This is true in low power triodes, but in case of directly heated power tubes, 220V is too much, and voltage drop in them adds to bias.
To answer the question the way you worded it, Yes. "when using a Center-Tapped filament transformer, the transformer needs to be 6.3VCT total, as in 3.15V-0-3.15" to drive a tube with a 6.3V filament.
However, if you have 2 tubes, each with a 6.3V filament, you could use a 12.6VCT ( 6.3-0-6.3) and connect each tube across one half of the secondary.
220 Ohm resistors...??
Regards
M. Gregg
Gregg: if unbypassed, they also adds NFB in cathode.
RGDS.
Gregg is talking about using two resistors (usually 100 - 220 ohm) to create a pseudo secondary center tap! No NFB here! Wake up!
Greg said:
Gregg is talking about using two resistors (usually 100 - 220 ohm) to create a pseudo secondary center tap! No NFB here! Wake up!
If the purpose of the resistor-bridge is to ground the cathode, and the plate current is going to flow through them to ground, because the transformer has no c.t., then they are indeed in the signal path too, but 220 ohms ?
The purpose of the relatively high resistor values is to prevent current flow from the winding through both of them, causing a draw and sag on the heater-voltage.
So there's the problem:
You want the resistors small to bring the cathode close to ground, but if the resistors are the "only" connection to ground, they in fact become the Cathode resistor (they will be in parallel from the view of the plate current).
If you tag the -ve return for the High Voltage Plate supply directly to the heaters, then the plate current does not flow through the resistors (or the winding) to ground. It flows straight back to the (semi-floating) HV supply.
If the resistors are in series with the tube in regard to the HV plate current, then indeed as cathode resistors, one might want to bypass them for the AC signal, for the purposes of altering the output Z of the stage.
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Maybe there is a language confusion here. In the UK we carefully distinguish between the filament in a directly heated valve, and the heater in an indirectly heated valve. It seems that this distinction is not always maintained in US English. So is the OP talking about what I, a Brit, would call a heater or a filament?
There is some fuzziness in regard to 'heater'/'filament'.
But the real point in the small debate above,
is that if you have a transformer with no center-tap,
the usual reason you make a 'pseudo-c.t.' is to provide
a balanced connection to ground, to minimize hum.
But when you do, the resistor-bridge becomes the 'cathode resistor', and joins the signal chain,
where it may want bypassing.
From this I conclude that all three posters were correct,
but misunderstood one another.
Here in Argentina, it is called "filamento" (Filament as in a EL34)) when indirectly heated tubes (real cathode isolated), and "calentamiento Directo" (Direct heating as 5U4) when no isolated cathode. In such a case, I understand from start, that the boy was making reference to a direct heating tube, in which case all I say was true.
Regards.
Regards.
Well just to mix it up a bit more..
I saw the word filament but assumed the question to quite newbie..as in 3.15 X 2 is this 6.3...for a filament so I assumed "Heater" and answered with this assumption... so vincent 77 was correct in his assumption about my answer, and I should not have made the assumption based upon my assumption. The reason for this is because when you assume it makes an As* out of U and me so I apologise for my assumption..Exit stage left <<<<<<
Regards
M. Gregg
I saw the word filament but assumed the question to quite newbie..as in 3.15 X 2 is this 6.3...for a filament so I assumed "Heater" and answered with this assumption... so vincent 77 was correct in his assumption about my answer, and I should not have made the assumption based upon my assumption. The reason for this is because when you assume it makes an As* out of U and me so I apologise for my assumption..Exit stage left <<<<<<
Regards
M. Gregg
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As a newbie is it possible to use this transformer on a 274b rectifier tube. Also how would I add the ct's to sec. #1 and sec.#2?
="https://tubedepot.com/products/r-core-20-plus-watt-tube-project-power-transformer"
="https://tubedepot.com/products/r-core-20-plus-watt-tube-project-power-transformer"
https://tubedepot.com/products/r-core-20-plus-watt-tube-project-power-transformer
you need 5volts at 2Amps, it looks like your traffo is not suitable...https://d1sjrnpi226dnf.cloudfront.net/spree/attached_files/tr-pw-879s.pdf?1382030398
Antek makes some decent, and quite affordable traffo's that ship immediately if they are in stock.
This one, with the secondaries wired in parallel, will suit your needs for 5V@2A if you drop about a volt (remember ohm's law and get average filament current from the datasheet), and add the center tap:
AN-0206 - 25VA 6V Transformer - AnTek Products Corp
A quick nomenclature explanation:
25VA is a traffo rating, basically it means that the traffo supplies approximately 25 watts of power at the rated voltages. Maybe the demi-gods around here can explain it better or why it's VA and not W...
If you have the patience, Edcor, who makes top notch products for their price point, makes the exact traffo you need:
https://www.edcorusa.com/lvp5-2-115
I have waited the full 8 weeks for them to begin construction of my order, I have also waited less than a week...so it's a question mark as to when they will build and ship it for you.
Hammond may make what you need but you'd have to go check mouser.com or the hammond mfg website, if they do odds are you can have it in the mail in a day or two.
You can also try eprey....epray....ebay for a transformer.
In all seriousness you can find good deals there on rca jacks, binding posts, umbilical sockets, isolation feet, tubes, and just about everything else.
I have to ask...what are you building or restoring?
This one, with the secondaries wired in parallel, will suit your needs for 5V@2A if you drop about a volt (remember ohm's law and get average filament current from the datasheet), and add the center tap:
AN-0206 - 25VA 6V Transformer - AnTek Products Corp
A quick nomenclature explanation:
25VA is a traffo rating, basically it means that the traffo supplies approximately 25 watts of power at the rated voltages. Maybe the demi-gods around here can explain it better or why it's VA and not W...
If you have the patience, Edcor, who makes top notch products for their price point, makes the exact traffo you need:
https://www.edcorusa.com/lvp5-2-115
I have waited the full 8 weeks for them to begin construction of my order, I have also waited less than a week...so it's a question mark as to when they will build and ship it for you.
Hammond may make what you need but you'd have to go check mouser.com or the hammond mfg website, if they do odds are you can have it in the mail in a day or two.
You can also try eprey....epray....ebay for a transformer.
In all seriousness you can find good deals there on rca jacks, binding posts, umbilical sockets, isolation feet, tubes, and just about everything else.
I have to ask...what are you building or restoring?
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