Check my Bias Supply Design?

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JoeTele
Posts: 127
Joined: Sun Oct 27, 2013 2:20 pm
Location: Milwaukee, WI

Check my Bias Supply Design?

Post by JoeTele »

I'm working up a tube tester design that will use real-world voltages. I'd like to have a pretty wide sweep on the negative bias supply and about .5 volts to 90 or so volts negative is what I've settled on. The following is what I've come up with. My main intent is to get the desired range without ever flowing too much current/voltage to burn up the main potentiometer (and, of course, the transformer).

1. I'll start with a 100 volt .5 amp Hammond filament transformer.

2. Bias will be adjusted by 2 Bourns pots in series to ground. The first will be a 50k 2-watt and the second a 10-turn 2k to fine tune (my go with a 10-turn for the 50k also).

3. The key to the circuit is a DPDT switch that will simultaneously act on both the input resistor to the diode and the limiting resistance in series with the pots.

-In position 1, the low end of the bias range (~.1v~48v) I'll have 6K resistance going into the diode and a 10 ohm limiting resistor in series with the bias pots. According to PSUD, with the pots turned all the way down to the 10 ohms of limiting resistance I'll have about 127 millivolts and 13 milliamps through the 10 ohm resistor. With the pots turned all the way up I'll have about 47 volts and 15 milliamps of current (.7 watts) through the pots and 10 ohm.

-For the upper end of the range (~45v~92v) I'll have 1.5k resistor going into the diode and a 3k limiting resistor in series with the pots. Per PSUD, with pots all the way off, will have about 45v and 15 milliamps through the pots (.7 watts) and with the pots all the way up/resistor in the circuit 92v and 1.7 milliamps (.16 watts).

A screen shot of this last scenario is included just to show how PSUD is set up. I've simulated the input resistance by making it internal resistance of the PT and "adjust the pots" by plugging different values into R1.

Is this legit? Safe? Is a regular 120v 3A DPDT mechanical switch ok here? Back to the drawing board?

As always, I'd appreciate any insights.

Thanks!

Joe
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