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23rd October 2006, 05:56 PM  #1 
diyAudio Member
Join Date: Jun 2005
Location: Los Angeles

How to calc Resistor Values to reduce Voltage?
B+ secondary reading is 257V. Using 6dj8 circuit, and this tube is rated .325 amp, and requires 200V...so I need to reduce 57V (257200)
So Ohm's Law says V=IR or 57V / .325 = R = 175 Ohm. And P = VI or 57V x .325 amp = 18.5 watts x 2 (to be safe) = 37 watt. I must be doing this way wrong as another fella suggested a 25K ohm resistor rated at 2 watts. Please set me straight.
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Cheers ~ Mike 
23rd October 2006, 06:22 PM  #2 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Hi,
what plate current will your tube operate at? Now calculate what resistor you need at the quiescent plate current. Now what range of plate currents flow when the tube is amplifying a signal. What happens to the plate voltage as the plate current varies. Is this good for an audio amplifier?
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regards Andrew T. 
23rd October 2006, 06:29 PM  #3 
diyAudio Member

Andrew is right. Your formula is correct, but it is not the RATED plate current that you need to plug in but the ACTUAL plate current in the circuit, of course. Makes sense, no? Same for the anode voltage. What is the ACTUAL voltage in your circuit? That determines (with the supply voltage) the number of volts you need to waste, and that gives you the R and the P.
Jan Didden
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23rd October 2006, 06:37 PM  #4 
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Join Date: Jun 2005
Location: Los Angeles

The voltage reading is 257 volts. 200 volts is the spec for the 6dj8 output stage I assembled.
I'm a beginner. I can measure voltage and ohms. But I haven't measured current yet. I'll have to read up on how to do this with my multimeter.
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Cheers ~ Mike 
24th October 2006, 05:31 AM  #5  
diyAudio Member
Join Date: Jan 2002
Location: somewhere in Australia

Quote:
to measure current, insert a 5W ceramic 10ohm resistor between B+ and plate. measure voltage. use ohm's law to compute for current. 

28th October 2006, 03:54 AM  #6 
diyAudio Member
Join Date: Jun 2005
Location: Los Angeles

measuring voltage "across" a cap?
Thanks Jarthel. I'll pick up a resistor tomorrow and try.
When someone says "measure the voltage across the cap" that is soldered to a circuit board, and the leads are not accessible (except by turning the board upside down) does this mean that you can place the DVM leads: (a) on the solder joints of the cap's leads on the underside of the board or (b) just on the cap itself respecting the positive and neutral orientation of the cap?
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Cheers ~ Mike 
31st October 2006, 12:58 AM  #7  
diyAudio Member
Join Date: Jan 2002
Location: somewhere in Australia

Re: measuring voltage "across" a cap?
Quote:
as for polarity if it's an elcap and you get negative voltage, just remove and the negative symbol and you get the proper voltage. 

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