The AM Forum

Working on laying out the front panel for the 807 modulator, I am looking at how to meter the main modulator current and have it switchable to meter each tube individually, or both together.

I have a few nice Simpson Model 27 meters that are direct reading, i.e. you put 100mA of current through them, they read 100mA. The one I want to use for the modulator reads to 250mA, which is plenty enough to make sure the meter isn't pegged out on modulation peaks.

I'm familiar with using meter shunts, but in this case, the shunt appears to be internal to the case-do I just open up the case and cut the shunt resistor out? That seems...barbaric. (but, I look the part so I shouldn't worry about acting the part...)

I had planned on putting 1 ohm shunt resistors in the cathode circuits of the 807's, so that by flipping the meter switch and adjusting the bias pots (accessible through insulated holes in the front panel) you could dial in any pair of 807's and not need matching pairs, or to bypass a safety switch to work on the modulator with B+ applied AND have the cabinet open.

I also plan on a 1 ohm shunt resistor in the B+ lead, to actuate a shutdown relay in case anything goes wrong and more than a certain amount of B+ current gets drawn (modulation transformer fault, tube fault, wire short, etc. I wanted to switch the meter to read across this as well.

Now, given that the 1 ohm resistors would give 0.25V for full scale, and the meter movement needs about .166V to read full scale (just directly on the power supply output, adjusting the current limit knob) That Ain't Gonna Work.

There was a good article on this somewhere, but I can't find it anymore. Anyone got a link or resource to share?

https://www.w8ji.com/metering_amplifier.htm

Thanks. Not the one I was looking for but it's got the info I need. I've got some catching up to do before winter gets here and I need to heat the office!