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Old 28th July 2008, 12:05 PM   #1
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Default Rheostat for adjustable "fixed" bias?

Has anyone tried using a rheostat for the cathode bias resistor in a SE amplifier? I've seen people use sets of resistors with a switch for changing bias resistor values for different tubes, but the rheostat gives infinite values (within it's range of course). I see that 750R 1/4 amp 50w rheostats can be had for $10 surplus, and the size isn't too large at 2-1/4" diameter. I suppose you'd need a panel mounted mA meter to watch the bias as you turn the rheostat.
I suppose the only risk of using a rheostat is that if you get intermittent connections as you turn the shaft, you could get no bias. Maybe it's better to have a rheostat in parallel with a fixed value resistor so the circuit never goes completely open?
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Old 28th July 2008, 12:23 PM   #2
SY is offline SY  United States
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If the inductance isn't too great, it should work fine.
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Old 28th July 2008, 02:18 PM   #3
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Good point Sy, I will have to measure that when I get the part in-hand.
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Old 28th July 2008, 06:57 PM   #4
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You mean self bias
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Old 28th July 2008, 07:13 PM   #5
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Yes, you are correct
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Old 28th July 2008, 08:29 PM   #6
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Known around here as cathode bias as well. Many older amplifier designs used cathode bias with rheostats or ceramic power resistors with sliding wiper arrangements. (adjustable tap vitreous enamel power resistors.. ) Works fine.. Do yourself a small favor and insert a small amount of fixed resistance say 1 - 10 ohms to make it easy to measure the cathode current - that way you don't need to know the actual resistance value of the rheostat to determine the current..

Go one step further and use fixed bias instead. No questionably quality bypass cap to deal with, and no concerns about inductance either.
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Old 28th July 2008, 08:38 PM   #7
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Yes, that's a good suggestion. I do the 1 ohm resistor tricks on my guitar amps (PP amps). Like you said, it makes the math easier, you can read the current right from the DMM.

Fixed bias would be nice, but that's a whole additional power transformer (i think), unless something clever could be made with the existing power transformer.

Glenn
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Old 28th July 2008, 10:15 PM   #8
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Default Re: Rheostat for adjustable "fixed" bias?

I suppose the only risk of using a rheostat is that if you get intermittent connections as you turn the shaft, you could get no bias. Maybe it's better to have a rheostat in parallel with a fixed value resistor so the circuit never goes completely open? [/B][/QUOTE]

If you use a potentiometer as a rheostat then you don't need to worry. If the wiper connection opens up then you'll just get the full value of the pot as your cathode resistor. So in choosing the pot value you just need to make sure it's not so high that if it's whole value ever does get control that the B+ rise (due to reduced current flow) won't do any harm.
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Old 29th July 2008, 12:48 AM   #9
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I used this in a Magnavox 6V6 amp. Parallel resistor to reduce the dissipation in the WW pot and a series one so that I didn't have to mark one end "Self-Destruct"...
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Old 29th July 2008, 01:51 AM   #10
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Quote:
Parallel resistor to reduce the dissipation in the WW pot and a series one so that I didn't have to mark one end "Self-Destruct"...
This is an excellent point. People worry that "you could get no bias" if there was intermittent contact. This is not the case. If the rheostat (or pot, or ordinary cathode resistor) went open the cathode voltage will rise to the point that the tube cuts off and stays there. Under normal circumstances nothing bad will happen. It is possible that the voltage is higher than the rating for the bypass cap, or that the B+ voltage will rise to an "unsafe value". The capacitors should be rated for this much voltage, but may not be. Consider this possibility when designing new equipment. It is also possible that a gassy tube will never reach cuttoff leading to excessive cathode voltage.

The real danger is having the ability to set the rheosat to zero resistance. In THIS case the tube WILL have NO bias and will be VERY unhappy! It may even look like these:

For my adjustable cathode bias experiments I use a big old wirewound pot. It is about 5 watts and about 800 ohms. I have a 330 ohm 5 watt resistor in series with it. I have two of these and I have had them for so long I don't even remember where I got them. It is wired so that it will go to full resistance if the wiper goes open. That is enough to prevent smoke.
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