Biasing Question

[Celery_Strat]

Hello,

Long time reader, first time poster... I recently completed a weber 6a20 (Fender BFDR) and it went off without a hitch first try and sounds AMAZING! Funny thing is, when I tried to bias the amp using the "shunt" method, I get 0mA on TWO seperate meters.... I can't figure this out! I connect to the center tap of the OT, and to Pin 3 of one of the 6V6's, and switch to "play" and nothing... I tested the meters on a battery, and the both measure DC current... What could be the problem here?

[selloutrr]

http://www.regiscoyne.com/tech/bias/

[Celery_Strat]

That is EXACTLY the site I learned to do it from... I guess it's because I don't have a "Fluke" meter...

[Jana]

My eyes are tired and I skimmed the article listed in that link but from what I can tell is that it is not accurate. It says to set the DMM to the 200 ma *current* reading but then gives instructions as if you were measuring a voltage (measure from center tap to pin 3). If you are going to measure from the center tap to pin three without disconnecting any wires and INSERTING the meter into the circuit then you should be measuring the voltage that is dropped across the OT winding. You use this voltage measurement (on each side) and with the known (measured) resistance of the windings then calculate the current.

[tubeswell]

What Jana said. measuring the mV drop across a known resistance will enable you to calculate the current flowing. I = E/R (or to put it another way Amps = Volts/Ohms, and milliamps = millivolts/ohms). You have to accurately know what the resistance of each side* of the OT primary is (which you measure with the amp switched off and the tubes unplugged), and then measure the (milli)voltage dropped across each side when the plates are idling.

i.e.: from the plate-end of each side of the OT primary to the centre tap of the OT primary

[Firestorm]

What Jana and tubeswell said is true, but kind of a hassle. If you have a good meter, the transformer shunt method works fine. Set to measure current, you meter will have an eensy weensy amount of resistance compared to half the transformer winding. 99.xx% of the current will bypass the winding and go through the meter instead. (You can hear the amp go dead silent when you do this).

Three things: 1) read your meter manual; many meters (like Flukes) require you to connect the leads differently to measure current (versus voltage and resistance), so if don't do that you'll get no reading. 2) The better the meter, the more accurate the reading. Some transformers have less winding resistance, so if your meter's resistance isn't essentially zero, the reading will be off by several percent. 3) This is dangerous. You're dealing with the highest voltage in the amp at a lethal amount of current. Connect the leads before you turn the amp on and keep one hand in your pocket while working.

[Celery_Strat]

Ok... I'm a bit confused... I removed the Power tubes, and measured the resistance of each side of the OT. (144Ohms) Now, do I measure the voltage with the tubes out, then with the tubes in, and use the difference as the V drop? DCV, correct?

[jjman]

You would now insert the tubes and attached one end of the meter to the CT and one to one plate. No hands should be used to hold either end on it's point since both ends have high voltage on them. The "other" end of the meter is hot because of that. Turn the amp on and note voltage (drop) on meter. Then use ohm's law.

I don't like either method mentioned since I find both to be inaccurate. Even a Fluke meter can provide enough of it's own resistance to skew the result on the "shunt method." And the voltage-drop method yields unbalanced math results for me on one side vs the other, whereas accurate cathode taps show balance.

I'd like to see the actual results on a given amp under all 3 methods.

[Celery_Strat]

I'd like to see the actual results on a given amp under all 3 methods.

Well I'd be glad to, if I could get this to work. (I feel really stupid, here.) When you say "drop", in relative to what measurement? I'm missing that part. Drop from what? With the tubes in and the amp on, I get 250 mV across pin3 and the CT.

[martin manning]

Be careful!

You don't need to remove the tubes, just turn the amp off, and drain the caps.
Measure the primary resistance R of each side of the OT.
Turn the amp on, standby switch on "play," and measure the DC voltage V across each side of the OT. This is often called voltage drop, as it is a reduction in voltage resulting from the DC current flowing through the resistance of the winding.
Calculate current for each side as V/R.
Divide by the number of tubes on each side to get current per tube.
Measure the plate to cathode voltage for each tube.
Multiply by the calculated current to get plate dissipation.

Using the OT primary resistance method should be dead-on, and gives plate current alone, not plate-plus-screen like a current sense resistor between cathode and ground. There will usually be a different resistance measured for each side of the OT, but if you use the measured values of R and V for each side that is properly accounted for. If there is more than one tube on each side, then the plate current and dissipation will be averaged, where individual current sense resistors will indicate individual (but total) current. Cathode current sense resistor method is quickest (if the amp has them) and safest due to the low voltages involved. The shunt method is the most risky, IMO.

[Celery_Strat]

OK...
Here is EXACTLY what I have done...

Measured resistance of OT =144Ohm per side.
Turned amp on, tubes in place.
Connect red test lead to center tap
Connect Black test lead to pin 3 of selected 6V6 tube
Set meter to 20VDC range. =.21V
Set meter to 2000mV range. =210mV (duh)

Now, if I divide .21/144 I get .00145
And obviously 210/144 gives me 1.45

What step am I missing here?

Please don't laugh at me! Leave that to me!

[Structo]

The transformer shunt method places you at unnecessary risk of shock and it is dependent on how good your multimeter is for accuracy of reading.

You are far better off soldering a precision 1 ohm resistor from each cathode to ground on the 6V6.

Buy two 1 Ohm 1% (or less) 1 watt resistors.

In place of the ground jumper on pin 8 of the 6V6 solder the 1 Ohm resistors.

Then set your meter to low voltage and measure the voltage drop across the 1 ohm resistor.
The 1 ohm resistor converts the amps to volts due to Ohm's law.

I = E/R so R =1 so whatever voltage you measure is the current.

This means black probe clipped to ground, red probe to pin 8.

Write that down.

Now measure the plate voltage at pin 3. Write that down

If your meter is not auto ranging set it to measure DC volts in the 500v range or so.

Most set the upper end of bias at 70% for safety.

(W * .7) / V = I

W = watts of 6V6GT
V is plate voltage
I is bias current

Figure 12 watts for the 6V6GT

12 * .7 = 8.4w

8.4/ what your plate voltage was.

Say it was 300

8.4/ 300 = .028 = 28ma

So 28ma on each tube would be the upper limit of your bias range.

Some amps sound better set lower.

Experiment. Probably don't want to go below 50%.

[martin manning]

Your calculation is correct, but the 1.45 mA can't be right if the amp sounds good. Was the standby switch set to "play" when you measured the OT voltage drop? You are looking for ~20mA, so you should get about 2.88V. What do you get measuring the voltage at pin 3 to ground? It should be 415V per the Fender schematic. I am a bit surprised that both sides of the OT have the same resistance, too.

[Celery_Strat]

Sorted! My mouser order came in just a bit ago. Soldered the resistors in place, and all is well. I definitely like that method better.! It just "feels" safer! Thanks so much guys!

[martin manning]

So what was the problem? The other way should work too.