Can I install the mA meter in this schematic directly in the circuit?

This is the high voltage plate supply and there is RF on it.

The diagram doesn't specify if there is any circuitry to adapt the meter to RF, if that is even needed?

The ammeter I purchased is just a square face, sweep meter, 200mA.


  • \$\begingroup\$ You don't say what your plate voltage is, but obviously, make sure that the meter's insulation is rated for it! \$\endgroup\$
    – Dave Tweed
    Commented Mar 2, 2019 at 16:39
  • \$\begingroup\$ Thanks Dave. Good point. Plate voltage is 600v. Meter is supposedly rated for 2kV. \$\endgroup\$
    – McMurdo
    Commented Mar 2, 2019 at 16:43
  • \$\begingroup\$ The meter will be fine fitted where you show in the schematic. There is no RF at that point as this is bypassed by a capacitor. \$\endgroup\$ Commented Mar 2, 2019 at 17:04

1 Answer 1


Yes. That 3n3 to ground ought to shunt a lot of RF away from the meter.

Then, there's nothing like a mechanical moving coil meter for acting as a low pass filter, reading the DC correctly, however much AC there is imposed on it. If that's what you mean by a 'sweep meter'.

If I'm not reading 'sweep meter' correctly and it is an electronic meter, then an RF bypass directly across it would be a good idea as well.

  • \$\begingroup\$ Hi Neil. Thanks so much! By "sweep meter," I just mean the old style analog meter with the needle. So this can just be directly installed? Thanks again! \$\endgroup\$
    – McMurdo
    Commented Mar 2, 2019 at 16:37
  • \$\begingroup\$ A bypass capacitor across a mechanical meter is a good idea, too. You don't want the meter coil spraying RF all around your ham shack. \$\endgroup\$
    – Dave Tweed
    Commented Mar 2, 2019 at 16:38
  • \$\begingroup\$ Ok, thanks Dave. Just a .01 should do the trick? What would you recommend? Thanks. \$\endgroup\$
    – McMurdo
    Commented Mar 2, 2019 at 16:40
  • \$\begingroup\$ Not critical. Just make sure that the capacitor's self-resonant frequency is well above your operating frequency. \$\endgroup\$
    – Dave Tweed
    Commented Mar 2, 2019 at 16:41
  • \$\begingroup\$ I would add a pair of anti parallel diodes (4007s or so) across the meter, that way when something arcs you might not destroy the meter! Also, a choke across the aerial connector is a really good idea, so that if the 3n3 at the output fails short, you don't wind up with ~750V on the long wire down the garden. A glitch resistor (maybe a few tens of ohms, typically carbon or wirewound) in series with the meter will also reduce the scope for the fuckup fairy to let the smoke out. Oh and a grid current meter is actually far more useful then plate current when tuning (Grids are delicate). \$\endgroup\$
    – Dan Mills
    Commented Mar 2, 2019 at 20:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.