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Old 9th April 2003, 11:54 PM   #11
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Default FROM WHAT I READ SO FAR...

Hi,

Quote:
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.
Yup.

And one poster seems to confuse the other on this very simple topic.

Nothing all that unusal around these places, it usually takes two pages to come to a point where you agree not agree here.

And if they can take it off topic they always will, won't they?

Blimey, center taps anyone?
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Old 10th April 2003, 12:50 AM   #12
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Quote:
And one poster seems to confuse the other on this very simple topic.
Count me in the very confused poster camp. Let's see if I understood any of this. If the filament transformer does not have a center tap, we need to make one (and ground it), either using 2 resistors or a pot. Is that correct? This is the type of circuit I'm used to seeing:

Click the image to open in full size.

Cathode bias, no center tap on the filament winding, hum pot wiper going to ground through cathode bias R and C. EC8010 said the transformer needs to be grounded directly to the chassis, not 0V. Does the above circuit achieve that? I don't see how. If we connect the hum pot wiper directly to the chassis, where do we put the cathode bias R and C?

Here's a different circuit:

Click the image to open in full size.

Now the transformer has a center tap winding, and the cathode bias R-C are attached to the center tap, and no hum nulling pot is used. Again, how do we ground directly to the chassis?

Thanks for being patient with my very newbie questions.
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Old 10th April 2003, 01:26 AM   #13
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Default SEE WHAT I MEAN?

Hi,

Quote:
If the filament transformer does not have a center tap, we need to make one (and ground it), either using 2 resistors or a pot. Is that correct? This is the type of circuit I'm used to seeing:
No, not necessarily.

Think of a center tapped xformer as a seried xV-0-0xV xformer where the 0 points are connected together.

The idea is to have a balancing mechanism so that the heater is fed in sort of a symmetrical way when applied to a DHT tube.

Say, when you need to feed a tube 6.3VAC you could split it up in twice 3.15VAC to feed your tube that does not have a centertap on the heater more equally.

The important part is achieving balance so that AC components balance against DC components and null.

No heater is ever perfectly balanced inside a tube and the resistors or trimpot allow you to trim this inbalance in such a way that AC pulses become neglectable, at least that's the reasoning behind it.

The wiper can be referenced to earth directly or to the grounded midpoint (CT) of the xformer, either way the goal is the same: balance and nulling AC.

All this is of importance to heat the cathode of that precious DHT equally so as not to create cold spots (islands) on the cathode surface.

You can achieve this just as well with a 0-xV xformer provided you create the virtual midpoint yourself using a trimpot in between the heaters' extremities with the wiper...you guessed it...referenced to ground.

This is by no means the whole story, just to give you some idea.

Ciao,
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Old 10th April 2003, 01:55 AM   #14
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OK, I think I understood this mostly correctly then. You want the heater to be +3.15/-3.15 instead of +6.3/0, that makes sense. What happens with cathode bias though? Don't the cathode bias resistor and capacitor have to connect "below" the wiper? Then the wiper connects to R||C, which then connects to ground - as far as cancelling noise is concerned, is this effectively the same thing as connecting the wiper directly to ground? If it is, then I think I've got it, if not, I'm still confused

Quote:
The wiper can be referenced to earth directly or to the grounded midpoint (CT) of the xformer, either way the goal is the same: balance and nulling AC.
If the CT and the wiper are both grounded... how does that work? I can understand how it works when there is no CT - the trimpot wiper sets the ground point, and adjusting the wiper "moves" the whole transformer voltage up and down relative to ground, and the objective is to set it so the transformer is symmetrical around ground, equal +ve and -ve voltages. In this case, there is only one ground reference, the wiper, and the rest of the heater circuit exists relative to that ground. But if the CT is also grounded, does the trimpot still work the same way?

Thanks,
Saurav
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Old 10th April 2003, 02:08 AM   #15
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Default OK, MY MISTAKE...

Hi,

When you use a cathode R to bias the tube, the wiper of the heater trimmer/ midpoint of the two Rs connects to the top of the cathode R.

Cheers,
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Old 10th April 2003, 02:15 AM   #16
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Thanks.
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Old 10th April 2003, 07:43 AM   #17
EC8010 is offline EC8010  United Kingdom
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Saurav,

glad you're sorted. DH valves cause a conflict. From an RF and hum point of view we want to connect our centre tap directly to chassis, but from a signal current point of view it must connect to 0V at the correct point along the 0V bus bar.
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Old 10th April 2003, 10:25 AM   #18
SHiFTY is offline SHiFTY  New Zealand
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Semi-OT, but I found I could not get rid of the hum on my 6B4 filaments (6B4 is a 6.3V version of a 2a3 DHT.) Hum pots made no difference.

So I went DC on the filaments, using a couple of $2 diode bridges and a couple of 6800uF caps with a dropping resistor. There is now no audible hum AT ALL, and I have wired the sockets in a mirror image, so I can swap them around every once in a while. (some people believe that DC on filaments causes more wear on one end)

Seemed to make no difference at all to the sound quality. (Apart from remove the annoying hum!)

Hope this might be helpful to you...
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Old 10th April 2003, 04:04 PM   #19
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I had to use DC filaments on my Foreplay preamp to get rid of hum too, but that was the first tube circuit I ever built, at first I didn't even know I had to twist the heater wires (somehow I missed that in the instructions, I had to go back and re-do that). Hopefully, I would do a better job this time, but I know that that's always an option if I can't get the hum down to an acceptable level using AC.
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Old 10th April 2003, 04:39 PM   #20
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hi,

I try the secondary winding with -filament CT tap.
My test result was in the input stage was effect quite large.
If thewe only use 3.15~0~3.15.
The 6.3V output but ct(0V) not ground if no hum not need to connect. But sometime if connect to geound we can hear the background will much slient in horn speaker.

thanks

Thomas
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