Bias Current?

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Searing
Junior tube assistant
Posts: 70
(2/6/04 11:10 am)
 Bias Current?
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 I keep reading to bias between 30-40ma, but I also have heard to go by the static dissipation or something like that, and use that method which brought my Sovteck 5881's up to about 46ma's at around 20v (the guy's papers said I can run these tubes at 24v max! I multiply my reading on pin 3 of the power tube socket by the Ma on the cathode resistor, adding .0 to it, So 428 X .045. This way was reccomended by the guy I got the tubes from, but everything I read keeps tellin' me between 30-40ma. What to do?
Chris
 
j allen shaw
Junior tube assistant
Posts: 3
(2/6/04 12:15 pm)
 Re: Bias Current?
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 Searing,
Try this link, it's a very informative article by "Lord Valve" that helped me understand bias a little better! Hope it helps!
J Allen
http://www.duncanamps.com/technical/lvbias.html
 
GroundhogKen
Forum Moderator
Posts: 2240
(2/6/04 2:38 pm)
 Re: Bias Current?
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 From reading your post I have deduced the following:

You have a cathode biased pair of 5881 which share a 220 ohm cathode resistor and have a plate voltage of about 428.

Step 1. Measure the plate voltage with respect to ground. 428 V

Step 2. Measure the cathode voltage with repect to ground. 20 V

Step 3. Calculate the cathode current.
20 V / 220 ohms = 91mA total or 45.5mA per tube

Step 4. Calculate the actual plate voltage with repect to the cathode.
428V - 20V = 408V.

Step 5. Calculate the plate dissipation ( heat being generated by the plate ).
408V x 45.5mA = 408V x 0.046 A = 18.8 Watts.

Step 6. Compare your plate dissipation with the max recommended for the tube ( from the data sheet--24 watts for 5881 )

18.8W / 24W= .78 = 78% of max.

You should usually shoot for about 70%, so you amp is running a little hot. If you want to cool it down a bit, increase the value of the cathode resistor--maybe to 250 ohms.

I'd say it looks OK the way it is.

Ken

Edited by: GroundhogKen at: 2/6/04 2:49 pm
 
Searing
Junior tube assistant
Posts: 70
(2/7/04 8:49 am)
 Re: Bias Current?
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 Thankyou Ken!
It makes perfect sense to me now!
Chris
 
 
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