[Amps] VTVM Diodes

[Will Matney]

David,

The tube type differences could be the gain, or voltages, I'm not sure. The probes could possibly work, but those are the ones that used an acorn tube in the tip aren't they? Here's the biggest problem. If you change the tube(s), the meter will about sure be out of calibration. What happenes is that as the meter was used from when it was new, the tubes gradually got weaker while the meter was sent out once a year for calibration. Each time it was re-calibrated, it was to the weaker tube signals. When you put a new tube in that is 100%, it throws off the calibration. Solid state meters don't do this to that extent. The yearly calibration for solid state I don't think is needed. Maybe every three years, and that's if they're really used a lot. However, if a solid state is repaired, it needs to be checked for calibration then unless the part was out of the same batch as the original. Even then, there's a possibility it could be off because of the gain difference. The tube meters though really need the calibration over shorter spans over the tubes gain dropping over use. How much a new tube could throw it off, I'm not sure.

What you could do is measure some standards if you have them, and write down the measured values. Really they don't have to be standards, just save the original parts you used to test back for the last test. Compare these measurements to the ones you get after you replace the tube, and adjust for them.

Best,

Will

*********** REPLY SEPARATOR  ***********

On 5/15/06 at 4:36 PM David C. Hallam wrote:

>I am trying to find an AC probe for my HP-410C.  I have a probe from a USM
>116 VTVM.  The 410C uses an EA53 diode in the probe and the 116 uses an
>EA52.  From what I have found on the internet, these two tubes seem to be
>similar, base connection, heater voltage, frequency range, etc.  Does any
>one know if I could use the probe from the USM 116 on the 410C?
>
>David
>KC2JD
>
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>
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