How do you measure bias and how do you determine the desired voltages? In a device with a power transformer, from the tube pins to the chassis, or from the tube pins to the cathode of that tube?

When a bias voltage is specified for a tube, it is measured from the control grid to the cathode of the tube.

Jay

After posting this, I found that Ted Webber has a bias calculator on their web site.

If you are talking audio output tubes, this calculator is a bit better than the Weber one.
https://robrobinette.com/Tube_Bias_Calculator.htm

Are you using self bias with a cathode resistor or fixed bias from a DC supply voltage?

Either way, the bias is measured between the cathode and grid.

Jay

Thanks for the replies to what is surely a very basic question to many people!

I'm pondering my 6L6GC amplifier and measured the following today:

6L6GC grid to cathode -2.28VDC, with 409 VDC on the plates

That tube must be close to melting. You are supposed to have about -30 volts on the grid at that plate voltage.

Jay

More measurements:

6L6GC
Plate to cathode 11.69VDC
plate to ground 409VDC
grid to cathode -2.28VDC
cathode to ground 39.1VDC

Stephen

Plate to cathode should be much higher. Maybe a bad meter connection?

Have you replaced the coupling capacitor to grid #1, pin #5? How about grid resistor on this pin?

Cathode to ground shows there is excessive bias. May be caused by grid #1 being slightly positive from bad cap or resistor. Try another 6L6. Internal tube leakage could also cause this problem. Be sure cathode resistor , pin #8, isn't open.

Something is wrong with the measurements since the Plate to cathode plus the cathode to ground should equal the plate to ground.
11.69 plus 39.1 does not equal 409.

Jay

This site describes ways to measure bias.

https://sites.google.com/site/yourtubea ... nt-methods

I recall in self biased amp designs adding the 1 ohm cathode resistor and then measuring voltage across it. A well matched pair of output tubes will have near equal bias currents.

The bias of 6L6 does vary substantially according to class & situation. The screen is often run at lower volts than the plate.

Control grid bias is always in respect of the cathode. The voltage across the self bias resistor is the grid bias. Where this varies (general) is where there is back bias and the cathode has no resistor, the voltage being developed by all of the cathode currents passing through the back bias resistor ct to chassis and the OP tube grid being on the centre tap.

If the cathode resistor fails, that normally results in the destruction of any bypass electrolytic. It either goes short, explodes, or goes open. Open is indicated by all grids going substantially positive. Grid 1 resistors on many of those OP tubes 6V6 especially have a high attrition rate, normally on the high side.

http://frank.yueksel.org/sheets/021/6/6L6.pdf This is a fairly informative sheet. To hopefully alleviate things?

Marc

when calculating the bias and static dissipation in watts, I use the cathode resistor method.

the key is to use precision 1 ohm resistors. they can be a bit expensive for a little one ohm resistor, but well worth it in the end.

I use 2 watt resistors b/c the leads are thicker and more sturdy for the probes.

http://www.duncanamps.com/technical/lvbias.html

https://www.mouser.com/ProductDetail/Vi ... 5VRW6K8%3D

this method has got me extremely close when matching tubes and for the standard 6V/K/L/F6 hifi or pa amplifier, this worked just fine.

steve

Ohms law does apply to a cathode resistor as it does for the rest. The voltage across it will tell you the current through it. That is a method (where cathode resistors are fitted) which is where "Duncan's amps" article is going; A measurement of voltage across a cathode resistor, will actually give an indication as to a tube working, or not.

A diagnostic often missed.

Marc

i use it all the time. it also helps identify when a tube will begin to run away and draw too much current..

i've seen that too.

steve

Tired, worked a long day Saturday and then more work on Sunday but now have better measurements:

Plate to Cathode 369VDC, Cathode to ground 39VDC, plate to ground 409VDC.

Whats the cathode resistor? Did you go to the website i posted?
https://robrobinette.com/Tube_Bias_Calculator.htm

Yes, that is a nice site. The resistor is 350 ohms, shared by both 6L6GC tubes.

Stephen

Voltage drop across the 350 ohm resistor gives you bias. You have 39 volts and a 350 ohm resistor.

39/350 = 111 ma current being drawn by the two 6L6 tubes. Is this what you are looking for?