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In Reply to: RE: How to add tube current meter to 300B amp?? posted by Alpha Al on March 20, 2014 at 09:47:08
I built that same design a long time ago, sourcing parts from SJS. I never liked the IT and later sold it (mine rolled off the highs). I also found it necessary to add grid stoppers to the 417a, which IIRC is not shown on the schematic. These use fixed bias via a seperate winding. All PS are choke loaded. Not sure if I can be of much help on the current meter, you'll need to know it's internal resistance so you can figure the shunt correctly. The 300b has DC heating, IIRC. Is the wiper of the hum pot grounded? If so, I'd put the meter there rather than the B+ side.
Follow Ups:
Yes, the wiper goes to ground. I planned on putting the meter there.
I just need to know how to shunt the meter to make it read 0-100mA.
Use a variable voltage source and pass a current so that 1ma flows. Measure the voltage. Internal R = v/i. That R becomes one part of a parallel circuit. You know 1ma across, at a particular V, so then at that V what R do you need to pass the other 99ma. Solve for that (will be a tiny resistance), and there you go.
The easy way to do this is to use a multimeter to measure the resistance of the meter. Shunt resistance should be 1/99th of the meter's resistance.
I thought about that but I didn't know if the voltage of the ohm meter might damage the 1ma meter, I guess not.
Start on a high resistance setting (the current will be very low). Switch to lower scales, watching the meter being measured. Stop before the needle goes off scale. That way you can be sure.
I'll be sling'in solder this weekend. Thx, Al
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