Toroids as O/P trannies - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

Ed_Dinning · 21st Jan 2007, 9:53 pm

Hi Folks, solid cores, as in lump of iron, would be increadibly lossy due to eddy currents. However, a ferrite has distributed "air gaps" due to its construction. The drawback is that it will only operate at 250/300mT, not the 1.5T of laminated cores (audio OP trannys operate at 1/3 of this for better linearity).
The main reasons for the air gap on OP trannys is to reduce saturation effects due to the DC flowing in the valve anode circuit in a Class A stage.
In Clas B/ push-pull the DC cancels out in the 2, 1/2 primaries and an airgap is not needed.

Ed

boiss · 22nd Jan 2007, 1:05 pm

OK Guys, Let me pickup a few points here. While 'each' winding on the transformer will not be wound over the entire core there will be an increase in Inductive leakage. However there will also be a decrease in inter winding capacitance because of the physical separation (Lateral on the core). If as stated energy is not transferred via the core at >10KHz then the coupling between primary and secondary will be abysmal. In practice I could obviously rewind the secondary over the primary should this be the case. I will wind it with a UL tap at 40% and this winding again will be sectioned laterally from the other 60%. What is the reason for the ratio of the UL tap effectively changing at higher frequencies? The open load impedance of the transformer will present a reasonable match to the valve at 20Hz without going over the top at higher frequencies hopefully such that there will be enough magnetising current at low volume levels to allow an undistorted transformer function.
I hope to get the wire tomorrow so that I can start winding and run some tests at the weekend with a bit of luck.
Anymore thoughts/comments very welcome especially before I start with the shuttle.
Thanks,Mike.

ppppenguin · 22nd Jan 2007, 1:22 pm

You can see much of Blumlein's ultralinear patent on line here:

http://www.european-patent-office.or...dos_search.htm

Do a search by number and enter GB496,883 in the search box. The comma is essential. Also tick the "family" box.

There is no doubt that Blumlein invented this technique and there are some reasonable explanations of why it's a good idea.

oldeurope · 22nd Jan 2007, 4:51 pm

Quote:

Originally Posted by ppppenguin

You can see much of Blumlein's ultralinear patent on line here:

http://www.european-patent-office.or...dos_search.htm

Do a search by number and enter GB496,883 in the search box. The comma is essential. Also tick the "family" box.

There is no doubt that Blumlein invented this technique and there are some reasonable explanations of why it's a good idea.

Hi Jeffrey, thanks for your attachement. I am not able to get it in the search box of the patent office.

In your attached text I can not find anything about ultralinear. Blumlein is not talking about ultralinear there. Maybe in the following text?
What I found is 1. Protection for the output transformer
2. Reducing the output Impedance to get any value between pentode and triode output.

Who tells that this is ultralinear? What is ultralinear? The voltage, the current or what

?

Hi Mike,
back to the output tranfrormer. The primary windings are not at the same place. The inner windings have another inductivity than the outer and the inductivity of the windings close the the secondary have another inductivity than the windings far away. The coubling of the primary windings to the secondary is not the same for all primary windings, the distance for example makes a change. The interleaving tries to reduce this (successfully!). This causes a change of the voltage ratio of high audio frequencies between the tappings. Conclusion, the UL circuit multiplies the mistakes of the output transformer. It reduces the output power too. Do you want this?

Kind regards,
Darius

ppppenguin · 22nd Jan 2007, 5:09 pm

I tried it and the EPO link doesn't work properly

I can't find a way to link to the correct search page. I had to hunt around the EPO site to find the correct search page.

I don't know who first used the word ultralinear. Blumlein certainly described the method but he did not use the word.

Ed_Dinning · 22nd Jan 2007, 8:22 pm

Hi Gents, just to throw in some more winding theoretical techniques.
Use very thin foil as the turns as it covers the entire width of the "window".
But on a more practical note (used in some switchmode PSU's) use a one turn transformer ~ this consists of a multitude of single turns all very closely coupled, actually interwound in a bifilar manner (watch the insulation resistance). They are then externally connected in series and parallel to give the turns ratio required. The practicalities of doing this for a UL, push pull OP trans are huge, but I have seen it work on SMPS at 500KHz with ferrites. The leakage inductance was under 0.1% of the primary inductance as bridge measured and evidenced by the overswing on the switching device.
Note that ferrites cores are still considered necessary in SMPS at 100KHz. They will work without but are much, much larger. I believe this also to be the case with audio transformers. It could however be that the energy being transfered at these frequencies is much lower and there is little AUDIBLE effect when the core is removed.
Any thoughts gents?

