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Old 5th June 2006, 04:33 PM   #21
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"Guys, can you point me to some direction how to use mains toroid with SE amp, i am about to make SE amp with GU-50.

"

What operating point will the GU-50 be at, what plate load will it need. What output power are you expecting.
Given that the GU-50 can deliver high output power and that you will need a toroidal of at least 4-6 times that power, you may well need a huge toroidal. If this is the case the high frequency response will probably suffer badly.

Shoog
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Old 5th June 2006, 05:03 PM   #22
Giaime is offline Giaime  Italy
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Hello Shoog, I would like to ask you a few questions since I know you have experience with toroidal OPTs.

From various sources, I read everything and its opposite.

- toroidals are bad because of high capacitance, big secondaries windings (wire diameter) that cause high-frequency losses.
- toroidals have very wide frequency response up to 100kHz, with no resonances in the audio band. This is in strong contrast.

- toroidals have low primary inductance. So big VA ratings are needed, and low driving impedance.
- toroidals are ideal in low frequency amplification in multiamplified systems. I think we're at the opposite ends here...

- toroidals can tollerate some small imbalanced DC current without any effect. So bias servos for PP amps using toroids are not needed, one just has to check the bias once a month or so.
- toroidals can never tolerate even a microampere of DC current without extreme losses in performance.

I think the truth lies in between, what about your experience?

I must add that I haven't made measurements yet, and I'm particulary oriented to PP using of toroidals, not parafeed.
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Old 5th June 2006, 05:56 PM   #23
gemby is offline gemby  Croatia
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Quote:
Originally posted by Shoog
"Guys, can you point me to some direction how to use mains toroid with SE amp, i am about to make SE amp with GU-50.

"

What operating point will the GU-50 be at, what plate load will it need. What output power are you expecting.
Given that the GU-50 can deliver high output power and that you will need a toroidal of at least 4-6 times that power, you may well need a huge toroidal. If this is the case the high frequency response will probably suffer badly.

Shoog
I would like to build something like this:

http://www.tubeland.de/images/gu50sc.gif

Or this:
http://www.jogis-roehrenbude.de/Lese...ltunggross.gif

I am pretty new with tubes, i made some solid state amps, but nothing too big or complicated, so i would like to made something proven
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Old 6th June 2006, 07:17 AM   #24
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Hi Giaime,

You ask all the relevant questions.
My experience so far with toroidals as output have been in two applications. The main one is in a 807 parafeed amp with the 110V tap used for ultralinear. They are about 120VA with 18V secondaries. I unwound two thirds of the secondaries to get the ratio I wanted. There is plate to plate feedback which keeps the output impedence well down. This allows you to drive the low primary inductance to low frequencies and I get response down to 20hz or lower. Using a high output impedence will produce worse results. The high frequency response is excellent and there is no roll off in the upper frequencies. A small amount of DC is tollerated because the UL arrangement works well, the 807 does draw unusually low screen current though. Overall in the parafeed arrangement they have worked extremely well.
I have also used them in a SE parafeed headphone amp using ECL82's with Plate to Plate feedback, but without UL. Again they have worked well, but the low frequency response isn't so good.

My conclusions would be;
To get low frequency response go for 10x the output power in VA rating.
The direction in which the windings are connected can have a big effect on the response. Connecting the primary in one direction produced low frequency roll off and a high frequency dip.
I have not found any issues with ringing. If there are unterminated windings ringing will become an issue.
Not all toroidals will work well, with high frequency roll off been the main victim.

It is early days with my experiements in toroidal PP's. So far I can report that the same toroidals work reasonably well. I am using 6080's which suffer from poor balance. The toroidals produce good high frequency response. In the lower frequencies they are satisfactory, but imbalance produces badly distorted low frequency waveforms which is especially noticable with square waves. Balancing eliminated this. This was achieved with seperate adjustable cathode resistors. If you do decide not to use a bias servo, seperate adjustable cathode resistors will produce better results.

I also am using toroidals as interstage phase splitters. This is where they have been most disapointing. Low frequency has been good, high frequency need a lot of current to drive the inter winding capacitance. I put this down to the 1:1+1 ratio. Step down ratios seem to work a lot better.

I would conclude by saying that it seems that all toroidals are not made equal. I have been lucky in having a good batch of toroidals which have performed really well.

I hope that helps, if you want clarification on any of those points - please ask.

Shoog
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Old 6th June 2006, 11:23 AM   #25
Giaime is offline Giaime  Italy
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Thank you very much Shoog, you have answered all my questions.

I'm planning to use toroids in a PP amp with EL36 in triode mode, as I understand they have a fairly low internal resistance, so I could be able to drive the transformers properly. Please check plate curves and typical characteristics (thank you Tom!!!):
http://www.tubes.mynetcologne.de/roe..._triode_v2.pdf

They will use fixed bias with a trimmer for each tube to balance currents.
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Old 6th June 2006, 12:56 PM   #26
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Quote:
[i]Originally post

To get low frequency response go for 10x the output power in VA rating.
[/B]

As i mentioned earlier, for low frequency Power, the VA rating is pretty much meaningless and going to a higher VA core will not increase the amount of voltage you can place across the primary.

I have also noted a direct relationship between primary inductance and VA rating and the smaller the core, the higher the inductance. This is with only maybe 10 samples or so, so i wouldn't consider it proof but rather a pattern.

If you consider a transformer with a fixed ratio from Full load to unloaded primary current (VA/line voltage)/ magnetizing current, as the VA rating goes down so must the no load current to keep the ratio the same. which requires an increase in inductance.

a simple test for this would simply be to measure the no load primary current of a few transformers and use 2piFL to detrmine the primary inductance. The transformer with the lowest current should have the most inductance.

dave
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Old 6th June 2006, 04:59 PM   #27
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So your conclusion is that smaller (to a point) would be better.

Shoog
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Old 6th June 2006, 06:04 PM   #28
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The formula for inductance of wound cores (N turns):
L = u0 *(Urelative) * (N*N) * (Core_Area) / (Core_length)

Voltage rating of winding (N turns):
V = 2*Pi*N*(Core_Area)*(Bmax)*(freq.)

If one doubles all linear dimensions of the core, core area quadruples, so the original N turns can be reduced by 4x for the same voltage rating.

Core length also doubles, so new inductance L' now becomes:

L' = u0 * (Urelative) * (N/4 *N/4) *(Core_Area *4) / (Core_length*2)

L' = L/8

Conclusion: Use a smaller xfmr for more inductance or use one with a higher voltage rating.

Don
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Old 22nd November 2006, 02:03 PM   #29
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How about a parafeed with a choke in a cathode circuit and a power transformer as output one? Does it solve a problem of impedance matching?
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Old 22nd November 2006, 03:12 PM   #30
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Choke in the anode would be better. Finding the right choke to set the bias would difficult, or you would need supplementary fixed bias to set it right. Also you would need to bypass the choke for AC.

I have got my 807 parafeed setup with Microwave Oven Transformers as the anode choke load. Works well - but not as good as a CCS.

Shoog
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