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Small tube audio amplifier at low anode voltage, filaments driving

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neazoi

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Hi there,
Here is a little audio amplifier I would like to try, since I have the tube and the headphones.
However, I would like to provide the filament voltage from the +12v one, so that a single 12v PSU can be used, without the need for the battery. I do not want to use semiconductors. The filament draws 240mA at 1.2V.

Can a simple voltage divider do the trick?
I worry about the isolation of the cathode and the anode, since this is direct heated, but also about the resistance of the filament prior to heating, which would lead to some unpredictable initial voltage on the filament?
 

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More simple, just a series resistor, calculated according to ohms law. It will keep the cathode potential.

But do you really want 240 mA drawn from the 12 V supply?

about the resistance of the filament prior to heating, which would lead to some unpredictable initial voltage on the filament?
Cold resistance is lower than hot resistance, there's no problem in using a series resistor.
 

More simple, just a series resistor, calculated according to ohms law. It will keep the cathode potential.
I do not know what is the hot resistance of the filament. It would be easier to try some series resistor values and I hope I won't burn the filaments out. Any estimation of the initial resistor value and wattage to start with?

But do you really want 240 mA drawn from the 12 V supply?

It draws 240mA at 1.2v, will this still be 240mA at 12v? Unless you mean the power dissipated by the series resistor?
 

I do not know what is the hot resistance of the filament.
According to the specification, it's 1.2V/240mA = 5 ohms. The nominal series resistor would be 10.8V/240mA = 45 ohms respectively.

It draws 240mA at 1.2v, will this still be 240mA at 12v?
Yes, the nature of a series circuit. A switch mode converter would be the only way to get away with less.

Unless you mean the power dissipated by the series resistor?
I didn't yet talk about power. My math says 2.6W.

In a datasheet, I see the heater current specifications 230mA min, 255mA typical, 280 mA max. So you'll better select the resistor empirically between 39 and 47 ohms or use a rheostat.

- - - Updated - - -

Just realized that the schematic in post #1 is probably wrong. The datasheet says Heater voltage 1.2 or 2.4V, the about 240 mA specification is for 1.2 V. According to my understanding, it's valid for a parallel connection of both filament halves, e.g. center "1" connected to ground, "2" and "6" connected to 1.2V. The shown series circuit would work with 2.4V.

Series resistor to be calculated respectively, 68 to 82 ohms.
 
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    neazoi

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According to the specification, it's 1.2V/240mA = 5 ohms. The nominal series resistor would be 10.8V/240mA = 45 ohms respectively.


Yes, the nature of a series circuit. A switch mode converter would be the only way to get away with less.


I didn't yet talk about power. My math says 2.6W.

In a datasheet, I see the heater current specifications 230mA min, 255mA typical, 280 mA max. So you'll better select the resistor empirically between 39 and 47 ohms or use a rheostat.

We cross posted, so I re-edit this post.

An estimation about the series resistor wattage?
 

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Last edited:

neazoi;
In the good old days, when efficiency was not an issue, the simplest tube circuits would use power resistors to drop the filament voltage to its proper level. I used that trick myself.

The only issue is that the tube will take a little longer to warm up. If that is no problem in your application, you are good to go.
Hint: calculate the power available to the heater with a cold-resistance value.
 
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    neazoi

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Power is about 1.2 W, inrush current is too short to be counted for continuous rating. I would use a 2W resistor.
 
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    neazoi

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A serious problem with your class-A TOOB amplifier is that if its continuous current in the headphones does not destroy them then the cones will be forced against one side and the headphones will not produce any sounds.
Never ever put DC through headphones or a speaker.
 
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    FvM

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It is a single heater but divided in two halves so that it can be fed from AC from the power transformer directly. The center tap is for the reference (anode) so that the power supply hum is effectively reduced (not audible). If you are feeding the heater with DC, you can as well connect them in series and use the lowest heater current spec (for max life) to calculate the series resistor. The heaters can be connected in parallel also but exact resistance match is not automatically established. The current specified is the operating current and you need not bother about cold/hot resistance. Usually the heater voltage in old tubes were 6.3V (I do not know why and how and from where this magic figure came) but perhaps low heater voltage produces less noise.

But you must attend to the point raised by Audioguru (#8 post).
 

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