Ed

boiss · 23rd Jan 2007, 12:15 am

There is quite a lot of info about Blumlein over the years that he was involved in audio amps. I believe that the name ultralinear was put forward by a well known Amp/Valve manufacturer around 195*, It was also referred to as Distributed Load (maybe by Blumlein and certainly in some magazines) which it certainly isn't as far as normal DL amps are concerned. (Broadband high gain/power). In fact what is happening is a form of negative feedback, producing a sort of halfway house between a Pentode/Tetrode and a Triode.
Because my amp will be a bit of a test bed I want to be able to try all 3 operation modes more or less at the turn of a switch.

As for power transfer, if the amp frequency response is flat then the O/P power will remain constant.

One thing that I have found with toroids at 50Hz is that the loaded O/P did not change no matter how I arranged the secondary winding, whether the turns were more or less adjacent to each other or spread out over half of the ring. This makes it very easy to add another winding for a new voltage in a power supply providing that the total VA rating is not exceeded. A good example was to use a 220V to 110V transformer (80VA) for a 1920s battery valve radio. The 110V O/P being rectified smoothed and regulated, there remained the problem of the 2V 1A heater supply. Simple, 30 turns wound on to the toroid and a rect/reg circuit. Sorry for the digression, I saw some stuff posted by a certain Igormak on DIY asylum regarding toroids as O/P transformers but couldn't view the pics. Interesting stuff, It appears that there are a lot of impoverished enthusiasts around that can't spend big bucks on exotic trannies. How does basically flat from 10Hz to 150KHz grab you with the resonance at 250KHz sorted out, all from a 10$ toroid. Ok It was only 3W. Away with the Lowthers I'll have to get a new driver for my 1925 BTH horn!!!!!!!!!!!
Cheers, Mike.

boiss · 23rd Jan 2007, 12:32 am

Hi Ed, The foil must obviously be insulated on one side otherwise it would constitute a shorted turn. As for bifilar or multifilar hand winding on a toroid, I reckon it would be a nightmare, It was bad enough unwinding multifilar windings. It could be OK for the secondary though instead of using a large dia single wire.
On the one SMPS ferrite tranny I have unwound, the multifilar winding was on a nice simple bobbin, easy peasy!
Mike.

oldeurope · 23rd Jan 2007, 9:46 am

Good morning,

Hi Ed, the working conditiones for an audio output transformer and an switched mode power supply transformer are very different. It is like using an smps transformer at VHF.

Hi Mike, did you measure the frequency response with or without negative feedback? Until now I did not find a (totoid) mains transformer that makes me happy using it as an audio transformer.

Kind regards,
Darius

boiss · 23rd Jan 2007, 12:35 pm

Hi Darius, No It wasn't me, I found a couple of posts from different sites, both were using small cheap power toroids in PP and the O/P levels were around 3W. Both quoted 3dB bandwidths of better than 20Hz to 100KHz and one was 10Hz to 150KHz. I would assume that there was quite heavy NFB in order to achieve these results and although not stated I would presume it was into a purely resistive load.
Cheers, Mike.

oldeurope · 30th Jan 2007, 12:55 pm

Quote:

Originally Posted by boiss

Hi Darius, No It wasn't me, I found a couple of posts from different sites, both were using small cheap power toroids in PP and the O/P levels were around 3W. Both quoted 3dB bandwidths of better than 20Hz to 100KHz and one was 10Hz to 150KHz. I would assume that there was quite heavy NFB in order to achieve these results and although not stated I would presume it was into a purely resistive load.
Cheers, Mike.

Hi Mike,
I tested an toroid mains transformer in my amp.

https://www.vintage-radio.net/forum/...t=13624&page=3

I used my transformers as a choke and connected the toroid via an 47uF Electrolytic between GND and plate. I got -3dB at 90KHz !

On the other hand, it will be less if you take much more windings. This is neccessary to get a higher induktivity.

