Toroids as O/P trannies - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

oldeurope · 7th May 2007, 7:38 am

Quote:

Originally Posted by boiss

Ok, did some testing on my newly wound toroid. I wanted to get some idea of the inductance but have only a scope, so I made a little hartley osc with a BC177. With some caps across the transformer I got a resonant freq of 28Hz giving me an inductance of 45H. Removing the caps the resonant frequency rose to about 60KHz giving a total capacitance in circuit of about 20pF, this includes the winding capacitance, the series 10pF in the base circuit, the collector capacitance and the scope probe capacitance.
Wire is 0.8mm,The winding resistance anode to anode is 40 Ohms.
I'm not an expert on transformers so what are your thoughts?
Mike.

Hi Mike,
the 60KHz seems strange to me. Maybe it is caused by the 10pF in the
base circuit. But since big toroid mains transformers work well in pp amps,
(GU50, PL5XY)
I think yours with much more windings will do it much better, I wish.

Good luck,

Darius

PS: Let us see some pics of the rewinding, thanks.

boiss · 7th May 2007, 5:45 pm

Hi Old Europe and all,
Made a mistake in the calcs, haven't used a hand calculator in yrs. I ran it on an online calculator a few minutes ago and with 22pF the res freq would be only 5KHz, at 60KHz the capacitance would have to be 0.15pF, an impossible figure.
At 28Hz with 0.69uF the 45H is correct. I have looked at specs on O/P transformers and the primary inductance ranges from 10H upwards to even 100H. From the calcs with 1H and 22pF resonance is at 33KHz and I don't think anyone would think of using a transformer with 1H inductance.
Something here is not correct!!!
The oscillator circuit I used drew about 2mA so that should not have caused any changes in inductance.
What did you think was strange with the 60KHz resonant frequency?
Mike

PJL · 9th May 2007, 10:26 am

Hi, I am no expert but I am pretty sure the winding capacitance occurs throughout the winding and therefore willl not contribute to resonance.

boiss · 9th May 2007, 6:17 pm

Hi PJL, the capacitance is between each turn and each layer of turns/windings etc, this will result in a very complex series and parallel capacitance arrangement and will have a bearing on the resonant frequency of the transformer. In this case there are some strange effects at play which I don't understand and don't make much sense either when professional transformers specs are consulted.
Mike.

PJL · 10th May 2007, 8:34 am

Thanks Mike, therefore I am not too sure what you are measuring when you allow the open primary to resonate? Just the in circuit capacitance?

Not sure if you found this supplier http://www.amplimo.nl/index.html?lan...arget=d21.html but there are some detailed specs which might help confirm your design.

valveaudio · 11th May 2007, 9:38 pm

Hi

You might find this site. www.amplimo.nl/ interesting it is a Dutch company which makes toriod O/P transformers.

Trevor

boiss · 23rd May 2007, 11:19 am

OK thanks, will take a look, can't do anything much at the moment as I'm in the Middle East for 12 weeks.
Mike.

7th May 2007, 5:45 pm	#62
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies Hi Old Europe and all, Made a mistake in the calcs, haven't used a hand calculator in yrs. I ran it on an online calculator a few minutes ago and with 22pF the res freq would be only 5KHz, at 60KHz the capacitance would have to be 0.15pF, an impossible figure. At 28Hz with 0.69uF the 45H is correct. I have looked at specs on O/P transformers and the primary inductance ranges from 10H upwards to even 100H. From the calcs with 1H and 22pF resonance is at 33KHz and I don't think anyone would think of using a transformer with 1H inductance. Something here is not correct!!! The oscillator circuit I used drew about 2mA so that should not have caused any changes in inductance. What did you think was strange with the 60KHz resonant frequency? Mike

9th May 2007, 10:26 am	#63
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Toroids as O/P trannies Hi, I am no expert but I am pretty sure the winding capacitance occurs throughout the winding and therefore willl not contribute to resonance.

9th May 2007, 6:17 pm	#64
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies Hi PJL, the capacitance is between each turn and each layer of turns/windings etc, this will result in a very complex series and parallel capacitance arrangement and will have a bearing on the resonant frequency of the transformer. In this case there are some strange effects at play which I don't understand and don't make much sense either when professional transformers specs are consulted. Mike.

10th May 2007, 8:34 am	#65
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Toroids as O/P trannies Thanks Mike, therefore I am not too sure what you are measuring when you allow the open primary to resonate? Just the in circuit capacitance? Not sure if you found this supplier http://www.amplimo.nl/index.html?lan...arget=d21.html but there are some detailed specs which might help confirm your design.

11th May 2007, 9:38 pm	#66
valveaudio Hexode Join Date: Feb 2006 Location: Diss, Norfolk, UK. Posts: 386	Re: Toroids as O/P trannies Hi You might find this site. www.amplimo.nl/ interesting it is a Dutch company which makes toriod O/P transformers. Trevor

23rd May 2007, 11:19 am	#67
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies OK thanks, will take a look, can't do anything much at the moment as I'm in the Middle East for 12 weeks. Mike.