resistor value

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ernie jr
Junior tube assistant
Posts: 86
(4/17/04 11:00 am)
 resistor value
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 i need to drop a b+ voltage after the choke from about 480 to around 435. any ideas on what value resistor to start with? is there a formula?
thanks for the help
ernie
 
Leevi
Junior tube assistant
Posts: 134
(4/17/04 11:21 am)
 Re: resistor value
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 It depends how much current is flowing through the resistor.
If we assume that the current is 200mA and you have to drop 45V so according to Ohm´s law, U=RI, you´ll need a 225 Ohm resistor. That is not so easy because you must take also the power loss into account. That is (P=UI) in your case 45V*0.2A=9VA. So with 200mA current you have to use ~220 Ohm/9W resistor. Note tha the resistor will run hot. There are other solutions to drop the voltage, hopefully someone else will tell you about them (zeners for instance).
/Leevi
 
Tiny Daddy
I will work on all amps
Posts: 617
(4/17/04 11:32 am)
 Re: resistor value
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 If you are "after the choke" then the current may be only 10mA so the resistor would be 45/.01 = 4500 ohms so you could try 4.7K 1 Watt and adjust from there. If you install the 4.7K you can measure the voltage on either side of the resistor, calculate the current and adjust the resistor as required.
 
ernie jr
Junior tube assistant
Posts: 87
(4/17/04 11:57 am)
 resistor value
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 thanks tiny daddy,
i figured it might be somewhere between 4k and 68k, 2 watt, based on other amps, amp will have 2 x 6550 pt
and a 6an8 and 12ax7 for the driver and preamp.
it is a modified dynaco/sunn amp.
thanks again
ernie
 
ernie jr
Junior tube assistant
Posts: 88
(4/17/04 12:21 pm)
 Re: resistor value
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 make 4k and 6.8k
sorry

 
 
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