Proper 6L6 Idle Current Test

[KingCameron]

Can someone please tell me the proper way to test the idle current on a 6L6 using a Fluke?

I know this makes me sound stupid, but I want to make sure there isn't a gotcha here...
Thanks

[Garydean]

Are you trying to match tubes or are you trying to bias your amp?

Gary

[KingCameron]

Bias the Bassman

I have been running my '67 Bassman at ~-25V bias voltage. It heated up my 6L6s super hot and would ultimately blow the fuse. I have since adjusted the negative voltage supply to the proper voltage (-44V)

However, I want to play with this a bit and see what current I am flowing at different settings. I don't have a proper bias tool. Soooo
I want to know what the best way to do this with a Fluke is..

[Garydean]

plate current can be measured with an ammeter in series with the plate connection pin 3 (caution HIGH voltage). Move your test leads on your fluke to measure current mA, not voltage, and adjust the dial as well to mA. Some would suggest to measure the voltage drop across a 1ohm resistor on the cathode pin 8, but your readings will be relative to the actual true value of the 1ohm resistor, and could be lower.

[mikeboone]

You need to buy a bias measuring device.

Several companies make bias meters. Ted Weber sells the Bias-Rite which is a really simple device that you can use with your DMM.

https://taweber.powweb.com/biasrite/br_page.htm

Look at the BR-AH (octal). That device or one like it is the simplest and most efficient way to accomplish this.

Otherwise you have to break the circuit, pass the voltage through your DVM to determine the bias current. There is considerable risk involved in that method.

[KingCameron]

Resistor check

Thanks, I'll try this method

[KingCameron]

Bias rite

Yup. I need to buy one...I was just curious if I could do some experimenting in the mean time.

[mikeboone]

To check it with the resistor...

Lift the ground on Pin 8 and solder a 1 ohm 2 watt resistor to it and connect the other side to ground.

Then measure the voltage present at pin 8 relative to ground. The voltage measured will equal the actual current since it is through 1 ohm.

Depending on your plate voltage and the exact tube type, you should measure approximately.035 to .040 VDC per tube. As long as you are in the ballpark you will not hurt anything, just maybe shorten the life of your tubes.

[Garydean]

I made my own bias plug/adaptor to the tune of about $6. Works great. Instructions upon request.

[Enzo]

1% tolerance resistors are not expensive, so that should solve the precision problem. Wire from pin 8 to ground on the power tube sockets.

Another crude measure is this. With power off, measure the DC resistance of the output transformer from center tap to each power tube plate - from the red wire to the blue wire and to the brown wire. Note down your reading, they probably won;t be the same. Now power up the amp and read DC voltage across each winding. Now plug the resistance you already knew and that voltage into Ohm's Law, and you get the current.

Let us say the primary half winding measured 88 ohms (I totally made up the number), and I get 3.0 volts across the half winding.

Ohm's Law tells me I=V/R=3/88=.034A=34ma.

There is also the shunt method. You set your meter to DC milliamps, then connect it between pin 3 of a power tube and the center tap of the OT. BE EXTREMELY CAREFUL, your meter acts like a short circuit in current mode, so if your probe slips and touches ground, you will ground off B+ through your meter. It will blow the expensive little fuse inside the meter at the least. Same deal if you stick the meter on the wrong socket pin. Only leave the meter there long enough for a reading. I highly recommend you use one of the other methods until you are an experinced tech.

[KingCameron]

Nice trick

Thanks Enzo,
I'll try the first method, and push for a raise in my allowance this month. Perhaps I'll just get the damn tool and forget about other methods.
I have more BF amps setting around than I know what to do with, and all my buddies are wanting that ambiguos special something out of them.

Thanks for the help..