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In Reply to: when you said "truly floating", what's the criteria for that? posted by jarthel on March 27, 2007 at 14:36:15:
That's fine, but why did you pick 1/4B+? It's the cathode voltage and the heater bias that you need to match. What is your DC cathode voltage, to ground?
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Brian
Follow Ups:
that's what Mr. Broskie suggested. I am building his aikido and the PDF in his site suggested to use 1/4B+.just as a clarification: the resistor divider is connected to the negative side of the DC supply. right?
thanks again
When the DC supply is floating, as it appears to be here, you can attach it to the reference DC voltage in any one of several ways. As Michael says above, you can tie the voltage to the center-tap of a resistor divider across the DC voltage, but to me, that’s usually not necessary with a floating DC supply (emphasis on “DC”), especially if you have a shunt cap across the output of the 12.6 volts supply that effectively connects both terminals together as far as noise is concerned. You can connect either the positive or the negative terminal of the 12.6 volts supply to the reference voltage, depending on what you want. Without dredging through all the Aikido schematics, let’s say that B+ is 400 volts. Then 1/2B+ = 100 volts. If you connect the negative terminal of the heater supply to this 100 volt reference, then the upper end will sit at 112.6 volts. If you connect the positive terminal to this 100 volt reference, then the negative terminal will sit at 87.4 volts. You can see why I can’t simply answer you with “connect to the positive” or “connect to the negative”. You could do either if it is appropriate. Now you need to look at your particular design and see where the cathodes sit (DC). Look at the heater-cathode voltage ratings for your chosen tubes. When feasible, I will tend to want to connect a DC supply slightly positive to the cathode to cut-off any weak heater-to-cathode emission diode that can create noise (yes, even hum in severe cases). If I had a cathode sitting at 100 volts, I would choose the connection that makes the heater span from 100 volts to 112.6 volts. I’m guessing that this is what Broskie is suggesting. But you could go even higher above the cathode. And no matter which terminal you attach to the reference voltage, you can always change the 1/4 ratio to suit whatever you want. With two tubes stacked, you need a compromise voltage. If you post an exact preamp schematic with voltages, we’ll try to be more specific. Hope that helps.By the way, I see your confusion in the word “floating”. If your supply has its own transformer or at least its own winding, and there is no connection to ground anywhere, then I’d call it “floating”. You can check it by placing an ohmmeter from the supply (anywhere) to ground, with everything turned off of course. You should read an “open”. Broskie talks about a floating supply option that is not referenced to anything. It uses a cap to ground and depends on heater-cathode leakage to bring the heater supply to where it needs to be. I refer to “floating” here at what the supply has to be BEFORE you nail it to the reference divider (after which it’s no longer “floating”).
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Brian
Brian
You are correct in that its usually not necessary with a floating Dc supply,however I did have two instances where I had to add the divider and Im not sure if it was a mechanical layout or the phase relationship of the transformer i was using but I needed the resistors to quiet the phono section in worst way..You are right tho usually the DC floating supply takes care of it.
"the resistor divider is connected to the negative side of the DC supply. right?"If you are referring to the negative side of the DC heater supply, I don't think it matters.
Mr. Broskie explains that he chose 1/4 of B+ because it positions the common heater of the lower and upper triodes in his schematic at a safe potential for the heater-cathode voltage rating. Presumably one cathode is at ground potential while the other is at about 1/2 of B+, so 1/4 of B+ is considered a safe compromise. This has nothing to do with minimizing hum, by the way.
http://www.tubecad.com/Nine-Pin_Aikido_PCB.pdfthank you.
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