between 1 and 2 k, and thus matching DC currents from 30 – 40 mA up to 70 – 80mA. A data sheet for the LL3720 will follow shortly
ISO FC-20S -> WE need IOS to lower the current
H-20-7U -> will not modfy the opt
Either ISO or H-20-7U is imprefect for 45 tube application unless they can modfy the opt.
I taked to Lundahl Last night
it seems that LL3720 is a good match. Mr Lundahl will make a 5K version for this project.
https://www.lundahltransformers.com/wp-content/uploads/datasheets/3720.pdf
From Lundahl
We are developing a new output transformer, LL3720 (6k : 8 ohms) specifically for single end application.By applying some tricks when winding the coils, the capacitive coupling between primary and secondary sections are minimized, which results in a better high frequency response. The transformer is suitable for triodes with plate resistance between 1 and 2 k, and thus matching DC currents from 30 – 40 mA up to 70 – 80mA. A data sheet for the LL3720 will follow shortly