Kind regards,
Darius

boiss · 31st Jan 2007, 12:45 am

Hi Darius, That sounds very promising, -3dB @ 90KHz. What was the VA rating of the transformer? No NFB?
Still not got the wire, I went on Friday and they told me I had to bring some scales in order to weigh the wire, will go back again on Friday.
Cheers,
Mike.

oldeurope · 31st Jan 2007, 8:40 am

Quote:

Originally Posted by boiss

Hi Darius, That sounds very promising, -3dB @ 90KHz. What was the VA rating of the transformer? No NFB?
Still not got the wire, I went on Friday and they told me I had to bring some scales in order to weigh the wire, will go back again on Friday.
Cheers,
Mike.

Hi Mike, no negative feedback. It is a 160VA (50-60Hz hi hi) made in the 1980th. A few years ago I tested a small one as a line level transformer. It was a desaster. The more power the toroid can handle, the less windings are neccessary and the higher is the breakfrequency, it seems. On the other hand I noted the signal more destored, but I'll measure it out and tell you the result.

Kind regards,
Darius

oldeurope · 31st Jan 2007, 6:25 pm

Hi, I worked it out.
I loaded both transformers with a load that gives the same resistance at the plate. 8 Ohms for the EI and 18 Ohms for the toroid mains transformer (2 x 20V in parallel as secondary)
At a plate AC level that gives 5% after the EI with air gap,
I got 6,5% with the toroid. Connecting the toroid in parallel to the "choke" without a coubling cap gives 10%. In this case the major part of the DC component goes over the toroid. Because of the low account of windings the toroid mains transformer (160VA) has a DC resistance of unbeleveable 8,7 Ohms against 180 Ohms for the EI.
At low levels it gets more and more dramatic. 0,5% EI and 1% toroid...

The reason is at low levels the triode gets more linear and the component of distortion caused by the output transformer is constant?!?

Conclusion:
The toroid gives a high efficency and a high HF breakpoint with non interleaved windings (mains transformer). The HF breakpoint is higher with more VAs. Try out!

The missing gap causes a nonlinearity, negative feedback is definately neccessary.
DC components must be eleminated or compensatet. A low DC component generates a big destortion.

(For me now it makes sense to use a small air or paper gap in push pull too.)

Thanks for the suggestiones in this thread, I learned much.

Kind regards,
Darius

boiss · 31st Jan 2007, 11:55 pm

Hi, Interesting results, one thought about the distortion, did you try it with just one output winding and not the 2 in parallel? What was the distortion or what did the waveform look like and did the distortion change with frequency?
From your description the toroid was effectively in parallel with the EI at all times, ie directly or with a DC blocking C (the supply rails being a low impedance path for the signal) and not as the only load for the valve.
My 300VA transformer measured slightly under 4 Ohms, when it is rewound it should be slightly under 5 Ohms and be around 100H.
Mike

boiss · 1st Feb 2007, 12:12 am

Darius, a couple of further points, running the two transformers in parallel would have some odd results regarding phasing due to the different magnetizing path lengths and characteristics, the second is that with the toroid the power output would have been just over 10% greater if I'm not mistaken for the same AC input.
Mike.

oldeurope · 1st Feb 2007, 9:36 am

Good morning Mike,
here some answers to your questions:

"Hi, Interesting results, one thought about the distortion, did you try it with just one output winding and not the 2 in parallel?"

I did it with both output windings in parallel.

"What was the distortion or what did the waveform look like and did the distortion change with frequency?"

I measured THD+N at 1KHz.

"From your description the toroid was effectively in parallel with the EI at all times, ie directly or with a DC blocking C (the supply rails being a low impedance path for the signal) and not as the only load for the valve."

I did the test with and without DC blocking C. I thaught I described this.

"My 300VA transformer measured slightly under 4 Ohms, when it is rewound it should be slightly under 5 Ohms and be around 100H."

This depends in the output stage and the output power. My amp needs 2K load. Is 100H neccessary? This takes much windings and reduces the HF breakpoint.

"Darius, a couple of further points, running the two transformers in parallel would have some odd results regarding phasing due to the different magnetizing path lengths and characteristics,"

Yes, I agree. I didn't run them in parallel. this is not possible and makes no sense to me.

"the second is that with the toroid the power output would have been just over 10% greater if I'm not mistaken for the same AC input."

