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Re: Where to buy 3W Bias Pot?

3/6/2006 11:18 AM
Todd
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Re: Where to buy 3W Bias Pot?
Just to throw an idea out there, why not use a safer setup with a more-common potentiometer value? Using a single, high-wattage pot will work to adjust cathode bias. However, if the pot ever fails or even gets dirty, you run the risk of an open connection or a short to ground with no resistance. You will probably lose your output tube if that happens.  
 
When I set up an adjustable cathode-bias, I use a common high-wattage, low-resistance resistor as if it were a non-adjustable amp. For example, use a 470-ohm, 5-watt resistor in a tweed 5F1 Champ. Then, in parallel to the big cathode resistor, I wire both a standard-wattage potentiometer and a 1/2-watt resistor in series. The resistance value of the pot and 1/2-watt resistor depends on the bias range you need but will likely be larger than the 470-ohm resistor. The safety benefits are great. On one hand, if you adjust the potentiometer, you will still adjust the cathode-bias value because the actual cathode-bias resistance will change with the parallel pot-and-1/2-watt-resistor resistance. On the other hand, if your potentiometer dies or gets dirty (as is common with potentiometers) and creates an open circuit on the potentiometer side, you still have the 470-ohm, 5-watt resistor as a cathode resistance to prevent your tube from dying. If the pot dies the other way and shorts as a closed circuit with no resistance, you still have the parallel resistance of the 5-watt and 1/2-watt resistors to keep the tube alive. The added benefit of using a parallel potentiometer setup is that the wattage of the potentiometer and 1/2-watt resistor do not have to be very high because of the voltage divider effect.  
 
To me, this has always been a cheaper and safer route than trying to find reliable, high-wattage potentiometers. While it may not matter with run-of-the-mill output tubes, it is important if you are trying to protect expensive NOS tubes. Good luck!

 
Replies:
anonymous If the pot dies the other way an... -- 3/23/2006 3:07 PM