Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Curranm on April 08, 2020, 10:11:37 am
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i am just starting to play with DC heaters. I am using a classic tone 40-18023 PT. http://www.classictone.net/40-18023.pdf
I am using a 50v/6A restifier (2)4700uF/26v caps. and a .15R/5w resistor with the heater CT referenced to ground.
I am taking the AC heaters (3.15v per heater wire), rectifying it and going into one 4700uF cap then the .15R resistor then into the 2nd 4700uF cap and then to the heaters of the (2) 12ax7's I am trying to heat.. my problem is I am only getting 2.57vDC +/- after the rectifier at the 1st cap. WHY am i not getting 6.5v like the merlin book says I should get?
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WHY am i not getting 6.5v like the merlin book says I should get?
what did you start with before DC?
are the filaments powering tubes or is your +/-2.57 without load?
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WHY am i not getting 6.5v like the merlin book says I should get?
Because you have it wired wrong. Disconnect the PT center tap and tap the end. Then connect the bridge negative terminal to ground.
PS... I should have mentioned that with your circuit wired as is you will have 2 x 2.57 = 5.14v ***ACROSS*** the first cap. But measuring voltage with one meter lead connected to chassis will only give 2.57v, as you've seen.
IMO, this is not enough to properly heat the 6.3V filaments.
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Because you have it wired wrong. Disconnect the PT center tap and tap the end. Then connect the bridge negative terminal to ground.
Huh?
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wired wrong............BUT this is the way the merlin book shows to wire it. I am fine with changing it, I am wondering why it is not delivering the voltage the book says it will. I can loft the heater CT no worries, and ground the negative of the rectifier. does this include the negative of the two caps and the ONE side of the tubes then? keep the (negative string) connected BUT put that to ground instead of the heater CT.
I started with 3.15vAC on each side of the ac heater tap.
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WHY am i not getting 6.5v like the merlin book says I should get?
what did you start with before DC?
are the filaments powering tubes or is your +/-2.57 without load?
started with 3.15vAC on each heater tap(this is also heating the PI and the two EL34's before going to the DC heater circuit. the DC reading is with tubes in the sockets.
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Your rectifier is dropping the voltage resulting in the observed DC voltage.
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Your rectifier is dropping the voltage resulting in the observed DC voltage.
OK, that is making SOME sense, so if I rewire it, I should then get my correct voltage?
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OK, that is making SOME sense, so if I rewire it, I should then get my correct voltage?
No. There is no need to rewire it. It just won't work because of the voltage drop across the diodes.
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so I rewired it and the DC voltage is now 6.5vDC. here is the changed schematic!
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so I rewired it and the DC voltage is now 6.5vDC. here is the changed schematic!
Never mind, then.
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so I rewired it and the DC voltage is now 6.5vDC. here is the changed schematic!
Never mind, then.
well I needed to get it to 6.3vDC to heat the tubes properly right? the other wya it was wires I was not getting enough voltage period(because of the voltage drop from the rectifier) to re-wiring it has gotten me my 6.3(6.5)vDC. so isn't that the idea for DC heaters? or is there another way to wire it up or utilize the way i HAD it wired up for heating the tube properly? thank you for chiming in and helping me get this right!
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...re-wiring it has gotten me my 6.3(6.5)vDC.
I don't see where you measured across the second capacitor when the center tap was still grounded. After you moved the ground, you did measure across the second capacitor. Unless there is some other path to ground, it will make no difference where you connect ground as far as the voltage across the second capacitor goes.