Yes, the effeciency of the toroid is better.
At full output power the difference between both is not that great because most destortion comes from the valve in this case.
The distortion problem takes more place at lower levels. So the effeciency is not that importand for me. I only want to have the destortion caused by the triode / triodelington to get the characteristical triode sound. This is why I don't make negative feedback. In an pentode amp (one EL84 or EL12, not push pull) with negative feedback a toroid output transformer and DC feeding with a separate choke makes much sense to me. I'll try this some time.

Kind regards,
Darius

oldeurope · 1st Feb 2007, 12:57 pm

Quote:

Originally Posted by boiss

OK, I have looked at this 300VA tranny and the tpv is approx 3, ie. with ten turns wound on the output was 3.24V. I was wrong about the secondary, it's not quadrifilar but sextofilar with wire around 0.8mm dia. The DC resistance of the primary is 4 Ohms. After doing some tests with a series resistor on the primary, I found that the inductance of the transformer off load is of the order of 73H, this would appear to be a pretty good ballpark figure for an Audio O/P transformer. The resistance is also very low at 4 Ohms.
Each turn on the toroid would be a tad under 8" so if a 12" long shuttle was used it would only need a third of the total number of turns on it.
As for the windings, if two separate primary windings are put on the toroid with each occupying only 180 degrees of the toroid they will be more matched than by putting one on top of the other.
Any suggestions?
Mike.

Hi Mike,
I don't beleve in your induktivity measurement. I tried to measure my 160VA toroid. At 220V input the current is destored very much and its value can not be used.
I reduced the primary voltage so that the waveform across the 1K shunt is nearly sinusodial. This is at 70VAC at primary. See what Jeffrey wrote in #34 !!!
An M core mains transformer without gap for example makes not that much destortion.
Testing the EI with gap gave a perfect sinwave voltage drop across the shunt at 260VAC input!.
70VAC 50Hz gives 2mA and the induktivity at50Hz is 111H.

But look what I got at 1KHz, only 4H. Parallelresonance?

The toroid makes much much more distortion than the EI because the lack of a gap. My oppinion:Forget toroid transformers as output transformers.

Kind regards,
Darius

boiss · 1st Feb 2007, 1:30 pm

Darius, In order to measure the inductance I used 2 series resistors, Unfortunately I can't repeat it because I have unwound it but the result was the same with each resistor at around 70H.
Mike.

boiss · 5th May 2007, 10:54 pm

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.

21st Jan 2007, 9:53 pm	#41
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Toroids as O/P trannies Hi Folks, solid cores, as in lump of iron, would be increadibly lossy due to eddy currents. However, a ferrite has distributed "air gaps" due to its construction. The drawback is that it will only operate at 250/300mT, not the 1.5T of laminated cores (audio OP trannys operate at 1/3 of this for better linearity). The main reasons for the air gap on OP trannys is to reduce saturation effects due to the DC flowing in the valve anode circuit in a Class A stage. In Clas B/ push-pull the DC cancels out in the 2, 1/2 primaries and an airgap is not needed. Ed

22nd Jan 2007, 1:05 pm	#42
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies OK Guys, Let me pickup a few points here. While 'each' winding on the transformer will not be wound over the entire core there will be an increase in Inductive leakage. However there will also be a decrease in inter winding capacitance because of the physical separation (Lateral on the core). If as stated energy is not transferred via the core at >10KHz then the coupling between primary and secondary will be abysmal. In practice I could obviously rewind the secondary over the primary should this be the case. I will wind it with a UL tap at 40% and this winding again will be sectioned laterally from the other 60%. What is the reason for the ratio of the UL tap effectively changing at higher frequencies? The open load impedance of the transformer will present a reasonable match to the valve at 20Hz without going over the top at higher frequencies hopefully such that there will be enough magnetising current at low volume levels to allow an undistorted transformer function. I hope to get the wire tomorrow so that I can start winding and run some tests at the weekend with a bit of luck. Anymore thoughts/comments very welcome especially before I start with the shuttle. Thanks,Mike.

22nd Jan 2007, 5:09 pm	#45
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Toroids as O/P trannies I tried it and the EPO link doesn't work properly I can't find a way to link to the correct search page. I had to hunt around the EPO site to find the correct search page. I don't know who first used the word ultralinear. Blumlein certainly described the method but he did not use the word.

