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Old 9th April 2003, 05:23 PM   #1
JBasham is offline JBasham  United States
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Default Wiring Filaments with Center Tapped Secondaries

I知 using a Hammond plate-and-filament transformer (373BX) that has center-tapped filament secondaries. All my previous experience is with transformers where the filament secondaries have no center tap.

I知 using the 5 volt/3 amp secondary to heat the filament of a (directly heated) 300B, with a hum pot. From what I致e gathered, I知 supposed to tie the center of the hum pot to signal ground (through the cathode bypass R/C network) and just wire-nut the center tap of the filament secondary. Sound right?

I知 using the 6.3 volt/3 amp secondary to heat the filament of an (indirectly heated) 6n1p driver tube. Do I tie one side of the secondary to pin 4, the other side to pin 5, tie pin 9 directly to ground, and wire-nut the center tap? Or do I use the center tap for the connection to pin 9?

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Old 9th April 2003, 06:07 PM   #2
Joel is offline Joel  United States
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Default Re: Wiring Filaments with Center Tapped Secondaries

Quote:
Originally posted by JBasham
...From what I致e gathered, I知 supposed to tie the center of the hum pot to signal ground (through the cathode bypass R/C network) and just wire-nut the center tap of the filament secondary. Sound right?
Yes.

As far as the 6N1p, just ground the center tap of that winding, and run both legs to the tube.
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Old 9th April 2003, 06:09 PM   #3
Joel is offline Joel  United States
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Although, you saying "signal ground" scares me a little. Ground is ground. Are you planning on floating the chassis? Just have one ground, where everything, including the chassis is tied. You will be glad for the extra shielding anyway.
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Old 9th April 2003, 06:25 PM   #4
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This may be a dumb question, but what is "wire nutting" a center tap? Do you mean just insulate it in some form and leave it unconnected? If that's correct, what is a wire nut and how does it achieve this function?

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Saurav
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Old 9th April 2003, 07:10 PM   #5
Joel is offline Joel  United States
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A "wire nut" is a plastic cap, with metal threads on the inside that you screw onto the end of a wire, or wires, to insultate them and/or make a connection.
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Old 9th April 2003, 07:14 PM   #6
PassFan is offline PassFan  United States
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Quote:
Originally posted by Saurav
This may be a dumb question, but what is "wire nutting" a center tap? Do you mean just insulate it in some form and leave it unconnected? If that's correct, what is a wire nut and how does it achieve this function?

Thanks,
Saurav
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Old 9th April 2003, 07:17 PM   #7
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Thanks! Didn't even know something like this existed. Looks like a much better solution than duct tape
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Old 9th April 2003, 07:45 PM   #8
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Default The center tap....

Quote:
I知 using the 5 volt/3 amp secondary to heat the filament of a (directly heated) 300B, with a hum pot. From what I致e gathered, I知 supposed to tie the center of the hum pot to signal ground (through the cathode bypass R/C network) and just wire-nut the center tap of the filament secondary. Sound right?
Yes!!...that sound right!...

If that mean that center tap isn't conected to anything!...it is left floating!

regards
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Old 9th April 2003, 07:48 PM   #9
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Concerning the 6N1Pi,yes,you should ground the center tap of the trafo winding.This is the nearest and most effective way to ground it.The other solution (grounding the pin no 9) is of no use in this respect,since this tube has NO center-tapped filament,as ECC 81-82-83 do have.Remember,the pin no.9 on the 6N1Pi tube is a shield between the two anodes!
Some techies would point that this shield should be grounded,too,for maximizing the isolation between the two sections of the tube,but this is totally unnecesary in audio applications.It' very useful in some HF apps,though...
The same story applies in the 6N2Pi tube (ECC 83 equivalent),and most other Russian double triodes,meaning that all those tubes have the heater between pins 4 and 5,and being made for 6,3 V only.
Please,try to consult a proper tube catalog,or watch for multiple sources if you're working with Russian tubes,at least for avoiding this kind of confusions!
Regarding the 300B filaments,I have another opinions:
1.You don't need AT ALL the center tap of the 5V winding!
If I understood corectly,you have a resistor and an electrolyt on the 300B cathode (or filament,how you like).This means that the 300B runs in auto-bias mode,wich means that on the filament you're supposed to have some positive potential (a few tenths of volts,depending on the Ua and Rk) between thw filament and ground.THIS IS,IN FACT,THE WHOLE STORY FOR THE 300B!
If you also ground the center tap of that winding filament,you only bypass the Rk/Ck ,wich results into another working point of the tube!
2.about the hum pot:get rid of it.
The hum pot is effective only in early stages (poorly-designed preamp ones),if you have un-tapped filament windings,but this is a completely different story.For output tubes,there is another way of managing the problem;in fact,it's the SIMETRIZATION of the filament ends that counts!
Therefore,you should apply the 5 V wires directly on filament,and use two ceramic resistors of around 50 Ohm each.Solder one of each resistor legs' on each filament leg,and the other two legs of the resistor together.On this point,solder the Rk/Ck ensemble.
This way,you'll have almost the same configuration as you were use the hum pot,but BETTER in every respect.
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Old 9th April 2003, 11:36 PM   #10
EC8010 is offline EC8010  United Kingdom
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Default Grounds, earths, and chassis

Various people have talked about grounding, but have not been specific about what it is.

Heater supply centre taps (whether they are transformer or resistor-derived centre taps) need to be directly "grounded" to chassis, rather than 0V. (Draw yourself a circuit diagram and consider where the noise currents flow, and you will soon see why.)

Once you start wondering about this sort of thing, it's worth reading the Henry W Ott book or the Ralph Morrison book. Both are expensive to buy, but libraries are cheap.
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