VDO Voltmeter

I have a 6v vintage car. I want to add a voltmeter under the dash but 6v voltmeters are scarce. So I thought to add a voltage doubler inline before the gauge and adjust the gauge for any voltage loss. I’ve built a voltage doubler with a 555 chip and some bits and pieces. The doubler seems to work perfect on the bench with my multimeter. With a 6v battery and the doubler, the multimeter reads 11.5v. With a 9v battery and the doubler, the multimeter reads 17v. The car has a 6v generator and puts out around 7v which measures 12v with the doubler and using the multimeter.
Using the VDO gauge I get unexpected results. Not using the doubler, VDO reads the voltage correctly, 12v battery reads 12v. 9v battery reads 9v. However when using the doubler I get very low results. 6v battery and doubler read 8. 9v battery and doubler reads 11v. So my 7v car generator doesn’t barely register on the VDO which starts at 8v. So... why does the doubler show different results between the multimeter and the VDO gauge?

 

When I do that, the multimeter reads about 8v. When I disconnect the VDO, the multimeter jumps up to 11v. Is this result suggesting that I can’t use an analog gauge with this doubler? Do you have a suggestion?

 

My understanding about these converters is that it outputs a constant higher voltage. I want to double my varying voltage. Am I wrong about that?

 

My goal is to use a gauge that looks proper and that would be a vintage VDO gauge. With that said, there are only 12v gauges available. I need to have a 12v gauge work with 6v input voltage, so I assume I need to double it. If you have another way to meet these requirements, I’d love to hear it. Thanks.

 

Looks like this inside. One big resistor like you said and there’s also a diode too. Can I replace that resistor with some sort of potentiometer so I can adjust the resistance?

 

I can’t measure the resistance without taking it apart. I will do that but it’s a process. Meanwhile I tested the voltage drop from the resistor, 12v in and 2v after the resistor. Assuming 68ohm R, I think I’ll need a 27ohm R to drop 6v to 2v. I should know more after I take it apart. But thinking that a 27ohm 1W resistor is the answer???

 

I know absolutely nothing about resistors except what I’m looking up on the web so I’m likely wrong. This resistor has 5 bands so the first 3 blue grey brown = 681 and then the gold multiplier = .1, so 68.1ohm total. Please tell me what you see. I’ll know for sure when I get the resistor out. Thanks.

 

Every table I look at for a 5 band resistor has the 4th column as the multiplier except your table which has the 3rd band as the multiplier. what am I doing wrong?
https://images.app.goo.gl/qPBtRR5fbZpyZzDo6

 

Ok. Don’t understand why the differing color code tables. This resistor is circa 1975. And like I said, I should be able to test it when I get it out. Thanks.