I’m 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’m using the 5 volt/3 amp secondary to heat the filament of a (directly heated) 300B, with a hum pot. From what I’ve gathered, I’m 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’m 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?
Thanks!
I’m using the 5 volt/3 amp secondary to heat the filament of a (directly heated) 300B, with a hum pot. From what I’ve gathered, I’m 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’m 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?
Thanks!
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.
Regards,
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.
Regards,
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.
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.
FROM WHAT I READ SO FAR...
Hi,
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?
Hi,
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?
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:
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:
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.
SEE WHAT I MEAN?
Hi,
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,
Hi,
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,
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
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
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
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...
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...
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.
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
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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