HOT cathode current monitoring - Zenith 25MC30

[cluelessgame]

A few months ago I picked up this little strip chart recorder. It uses a 50uA meter, which by itself isn't very useful for most things I work on, but with proper limiting/shunting it appears to be pretty versatile. I have been wanting to use it to graph the Horizontal Output tube cathode current on several of my sets, but primarily on my Zenith 25MC30, who's current starts out at about 202mA and over time drops to about 190mA. I want to make sure it won't fry the recorder, or alter the set's performance and/or damage it. I've laid out a rough simulation of the shunt circuit I came up in falstad. It works to where If ~212mA is going to the plate (higher than what any of my sets actually operate at) I get ~21.2uA across where my strip chart meter would be (in series with the 100k). The total shunt network resistance is just shy of 10ohms.

Would adding ~10ohms to the cathode of the HOT be detrimental to the TV?

[old_tv_nut]

Not answering your question, but do you know the resistance of your Simpson? Your diagram seems to assume it's near zero.

[cluelessgame]

Unfortunately no. The manual is very vague/practically useless for most info. I'm not too concerned about it being numarically accurate, more I just want to observe the general trace over time to monitor fluctuation. I'll have my DMM hooked up in series before the shunt network to get accurate values

[Electronic M]

Got a variable bench supply with a constant/regulated current mode?... If so connect your meter across the supply in constant current mode and slowly increase the current until you hit full scale, note current being supplied with a second meter if you can (to make sure full scale is accurate) and also measure voltage across the Simpson....if you have it's full scale current and voltage, and know ohms law you are 1 math problem away from having the meter resistance... It would probably be a lot more useful if your graphs were calibrated so you could read the actual current value over time....if you design the shunt to give you 250-300mA full scale (most sets that are not redplating never touch 250mA) you can get an accurate value off the chart.

[maxhifi]

Measure the DC resistance of the Simpson meter movement, with a DMM. Then use the following formula:

The resistance RP to be connected in parallel with an ammeter of full scale current IA and full scale voltage drop VA to increase the full scale current to I is:

RP = VA / (I - IA)

The power P dissipated by the resistance RP with voltage drop VA carrying current (I - IA) is:

P = VA2 / RP = VA(I - IA) = (I - IA)2RP

Note: Use I=250mA, IA=50uA, VA = R(Measured) * 50uA