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Old 23rd April 2011, 07:58 PM   #1
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Default Edcor Output Transformer Resistance

Greetings everyone,
So, I seem to be at a loss, I have 4 Edcor 3.3KΩ/100W transformers. When I measure the resistance between the red and blue leads and then the red and brown leads, there seems to be a difference of 10-15 ohms. Is this by design or is this manufacturing tolerances?

Thanks,
Ray
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Old 23rd April 2011, 08:10 PM   #2
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What you measure is an inaccurate CT (red) caused by the winding scheme; typical for budget transformers.
However 10-15 ohms is pretty much.
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Old 23rd April 2011, 10:26 PM   #3
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What you're measuring is not necessarily an inaccuracy, but has to do with the wire length as the coil build-up becomes larger. The outer layers require more length per turn so the resistance increases. A high quality transformer will have progressively slightly larger wire size to compensate for this as the build-up expands. If you use low voltage AC on the secondary to measure each half of the primary, you'll probably find a balanced CT.
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Old 23rd April 2011, 11:20 PM   #4
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Originally Posted by HollowState View Post
A high quality transformer will have progressively slightly larger wire size to compensate for this as the build-up expands. If you use low voltage AC on the secondary to measure each half of the primary, you'll probably find a balanced CT.
There are other (and better) ways to ensure accurate CT's on push-pull transformers.
Measuring the primary through a voltage source on the secondary does not tell the whole story, as in this case there is no DC current flowing in the primary.
Inaccurate CT's will cause extra odd order harmonic distortion.
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Old 24th April 2011, 03:26 AM   #5
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High quality OTs use a split bobbin with equal resistance windups for each primary side. The cheaper ones (most) just use a single bobbin, so one side of the primary is wound at larger diameter than the other. So the resistances come out unequal unless different wire sizes are used to compensate. Good amp designs will then use an equalizing resistor in series with the lower R winding to equalize the sides. This equalization helps to reduce distortion (gain symmetry) but will increase losses and output Z a little.

Another factor to check for is unequal leakage inductance on the two primary sides. This causes some distortion at high frequencies. Typically the inner UL 40% winding sections are between secondaries and have the best coupling. The 60 % sections have a large difference in leakage L, since one is wound at the beginning of the bobbin and the other is wound on the outside.

This is one reason I like the "Elliptron" configuration, since each tube uses both of the good 40% sections, with the CFB taken off of one 40% section for each tube. Using the 40% taps for differential local feedbacks in the amp works out well too, since they are both always driven windings (each tube drives both). Whereas, the typical local plate "Schade" feedback scheme uses feedback points that have a large difference in leakage L and only one of them is driven at any time (in class AB). So the other feedback is really not local since it has to pass thru the xfmr.
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Last edited by smoking-amp; 24th April 2011 at 03:42 AM.
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Old 24th April 2011, 03:49 AM   #6
Matt BH is offline Matt BH  United Kingdom
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OK, I have seen the introduction of some series R to balance unequal primary DC resistance either side of the CT reduce distortion in small signal stuff (even this was debatable) but introducing a series resistance in a power application ie. an output stage is daft.

If the CT is truely not a CT then send it back.

I would love to see some measurements on a 100W output tranny with unequal primary DC RESISTANCE that has been balanced with some series R that show less distortion in this configuration.

Anyway, I have no experience of Edcor trannys and have only read good things about them. If the OP is not happy with his 3K3 trannys I am sure I can make use of them, send them this way for proper disposal

Cheers Matt.
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Old 24th April 2011, 03:53 AM   #7
Matt BH is offline Matt BH  United Kingdom
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Sorry Smoking Amp,
Seems we were posting in sync

So does the Eliptron work?

I have a couple of 120W 2K2 UL PP trannys that would be ideal with some GU50 and two 500V supplys.

Cheers Matt.
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Old 24th April 2011, 04:37 AM   #8
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The conventional Elliptron configuration is just a Circlotron with less cathode winding %. A 40% UL tap gives (.4/1.4 = .286 or 28.6 % CFB). No question it works, but requires two inconvenient floating power supplies. I use some Lambda power factor modules to get 360 V regulated from some split bobbin isolation xfmrs (low common mode capacitance). A big advantage is that Elliptron mode essentially doubles (1.4 squared) the primary impedance seen by the tubes. Some of those 10 K Ohm P-P xfmrs out there (for GU50 say) don't have any bandwidth, but a 5K P-P Elliptron'd (to 10K) OT is quite practical.

Things get more tricky when trying to get by with just one power supply in the Elliptron. I have just acquired some high bandwidth balancing inductors to try the cathode DC feed scheme. An issue comes up here concerning DC resistance of the inductor feeds, since they set up a time constant with the two floating power capacitors, which affects the tube biasing with class AB and signal evelope bursts. One can try to brute force solve that by using a low DC resistance balancing inductor to feed the two floating power caps, but this gets expensive to also keep the magnetizing current low (inductance high).

I'm exploring a servo circuit from a small negative supply to regulate the cap voltages and hence the tube biasing. So this is still under development. Although there is an alternate approach of using drivers with their plate loads bootstrapped from the output tube cathodes to effectively remove the bias shifts due to the floating power cap voltage variations. I have no doubt the single supply scheme can be made to work, its just a question of finding the most effective and cost effective approach.

----------

The balancing resistor for the OTs are typically only 15 Ohms or so as Rsumperl mentioned. Not a problem in kiloOhm level outputs. People put that much resistance in the cathode circuits just to measure idle current. Certainly depends on the standards one wants to achieve as to distortion too. The usual couple percent dist. amps won't notice any improvement from balancing R. 15 Ohms imbalance for 1000 Ohms per side (a 4K P-P OT) is a 1.5% gain error, that makes real distortion. One can try to null that out with an AC gain balance pot in the driver stage, but that only works for a fixed load impedance. Real speakers have varying Zload with frequency.
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Last edited by smoking-amp; 24th April 2011 at 04:58 AM.
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Old 24th April 2011, 11:01 AM   #9
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Quote:
Originally Posted by smoking-amp View Post
High quality OTs use a split bobbin with equal resistance windups for each primary side. The cheaper ones (most) just use a single bobbin, so one side of the primary is wound at larger diameter than the other. So the resistances come out unequal unless different wire sizes are used to compensate.
Split bobbins or two identical coils on a c-core are techniques to ensure accurate CT's.
But even with the single bobbin and the same wire size it is not that difficult to get an accurate CT.
How about this one to tease your brains: 1+4=2+3, or 1+4+5+8=2+3+6+7.
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Old 24th April 2011, 11:35 AM   #10
AJT is offline AJT  Philippines
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i have used 1+4 = 2+3, resistance is within 1 to 3 ohms of each other...

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Last edited by AJT; 24th April 2011 at 11:55 AM.
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