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Old 14th July 2009, 11:15 PM   #1
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Default Toroidal Output Tranformers

Hello all, I recently had the good luck to pick up a couple of Toroidal OPTs at a good price. I did some searching here, and it seems they are somewhat temperamental about DC balance/bias currents. A friend at another forum suggested I try the Bernig EA-230 circuit, due to it's low bias current. I thought it would be a good idea to ask the experts here, about that circuit, other circuits, and building with Toroidal outputs in general. I guess I'm somewhat apprehensive about building with sweep tubes (the 6JN6) that are used in that circuit. There's no real reason for this apprehension, it's just strange to me. The transformers in question are PP Tomiko units with a 5K primary Z. There is no watt rating on them, but from their size I would guess somewhere between 20 and 30 watts. Thank you all very much in advance for any and all discussion on this topic. Jay
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Old 14th July 2009, 11:22 PM   #2
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Bias current does not matter since it may be asymmetrically driven by signal.
Go for it, try, get your experience. The devil is not so scary as is often drawn on pictures.
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Old 15th July 2009, 12:27 AM   #3
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The bias current level itself,whether high or low,isn't really important (as long as it is within the transformers specs!).
What DOES matter is the balance,each 'half' of the primary must have the exact same current running through it,as any imbalance will magnetize the core. Even a couple of ma difference can affect transformer performance.
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Old 15th July 2009, 12:38 AM   #4
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Ah, I see what you mean. That brings up an interesting point that has me wondering. When I ohm out the primary of these, one side to center measures 38 ohms, while the other side measures 26. I've seen this in regular transformers and understand that it is the length of wire as one is wound on top of the other, but how will this affect my trying to keep the "sides" balanced current wise? If my thinking is right, it seems to me that I would want a circuit with provisions for separate bias, and balance controls.
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Old 19th July 2009, 04:03 PM   #5
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Hi again, I've decided to go with the Berning EA-230 circuit, and use 6JM6 tubes for the output. I was just wondering if anyone here has used these (or 6JN6s), and if GE was a decent brand. I'm also having trouble locating the smallish anode caps in something that is insulated (ceramic maybe). Thanks, Jay
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Old 19th July 2009, 04:11 PM   #6
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Remember, the small difference in plate voltage because of the differing drops side to side will not be very significant; tube-to-tube variation will overwhelm it.

I've used 6JN6s in a bench prototype of a screen drive amp (my actual amp uses 6LF6, now difficult and expensive to get- the 6JN6 doubled up is being tested as a substitute) and saw no real difference between brands. They may even be the same tube, just with different printing.
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Old 19th July 2009, 04:35 PM   #7
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ESRC sells small tube caps.

There is a couple interesting threads brewing that discuss screen drive sweeps tubes. I believe Bernings design you mention is a screen drive. Try searching for "screen drive". In case you're interested.

What ratings are on those OPTs?
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Old 19th July 2009, 05:22 PM   #8
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Thnks guys, the transformer ratings are 5k pri. Z, 0, 4, and 8 ohm secondaries, 10hz-38khz @-3db or 10hz-26k @ -1db with an 80 watt max rating. I don't know how accurate that is, because I have never heard of the maker before (Tomiko), and have not done any testing of my own yet. I may run a square wave through them later today just to have a look.
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Old 21st February 2011, 12:38 AM   #9
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Jaymanaa, did you get around to testing those Tomiko OTs? I'm interested in buying a pair but want to make sure they are decent before I buy.
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