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Dropping voltage across a resistor


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5/4/2006 12:23 AM
James
Dropping voltage across a resistor
I want to try putting an EF86 w/bandaxall TS in the first channel of my Pro Rev. How do I figure what size resistor to use to drop the voltage down to 70v or so like an EF86 wants to see. Also do I then need to add another filter cap there?
 
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5/4/2006 1:20 AM
Carl Gigun
I guess you're looking at a specific schematic that runs an EF86 from 70v? In general though they run from supply voltages normally found in tube pre-amps, 150v-400v.  
 
To figure your dropping resistor assume the current draw will be in the ballpark of 1.5ma, or look up a more accurate figure from the EF86 charts. You will need a cap after the R for this new voltage.
 
5/4/2006 2:51 PM
James

Thanks Carl! The data sheet says  
Pentode Connection  
HT Supply Voltage (volts) 100 - 200  
----Cathode Current (mA) 1.0 - 1.7  
 
 
Is that what I'm looking for?
 
5/4/2006 3:17 PM
Carl Gigun
Yep cathode current is the right one. The current will depend just as much on the circuit and resistors used as the supply voltage. The screen voltage in particular will be a big factor so try to find a circuit in the sheet that matches yours as much as possible.  
 
Of course you can always try a few different dropping resistors so you don't have to be bang on with the current on the first try. Personally I use a trimmer when doing something like this and replace it with a resistor later but it's not the safest way so I'm not officially recommending it ;)
 
5/4/2006 4:00 PM
James

It seems like I could just use 220K-330K resistor on the 390V B+ rail and be fine. Does this seem correct to you?
 
5/4/2006 7:29 PM
Carl Gigun
sounds close but maybe just a little big? I'm not sure what current you are expecting through the EF86, but lets say it's 1ma:  
 
milli's and kilo's cancel out conveniently (200,000 x 0.001 = 200 x 1) so you're looking at 220 to 330 volts dropped @ 1ma, or a supply voltage of 60v to 170v.  
 
70v supply sounds very unusual to me. I'd double check the schematic you're working from to make sure the 70v is really the supply voltage and not the plate voltage. You'll need more like 150v supply to have 70v at the plate.
 
5/4/2006 11:28 PM
James

My suppy voltage is 390V. I'm wanting to end up with 70v-80v on the plate.
 

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