Bias setting: Shunt vs Chathode resistor

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Bias setting: Shunt vs Chathode resistor

Postby maxwedge on Tue Mar 24, 2009 9:57 pm

So I've built 1 amp, totally redone another and messed with the rest of them.
I've put 1% 1w 1 ohm resistors on 2 of the amps to read the pwr tube current in mv and the reading is always higher than when I check them using the OP trans shunt method, in ma.
With my Twin I get 43mv for each tube on one side across the resistor and 69ma shunting the same side of the OPT. I realize there's 2 tubes involved when I shunt it but that's 34.5ma per tube as opposed to 43mv across the resistor.
I would think the shunt method is more accurate but can anyone shed any light as to why I see such a difference?
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74 Marshall 50 watt
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76 Ampeg V4
77 Hiwatt DR504
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Re: Bias setting: Shunt vs Chathode resistor

Postby gusfinley on Mon Mar 30, 2009 5:53 pm

That is because the cathode current includes the screen current as well as the plate current. My opinion is that 1 ohm resistors at the cathode is not an accurate bias method. I have installed 1 ohm resistors at the plates of the output tubes with much better results. If you're interested in plate current, why not just read it from the plates??
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Re: Bias setting: Shunt vs Chathode resistor

Postby PaoloJM on Sun May 03, 2009 1:25 pm

Putting the 1 ohm resistor on the plates would be more accurate but would also be a helluva a lot more dangerous, even for an experienced tech. It puts your meter at potentials of up to 500Vdc (depending on the amp) above the ground you are standing on or the chassis of the amp.
I would suggest to use the cathode resistor method and refer to the valve datasheet to see what ratio plate and screen currents typically occur under quescent conditions for the valve you are biasing. This will give you an accurate result for plate current to within a mA or two.
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