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Old 3rd May 2012, 04:58 PM   #1
jamesdb is offline jamesdb  United States
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Default Filament Voltages CT vs Non-CT

Is it correct that for a tube that has a 6.3V filament, when using a Center-Tapped filament transformer, that the transformer needs to be 6.3VCT total, as in 3.15V-0-3.15?
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Old 3rd May 2012, 05:01 PM   #2
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Yes, but check that the entirely secondary may carry the current needed by all tubes it gets power, 1 or more of them.Some 6+6v xformers are designed to full wave rectification, and then each half secondary carried half output current.
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Old 3rd May 2012, 05:05 PM   #3
M Gregg is offline M Gregg  United Kingdom
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Or you can have 6.3V with a psudo CT a 220 ohm resistor from each side to Gnd.

Unless the tube cathode voltage is elevated for some reason.

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Old 3rd May 2012, 05:15 PM   #4
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Quote:
Originally Posted by M Gregg View Post
Or you can have 6.3V with a psudo CT a 220 ohm resistor from each side to Gnd.
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.
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Old 3rd May 2012, 05:17 PM   #5
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Osvaldo de Banfield View Post
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.
220 Ohm resistors...??

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Old 3rd May 2012, 06:44 PM   #6
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Quote:
Originally Posted by jamesdb View Post
Is it correct that for a tube that has a 6.3V filament, when using a Center-Tapped filament transformer, that the transformer needs to be 6.3VCT total, as in 3.15V-0-3.15?
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.
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Old 3rd May 2012, 06:59 PM   #7
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Quote:
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220 Ohm resistors...??

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Gregg: if unbypassed, they also adds NFB in cathode.

RGDS.
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Old 4th May 2012, 01:40 AM   #8
jamesdb is offline jamesdb  United States
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Thanks very much everyone, for the informative answers!
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Old 4th May 2012, 10:32 AM   #9
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Quote:
Originally Posted by Osvaldo de Banfield View Post
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!
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Old 4th May 2012, 11:43 AM   #10
nazaroo is offline nazaroo  Canada
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Quote:
Originally Posted by Greg
Or you can have 6.3V with a psudo CT a 220 ohm resistor from each side to Gnd.
Quote:
Originally Posted by Osvaldo de Banfield View Post
Gregg: if unbypassed, they also adds NFB in cathode.

Quote:
Originally Posted by Vincent77 View Post
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.

Last edited by nazaroo; 4th May 2012 at 11:53 AM.
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