How to measure tube cathode current?

[pbman1953]

I'm learning that my Bias Rite meter is not as accurate as it should be.


I know I can get the voltage figure from pin 3 from all tubes.I do notice that my meter measures ,on the tube socket side, all the same for all pins 3's. The issue I have with the BR is that not all positions measure the same. I'm in contact with them about that.


How do I get the cathode current to figure out the dissipation I want or if I want to dial in cathode current?


Thanks

[Papa Jim]

Somewhere in the cathode wire going to the socket pin in series put a 1 ohm resistor.  Measure the the voltage drop across the resistor with your meter on the mV scale. Every 1mv means 1ma of cathode current. I'm pretty sure you only need to measure the output tubes cathode current.  from others on that though to be sure.

[sluckey]

If it's a cathode biased amp simply divide the measured voltage at the cathode by the measured value of the cathode resistor.

If it's a fixed bias amp, disconnect the cathode of the power tube from ground and connect a 1Ω/1W resistor between the cathode and ground. Do this for each power tube. Now set your voltmeter to read millivolts and measure the voltage at the cathode. Let's say you measure 40mV. That's the same as 40mA flowing into the cathode.

[pbman1953]

I had a feeling you'd say this. I'm not with the amps now, But I think both amps, pin 1 & 8 , are tied to ground. This means that there's a 1 ohm/1 watt in the tube adapter of the Bias Rite.


I think i have get with them if I want to trust the BR. Or separate each pin 8 ground in order to attach it to the 1 ohm/1 watt to meter to ground.


Do I have this right?


Thanks

[sluckey]

Maybe you can post a schematic for Weber's Bias Rite? There does not necessarily have to be a 1Ω resistor in the bias rite. Some bias probes actually measure current rather than voltage across a 1Ω resistor. Hoffman's bias probes are like this.

[Papa Jim]

I'm just curious because I am trying to learn more about amps. What tube is it, or what amp?

[pbman1953]

I have a Traynor YBA-3 Custom and a Sunn 2000s


Traynor- 4- 6550's


Sunn- 4- KT120

[shooter]

Traynor- 4- 6550's
Sunn- 4- KT120

That's enough to get the neighbors attention 

[zozoe]

OK now, After draing all the filter caps🥵, with my handy DVM, which pins on the power tube do I check the voltage over? I was told to check the MV across the 1 ohm resistors & on each power tube cathode.... (the sizable green ones)...
Kindly highlight the pins for me, or am I to put the leads across that resistor, then power up?
Thnx ॐ💃

[sluckey]

Pin 8

[zozoe]

Are the pins numbered on the socket?
Pin 8 & where, the hot end of that 1ohm resistor?  thnx

[sluckey]

Connect the red meter lead to pin 8. That's the pin with the big green resistor connected. Connect the black meter lead to chassis.

[zozoe]

GOTCHA!!  Thnx, Kenny ॐ💃