Favorite bias measuring and metering methods?

[Gabevee]

Hello all!

A question... Does not an ammeter neet to be installed in series with the circuit?

If so, the ammeter will add its own resistance to the circuit. If so, it will cause a slight change to the circuit when removed from the circuit. If so, balance may go off a bit, if the tubes are not dynamically matched... no? Am I way off and don't know what the heck I am talking about (which could be the case)?

Me, I would use a high impedance or digital volt meter to measure the voltage across the grid resistor, if self biasing is used, or, if a small resistor is inserted to each cathode then to a common cathode resistor, across each small value resistor. This way when the adjustments are made, they are pretty much that way for good, because the DVM will present no load changes when removed from the circuit. Current can be calculated easily with Ohm's law.

Unless one plans to keep the ammeter in the circuit.

All of this depends on the type of bias scheme and push pull connection used, whether the cathodes share a common resistor or not... etc. etc. etc.

Just a thought.

Gabe

[SY]

Well, the idea of using the ammeter in series is that its resistance is quite low, certainly lower than the usual cathode resistor. And the meter can be easily switched (or plugged) in and out of the circuit.

Michael, I'd use several opamps (or, realistically, a single quad unit) and vary the output connections, simply because I prefer not to spray signal around the chassis when avoidable. Since you're doing a measurement that's always positive, you can get away with a single-ended positive supply if you choose the right chip.

[djmiddelkoop]

when we balance out between both tubes in a PP, we assume both tubes are equal in behaviour.

Yes we do need matched tubes, but they are measured when new.

Have we considered what happens when the tubes age ?
Is balancing still ok ?
Shouldend we match on transconductance ?


Dick

[Original Burnedfingers]

Michael

The Quicksilvers bias is set for the tube pair, not individually . The meter is a mA meter sold with the amps or aftermarket. I wish they had separate bias but I would hate to modify them and loose some of the value.

Joe

[audiobot]

Hi All,

I dug into my junk pile today and brought out the power supply that has a meter that reads from 0-20uA, 0-200uA, 0-2mA and 0-200mA with different switch settings. That is as well as 0-5kV and pressure in torr. The resistance through the meter with the lead marked positive disconnected is 5.6k and it appears to be always shunted with 4.6k when connected to the switch. Other than that the ranges seem to be selected by bringing into the circuit somehow (this is all on a multipole switch and not the easiest to figure out) a decade series of resistors, ie. 1, 10, 100. 1000 ohms and HV setting seems to be a voltage divider. When I put my ohmmeter across the meter the needle (on the meter not the ohmmeter) goes to the stop. My question, what kind of meter is this and what do you think the native range is?

Thanks,
Michael