Digit Brightness, Classic Rev6, IN-8 Tubes

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8 months 1 week ago #12736 by Jedidiah
I've put together a Classic Rev6 with WiFi using IN-8 tubes. Everything works as expected except that digit brightness seems to be a bit weak compared to an IN-8 being directly driven from a 170 volt power supply. I've disabled the LDR but it makes no difference. Is there anything I can do to address this or is it just a symptom of the multiplexed design?

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8 months 1 week ago - 8 months 1 week ago #12737 by BS_Jim
When you compare it to a directly driven tube on a 170v supply, what anode resistor are you using? The 2.7k resistors supplied/recommended take into account the multiplexing action of the circuit. They supply about 10ma to each tube. If you used the same resistor on a direct drive circuit, it would supply 60ma - quite excessive for any tube longevity with IN-8's, but definitely much brighter. For comparison purposes, you should use an 18k resistor on your direct drive circuit.
Last edit: 8 months 1 week ago by BS_Jim.

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8 months 1 week ago #12738 by Jedidiah
I'm using 15K for the anode resistor on the direct drive for comparison. That resistor is dropping 37 volts, so the current is about 2.5ma, which is on the low end of the spec for an IN-8.

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8 months 1 week ago - 8 months 1 week ago #12739 by BS_Jim
Ok, sorry my quick math was off. The basic idea is you need a resistor of 6x the value of the one in a multiplexed circuit for your direct drive circuit to be comparable. That works out to 16.2k. If you really want your tubes brighter you could lower the anode resistors in the clock to maybe 2.2k. That would probably make them a little brighter at the expense of tube life. Not worth it in my opinion, but if you've got enough spares then go ahead.

Edit: I just had a look at the translated datasheet for IN-8's on Dieter's site (IN-8 datasheet) and 2.5ma is right or maybe even a little high for >10,000 hrs. lifetime. I believe Ian supplies 3k resistors with his kits. You could replace them with 2.7k or even 2.5k (if you can find them) to get the brightness you expect from your test circuit.

The tubes in my avatar photo are IN-8-2's with 3k resistors in indirect afternoon light.
Last edit: 8 months 1 week ago by BS_Jim.

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8 months 1 week ago #12740 by Jedidiah
Thank you, BS_Jim. The anode resistors are indeed 3K. If I measure the voltage across any of them while the clock is running and the LDR is disabled, I get about 3.5 volts at the most while the digit "8" is displayed. This would give an average current of about 1.2 ma at the most, and even less for other digits. If I use an anode resistor of 30K on my test stand (to duplicate the 1.2 ma), I get about the same brightness. So it would seem that an an adjustment to the anode resistor values -or- an adjustment to the MUX timing might possibly work. Any other thoughts?

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8 months 1 week ago #12741 by BS_Jim
Whatever value resistors you decide upon, check the voltage across them in the clock circuit and calculate the current. You'll want to keep it below 2.5ma to get the rated life out of the tubes. You could set up a test jig on a breadboard on one of the clock digits and try different values to decide what's best.

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