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- Thread starter sbelyo
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sbelyo said:I've got 7.7 VAC on a 6.3 tap thanks to high ac lline voltage.

I obviously need to lower it 1.2 volts

Is that connected to the heater, or not? If not, connect it and that will drop some.

sbelyo said:I have 2 100 ohm 3 watt resistors and several 500 ohm 3/4 watt laying around

What wattage value will I need if 1.8 amps is being drawn from the circuit?

W = VI = IIR

Double it for safety.

With .6 amp draw it's at 7.7dsavitsk said:Is that connected to the heater, or not? If not, connect it and that will drop some.

W = VI = IIR

Double it for safety.

with all 4 tubes I expect 1.8 amps

Now the V in the equation, is it the voltage dropped accross the resistor or the voltage of the circuit?

You have not given much indication of what you are doing. If you are building an amp from scratch, you are probably doing the heater wiring first. If this is the case, wire up allowing for the addition of a dropping resistor. Put all your valves in and fire her up. Measure the voltage with the full load, and then use Ohm's law to calculate the resistance required for the voltage drop required. Use Ohm's law again to calculate the power dissipation of the resistor, then choose a conservative rating. Dsavitsk recommends doubling for safety...

V=I/R, R=V/I

So, to drop 1.2 volts (you may need a different amount when you measure under load) with 1.8 Amps, you will need 0.666 Ohms and dissipate 2.16 Watts, so go with a 5 Watt resistor.

Not sure what valves you are using, I have looked up heater voltage ratings and not many show a +/- value. THe Russian valves do, and usually have 5.7-6.9 volts listed, so if it is well inside this I would not worry, though higher voltage may lead to shorter valve life.

Chris

Got itdsavitsk said:With 1.8A draw the voltage will be lower.

It's the V across the resistor as the resistor does not need to dissipate the heat dissipated in the heater.

Yes, building a headphone amp from scratch.chrish said:

You have not given much indication of what you are doing. If you are building an amp from scratch, you are probably doing the heater wiring first. If this is the case, wire up allowing for the addition of a dropping resistor. Put all your valves in and fire her up. Measure the voltage with the full load, and then use Ohm's law to calculate the resistance required for the voltage drop required. Use Ohm's law again to calculate the power dissipation of the resistor, then choose a conservative rating. Dsavitsk recommends doubling for safety...

V=I/R, R=V/I

So, to drop 1.2 volts (you may need a different amount when you measure under load) with 1.8 Amps, you will need 0.666 Ohms and dissipate 2.16 Watts, so go with a 5 Watt resistor.

Not sure what valves you are using, I have looked up heater voltage ratings and not many show a +/- value. THe Russian valves do, and usually have 5.7-6.9 volts listed, so if it is well inside this I would not worry, though higher voltage may lead to shorter valve life.

Chris

I'll plug in all the valves and turn it on just too see what it drops to. I only had 2 of the 4 in when I measured.

Once I find the value of the dropping resistor, do I put it on one leg of the tap or both?

There is a resistor on the board to drop the unregulated DC voltage for the 5842's (R3). After the amp is all finished and working you can measure the voltage on the 5842's and adjust its value if needed. This is usually only needed if using a power transformer with a over rated filament winding (especially if it is a Hammond).

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