Hi,
I'm currently window shopping parts for for an upcoming 300B SET project. I already have the tubes, output transformers and interstage transformers but no suitable mains transformer.
I recently used some transformers from the (seemingly rather unknown) Italian brand Piemme in a project and I'm quite happy with the price/quality ratio of their iron.
They have a few mains transformers that would suit my needs and this one in particular would be perfect, with its high-ish voltage bias winding, high current heater winding for the damper diodes I intend to use as rectifiers (in a hybrid bridge) and a HV winding with several taps that would let me to adjust the B+:
What bothers me is the CT on the HT winding. Using this transformer with a FWCT rectifier wouldn't produce anywhere near the voltages I need (~400VDC) but in theory it would be simple enough to just ignore the CT and use the full winding with a hybrid Graetz rectifier bridge.
The question is: Would in be safe? Not grounding the CT must put more strain on the inter-winding insulation, am I right?
I guess the right thing to do is to ask the manufacturer, but I'd like to hear any opinions the good people here might have on this subject first.
I'm currently window shopping parts for for an upcoming 300B SET project. I already have the tubes, output transformers and interstage transformers but no suitable mains transformer.
I recently used some transformers from the (seemingly rather unknown) Italian brand Piemme in a project and I'm quite happy with the price/quality ratio of their iron.
They have a few mains transformers that would suit my needs and this one in particular would be perfect, with its high-ish voltage bias winding, high current heater winding for the damper diodes I intend to use as rectifiers (in a hybrid bridge) and a HV winding with several taps that would let me to adjust the B+:
What bothers me is the CT on the HT winding. Using this transformer with a FWCT rectifier wouldn't produce anywhere near the voltages I need (~400VDC) but in theory it would be simple enough to just ignore the CT and use the full winding with a hybrid Graetz rectifier bridge.
The question is: Would in be safe? Not grounding the CT must put more strain on the inter-winding insulation, am I right?
I guess the right thing to do is to ask the manufacturer, but I'd like to hear any opinions the good people here might have on this subject first.
It will be fine just rectifying it as a 360 volt winding using a 4 diode bridge. Even using a FWCT it still sees 510 volts peak end to end and has to be insulated for it. Plus margin. And heater windings are almost always on separate layers, with paper or tape between those and the other windings.
Thanks!
I wouldn't expect any troubles using this transformer with a bridge rectifier but it still bothers me a bit to use it in a way that it, possibly, is not intended for.
If I'm visualizing things correctly, the voltage between each end of the HT winding and the winding (or core) next to it must be doubled when used with a bridge rectifier rathed than with the CT grounded?
I just found out they have another transformer that is pretty much the same thing but with a slightly higher VA rating and a single 360V winding without CT or other taps. That one would work, but I'd miss the ability to tune the B+ a bit using different taps on the HT winding.
I wouldn't expect any troubles using this transformer with a bridge rectifier but it still bothers me a bit to use it in a way that it, possibly, is not intended for.
If I'm visualizing things correctly, the voltage between each end of the HT winding and the winding (or core) next to it must be doubled when used with a bridge rectifier rathed than with the CT grounded?
I just found out they have another transformer that is pretty much the same thing but with a slightly higher VA rating and a single 360V winding without CT or other taps. That one would work, but I'd miss the ability to tune the B+ a bit using different taps on the HT winding.
With CT one has more options. FWB or FWCT. No need to connect CT in a FWB or for safety.
More read here:
https://www.diyaudio.com/community/threads/full-wave-rectifiers-center-tapped-vs-bridged.239389/
More read here:
https://www.diyaudio.com/community/threads/full-wave-rectifiers-center-tapped-vs-bridged.239389/
The interwinding voltages won't change, they are what they are, determined by construction and mains voltage, independently what you do with them externally.
So no change there.
Jan
Trying to wrap my head around the last part. If the 100V winding is used to produce a -140VDC bias supply and the heater winding is grounded through its CT, shouldn't there be more stress on the insulation between the windings if the HT winding is rectified in a way that results in over 500VDC rather than 250VDC?
Perhaps I'm just visualizing things wrong, it surely wouldn't be the first time🙂
200VA, just over 100 Euros, and the shipping costs from Italy isn't too bad.
I was talking about the interwinding voltages of the secondary as that was what you were referring to.
If you mean the voltages between primary and secondary, then you are correct that that is depending on wjhat you do with the secondary.
Jan
If you mean the voltages between primary and secondary, then you are correct that that is depending on wjhat you do with the secondary.
Jan
The prim-sec insulation should be good for at least a few kV if the manufacturer knows what they are doing, so that part should be fine.
The whole question is probably a non-issue but I saw the center tap and remembered a discussion some twenty years ago with a guy who was building a 211 SET with a stacked PSU made from two 325-0-325V with hybrid bridge rectifiers.
That was an extreme case though.
Thanks for your replies!
The whole question is probably a non-issue but I saw the center tap and remembered a discussion some twenty years ago with a guy who was building a 211 SET with a stacked PSU made from two 325-0-325V with hybrid bridge rectifiers.
That was an extreme case though.
Thanks for your replies!
Transformers for 240 VAC operation are typically hipot tested for 1800VAC or so primary to core, secondary to secondary, and twice that primary to secondaries. Likely higher with a high-voltage secondary like this. Transformer maker can tell you - I'm sure a complete spec sheet would show it.