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Where to buy 3W Bias Pot?


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3/5/2006 6:18 PM
Peter Where to buy 3W Bias Pot?
Dear y'all,  
 
where can I get 100 and 200 Ohm, 3W - 5W bias pots for use in cathode biased amps. European sources are welcome, too. 2W would work too, although a higher wattage would be safer.  
 
I can't seem find any from the regular tube-amp sources or Mouser.  
 
I need the pots for my DIY Musicmaster and 5f1, because they both can run with either 6v6 and 6l6.  
(e.g. 5f1 & 6l6: optimal bias between cutoff and saturation requires a 330 Ohm cathode resistance.)  
 
Peter
 
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3/5/2006 8:51 PM
Crackerhead

Try Weber. I found a 500 Ohm variable rheostat that he advertises just for this purpose. It's rated at 25 watts and the part number is RHEO500-25.  
 
Hope this helps........Bryan
 
3/5/2006 9:04 PM
Jeff E

Check Mouser again - these might do it.  
 
588-93J100  
588-93J200
 
3/6/2006 11:18 AM
Todd

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!
 
3/23/2006 3:07 PM
anonymous
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.  
 
I appreciate the attempt to be helpful, but this statement is just plain wrong. Would'nt this scenario short the cathode resistor, bypassing all of the tube's current directly to ground, causing the tube to redplate rather quickly?
 
3/23/2006 7:40 PM
Todd
No, read my post more carefully...
It is not plain wrong if you reread my post and think of Ohms law. The pot is in SERIES with a 1/2-watt resistor, and those two components in series are PARALLEL to the larger, high-wattage cathode resistor. Thus, if the pot shorts to an open circuit, both the damaged pot and 1/2-watt series resistor are taken out of the circuit because of the broken connection, leaving only the high-wattage cathode resistor to set bias.  
 
If the pot shorts to a closed circuit -- zero ohms of resistance -- it is still in series with the high-ohm, 1/2-watt resistor. Those two series components are still in parallel with the high-wattage cathode resistor. Series resistances add, parallel resistors add reciporically. As a result, you still have bias through the parallel resistance of the high-wattage cathode resistor and the 1/2-watt resistor (because the 1/2-watt resistor resistance plus the 0-ohm pot resistance in series equals the 1/2-watt resistor resistance).  
 
Let's use some numbers to make it clear: Assume your high-wattage cathode resistor is a 5 watts, 500 ohms. Your pot is a standard-wattage pot, 5K ohms. Your series resistor is 1/2 watts, 5K ohms.  
 
With a working pot set at maximum resistance, you have the sum of the series pot and 1/2-watt resistor (5K + 5K = 10K ohms) in parallel with the cathode resistor (500 ohms) for about 476 ohms of total cathode-bias resistance.  
 
With a pot which dies open, the series pot and 1/2-watt resistor is an open circuit, leaving only the cathode resistor for 500 ohms of total cathode-bias resistance.  
 
With a pot which dies closed (offering a short circuit with 0 ohms of resistance), you have the sum of the series pot and 1/2-watt resistor (0 + 5K = 5K ohms) in parallel with the cathode resistor (500 ohms) for about 454 ohms of total cathode-bias resistance.  
 
Obviously, adjusting the pot lower than 5K resistance would give you values in between 454 and 476 ohms. While these numbers are just examples, in the real world you determine the ohm and wattage value of each of these three components depending on the adjustable bias range you need. No shorting the cathode to ground, no red-plating. Try it, it works great!
 
3/23/2006 8:20 PM
Enzo

If the pot opens, it simply turns the tube off and no damage will result. In fact opening the cathode leg is one way of wiring a standby switch. It is also sometimes used to disable two tubes in a four tube setup as a half power switch.  
 
SHorting the cathode to ground would red plate the tubes, but how often do pots fail short? Especially a wire wound. Those rheostats are darned sturdy. A 25 watt rheostat would be just about the LAST thing I would expect to fail in an amp so wired.  
 
In my humble experience, shorting power pots is not an issue. The only shorting of pots I ever encounter is when debris gets in them, or when they get their shafts punched in - as when you step on one in a pedal. Chances are that an adjustable cathode bias pot is not going to be on the floor gathering dirt, nor is it going to be on a panel or someplace accessible for the shaft to get punched in.
 

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