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Old 3rd July 2005, 02:57 PM   #31
amperex is offline amperex  United States
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Default Clean bias supply........

Start out with twice the required bias DC output voltage via a full-wave rectifier & capacitor. (If you need -50 vdc bias, start with -100 vdc).

Supply that semi-filtered voltage in a RCRC filter. Drop voltage approximately 15% in the first resistor & 15% in the second resistor. With 100 volts in, 70 volts is the output. Feed that voltage into a potentiometer that supplies a few millampere load to ground. The wiper on the pot is the adjustable bias voltage out. For better resolution on output voltage, the pot can be in series with another resistor to ground. The first three capacitors are 50uF and I prefer a 10uF poly as the last capacitor connected to the pot wiper output. Adjust the wiper on pot for -50 vdc in this example.

The end result is actually a CRCRC to a RC with the pot acting as the last RC filter with adjustable fixed bias. It is a very simple circuit & provides very clean DC voltage.
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Old 3rd July 2005, 03:54 PM   #32
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Default Re: Clean bias supply........

Quote:
Originally posted by amperex
Start out with twice the required bias DC output voltage via a full-wave rectifier & capacitor. (If you need -50 vdc bias, start with -100 vdc).

Supply that semi-filtered voltage in a RCRC filter. Drop voltage approximately 15% in the first resistor & 15% in the second resistor. With 100 volts in, 70 volts is the output. Feed that voltage into a potentiometer that supplies a few millampere load to ground. The wiper on the pot is the adjustable bias voltage out. For better resolution on output voltage, the pot can be in series with another resistor to ground. The first three capacitors are 50uF and I prefer a 10uF poly as the last capacitor connected to the pot wiper output. Adjust the wiper on pot for -50 vdc in this example.

The end result is actually a CRCRC to a RC with the pot acting as the last RC filter with adjustable fixed bias. It is a very simple circuit & provides very clean DC voltage.
Any chance of a schematic?
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Old 3rd July 2005, 08:33 PM   #33
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HI

Why don't you make use of the automatic-fixed bias...
It's a design by mattijs from machmat (www.machmat.com/sales/kits/index.htm)

you can manually set the bias current... and whatever changes (even the mains voltage changes within 5-10%) the bias current will be kept on the right level....no more milli-amp meters in your amp!!

i tried it and it was really worth it!! the sound got much better....

i'd really like to hear your comments on this

henk
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Old 5th July 2005, 06:16 PM   #34
PRR is offline PRR  United States
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> how do you calculate the grid resistance needed when you use combination bias?

By the percent of self-bias.

If the book says 500K self-bias, 50K fixed-bias, and you run 50% fixed bias, use 250K max grid resistance.

No reason you can't use your 0.1uFd caps.

The above derivation ignore the fact that the fixed-bias rating isn't zero ohms. I suspect you could apply the factor to both ratings and add: 250K+25K= 275K max with 50% self bias.

I don't see any point in precision math here. The max grid resistor rating is a worst-case. Look around, you find many working amplifiers using "illegal" grid resistors, and living. Mostly. 90% to 99% of tubes will have an actual grid current much lower than the one used to set the max grid resistor rating. In DIY, you can often "cheat", being prepared to replace any tubes that get unhappy with too-large grid resistors. In mass-production, such tube selection (and tube aging inside the warranty period) is just too expensive to consider, so production gear usually stays with the rating.

There is a special case for triodes with large plate resistors. Even if the grid current and voltage runs-away, they can't melt-down. You can run them without cathode bias and let the grid find its own level. 10Meg and 22Meg were common grid resistors in this use.
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