22nd Jan 2007, 8:22 pm	#46
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Toroids as O/P trannies Hi Gents, just to throw in some more winding theoretical techniques. Use very thin foil as the turns as it covers the entire width of the "window". But on a more practical note (used in some switchmode PSU's) use a one turn transformer ~ this consists of a multitude of single turns all very closely coupled, actually interwound in a bifilar manner (watch the insulation resistance). They are then externally connected in series and parallel to give the turns ratio required. The practicalities of doing this for a UL, push pull OP trans are huge, but I have seen it work on SMPS at 500KHz with ferrites. The leakage inductance was under 0.1% of the primary inductance as bridge measured and evidenced by the overswing on the switching device. Note that ferrites cores are still considered necessary in SMPS at 100KHz. They will work without but are much, much larger. I believe this also to be the case with audio transformers. It could however be that the energy being transfered at these frequencies is much lower and there is little AUDIBLE effect when the core is removed. Any thoughts gents? Ed

23rd Jan 2007, 12:15 am	#47
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies There is quite a lot of info about Blumlein over the years that he was involved in audio amps. I believe that the name ultralinear was put forward by a well known Amp/Valve manufacturer around 195*, It was also referred to as Distributed Load (maybe by Blumlein and certainly in some magazines) which it certainly isn't as far as normal DL amps are concerned. (Broadband high gain/power). In fact what is happening is a form of negative feedback, producing a sort of halfway house between a Pentode/Tetrode and a Triode. Because my amp will be a bit of a test bed I want to be able to try all 3 operation modes more or less at the turn of a switch. As for power transfer, if the amp frequency response is flat then the O/P power will remain constant. One thing that I have found with toroids at 50Hz is that the loaded O/P did not change no matter how I arranged the secondary winding, whether the turns were more or less adjacent to each other or spread out over half of the ring. This makes it very easy to add another winding for a new voltage in a power supply providing that the total VA rating is not exceeded. A good example was to use a 220V to 110V transformer (80VA) for a 1920s battery valve radio. The 110V O/P being rectified smoothed and regulated, there remained the problem of the 2V 1A heater supply. Simple, 30 turns wound on to the toroid and a rect/reg circuit. Sorry for the digression, I saw some stuff posted by a certain Igormak on DIY asylum regarding toroids as O/P transformers but couldn't view the pics. Interesting stuff, It appears that there are a lot of impoverished enthusiasts around that can't spend big bucks on exotic trannies. How does basically flat from 10Hz to 150KHz grab you with the resonance at 250KHz sorted out, all from a 10$ toroid. Ok It was only 3W. Away with the Lowthers I'll have to get a new driver for my 1925 BTH horn!!!!!!!!!!! Cheers, Mike.

23rd Jan 2007, 12:32 am	#48
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies Hi Ed, The foil must obviously be insulated on one side otherwise it would constitute a shorted turn. As for bifilar or multifilar hand winding on a toroid, I reckon it would be a nightmare, It was bad enough unwinding multifilar windings. It could be OK for the secondary though instead of using a large dia single wire. On the one SMPS ferrite tranny I have unwound, the multifilar winding was on a nice simple bobbin, easy peasy! Mike.

23rd Jan 2007, 9:46 am	#49
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: Toroids as O/P trannies Good morning, Hi Ed, the working conditiones for an audio output transformer and an switched mode power supply transformer are very different. It is like using an smps transformer at VHF. Hi Mike, did you measure the frequency response with or without negative feedback? Until now I did not find a (totoid) mains transformer that makes me happy using it as an audio transformer. Kind regards, Darius

23rd Jan 2007, 12:35 pm	#50
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies Hi Darius, No It wasn't me, I found a couple of posts from different sites, both were using small cheap power toroids in PP and the O/P levels were around 3W. Both quoted 3dB bandwidths of better than 20Hz to 100KHz and one was 10Hz to 150KHz. I would assume that there was quite heavy NFB in order to achieve these results and although not stated I would presume it was into a purely resistive load. Cheers, Mike.

31st Jan 2007, 12:45 am	#52
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies Hi Darius, That sounds very promising, -3dB @ 90KHz. What was the VA rating of the transformer? No NFB? Still not got the wire, I went on Friday and they told me I had to bring some scales in order to weigh the wire, will go back again on Friday. Cheers, Mike.

