# Modify a 1629 magic eye tube circuit to a 6E5S tube?

I've an EICO 950B capacitance tester with me. It has an aging 1629 magic eye tube. I have a bunch of 6E5S tube with me. Can you guys guide me with the modification?

I know the first step to this is to reduce the filament voltage to 6.3V for the 6E5S. But something that bugs me is, can a simple resistor voltage drop get it done? I'm worried that this path might have issue which might kill the tubes very fast. Hoping to get some expert opinions here.

1629 datasheet: http://www.r-type.org/pdfs/1629.pdf

• Do you have datasheets for these tubes and the capacitance tester? – Hearth Oct 13 '18 at 20:26
• Have you seen this site? It seems to say that they can be adapted, but aren't directly replaceable (even aside from the filament voltage.) It also says it is more common for substituting in the other direction. You can use a 1629 in place of the 6e5s, but it is not common to use a 6e5s in place of a 1629. – JRE Oct 13 '18 at 22:34
• @JRE I agree with that. Most of the articles just mention about doubling the voltage to 12.6 to run the 1629, not the other way. But in the 950B there is already a center-tap. So, I can get the 6.3V filament voltage. But I'm not sure if the other connectons can just be wired like the 1629. – The_Vintage_Collector Oct 14 '18 at 5:43

No, it can't. Note that the filament supply is 12.6 V @ 0.15 A for the 1629, while it's 6.3 V @ 0.30 A for the 6E5S — half the voltage, but twice the current. The power transformer has a 6.3V tap on the filament winding; all you need to do is connect the filament between X and Y on the transformer (instead of between Y and ground).