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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: safest way of biasing an amplifier internally posted by afg on January 25, 2006 at 00:47:45:
Take care because with power off, the amp can still have the capacitors charged and kill you!
- first ensure 0V at input: if it has volume control, set it to minimum. if doesn't have one, connect a shorted RCA (ground the input).
- take care if you have long hair.
- do not touch anything.
- with the amp turned off, measure the cathode resistance about its value. Ex my amp: 10 ohms.
- turn the amp on and measure the voltage across this resistance. Use a normal tester to do this. use the two hands, if you wish. Use the correct scale (V or mV) on tester to get the better accuracy.
- Use ohms law to determine the Cathode current.
Ex. for my amp: Rk= 10, VRk = 800mV => I=U/R => 0.8/10 = 0.08 A = 80mA.
- Adjust the bias pot to the correct value.
- If your amp do not has bias pots the bias is self-regulated and not necessary to calibrate. if still you want to change the bias point in this case, you will have to change this resistor.
I dont know if you amp is SE or P-P nor the tube you're using
How to know the maximum Anode current for a tube?
- measure now your B+ (remember to set the tester to high DC voltage scale, or you will burn your tester).
P = UI => I = P/U => Ianode = Plate dissip / B+
For KT88, max plate dissipation is 42W. 25W for EL34. look at datasheets...
Ex for my amp (kt88 at B+ = 400V):
Imax idle (without signal)= 42 / 400 = 105mA
To extend the tube life, use less current:
Use 70-80% of this value if the amp is SE or 50-60% if it is P-P.
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Follow Ups
- Don't think if power is off you will be safe!! - SandroN 15:29:45 01/25/06 (0)
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