31st Jan 2007, 6:25 pm	#54
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	linearity of the toroid makes me not happy, missing gap Hi, I worked it out. I loaded both transformers with a load that gives the same resistance at the plate. 8 Ohms for the EI and 18 Ohms for the toroid mains transformer (2 x 20V in parallel as secondary) At a plate AC level that gives 5% after the EI with air gap, I got 6,5% with the toroid. Connecting the toroid in parallel to the "choke" without a coubling cap gives 10%. In this case the major part of the DC component goes over the toroid. Because of the low account of windings the toroid mains transformer (160VA) has a DC resistance of unbeleveable 8,7 Ohms against 180 Ohms for the EI. At low levels it gets more and more dramatic. 0,5% EI and 1% toroid... The reason is at low levels the triode gets more linear and the component of distortion caused by the output transformer is constant?!? Conclusion: The toroid gives a high efficency and a high HF breakpoint with non interleaved windings (mains transformer). The HF breakpoint is higher with more VAs. Try out! The missing gap causes a nonlinearity, negative feedback is definately neccessary. DC components must be eleminated or compensatet. A low DC component generates a big destortion. (For me now it makes sense to use a small air or paper gap in push pull too.) Thanks for the suggestiones in this thread, I learned much. Kind regards, Darius Last edited by oldeurope; 31st Jan 2007 at 6:32 pm. Reason: Text

31st Jan 2007, 11:55 pm	#55
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies Hi, Interesting results, one thought about the distortion, did you try it with just one output winding and not the 2 in parallel? What was the distortion or what did the waveform look like and did the distortion change with frequency? From your description the toroid was effectively in parallel with the EI at all times, ie directly or with a DC blocking C (the supply rails being a low impedance path for the signal) and not as the only load for the valve. My 300VA transformer measured slightly under 4 Ohms, when it is rewound it should be slightly under 5 Ohms and be around 100H. Mike

1st Feb 2007, 12:12 am	#56
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies Darius, a couple of further points, running the two transformers in parallel would have some odd results regarding phasing due to the different magnetizing path lengths and characteristics, the second is that with the toroid the power output would have been just over 10% greater if I'm not mistaken for the same AC input. Mike.

1st Feb 2007, 9:36 am	#57
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: Toroids as O/P trannies Good morning Mike, here some answers to your questions: "Hi, Interesting results, one thought about the distortion, did you try it with just one output winding and not the 2 in parallel?" I did it with both output windings in parallel. "What was the distortion or what did the waveform look like and did the distortion change with frequency?" I measured THD+N at 1KHz. "From your description the toroid was effectively in parallel with the EI at all times, ie directly or with a DC blocking C (the supply rails being a low impedance path for the signal) and not as the only load for the valve." I did the test with and without DC blocking C. I thaught I described this. "My 300VA transformer measured slightly under 4 Ohms, when it is rewound it should be slightly under 5 Ohms and be around 100H." This depends in the output stage and the output power. My amp needs 2K load. Is 100H neccessary? This takes much windings and reduces the HF breakpoint. "Darius, a couple of further points, running the two transformers in parallel would have some odd results regarding phasing due to the different magnetizing path lengths and characteristics," Yes, I agree. I didn't run them in parallel. this is not possible and makes no sense to me. "the second is that with the toroid the power output would have been just over 10% greater if I'm not mistaken for the same AC input." Yes, the effeciency of the toroid is better. At full output power the difference between both is not that great because most destortion comes from the valve in this case. The distortion problem takes more place at lower levels. So the effeciency is not that importand for me. I only want to have the destortion caused by the triode / triodelington to get the characteristical triode sound. This is why I don't make negative feedback. In an pentode amp (one EL84 or EL12, not push pull) with negative feedback a toroid output transformer and DC feeding with a separate choke makes much sense to me. I'll try this some time. Kind regards, Darius

1st Feb 2007, 1:30 pm	#59
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies Darius, In order to measure the inductance I used 2 series resistors, Unfortunately I can't repeat it because I have unwound it but the result was the same with each resistor at around 70H. Mike.

5th May 2007, 10:54 pm	#60
boiss Pentode Join Date: Jun 2006 Location: Malaga, Spain. Posts: 235	Re: Toroids as O/P trannies 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.