Here's a guide on biasing an Engl Powerball that I posted at Lord Riffenstein's Engl Forum:
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Biasing the Engl Powerball
Revision 2.2
Alright, as I wasn't fully satisfied with the guides on the net and here, I decided to put a list up of the steps now that I saw how to do it, first-hand. As I may have missed a step or something important, I'd appreciate if you guys could correct any mistakes on this
1. Remove the amp from the wooden shell
When you turn the head upside down, you'll see some screws
next to the rubber legs (not in the middle of the rubber legs). Unscrew those to loosen the amp from the wooden case. Next, unscrew and remove the metal plate (above the speaker connectors etc) on the rear of the amp. Now, you can wiggle the amp a little so that it is loosened from the case a little, and then push the front plate (where the gain/EQ/volume etc are) towards the back of the amp, and take the amp outside.
2. Removing old tubes
Whenever you handle the tubes, be sure to wear some plastic gloves or similar to protect them. If you handle the tubes with bare hands, you will leave grease on the tubes. When the tubes heat up, the grease will burn through the tube and destroy it. Now that you have the amp outside the shell, measure the bias current of your old tubes and write it down. After that, adjust the bias pot to the maximum negative value. Then pull out the old power tubes (don't put the new ones in yet). And the lastly, put the amp upside down on top of books or something similar. The tubes will be facing down, and you will see the electronics of the amp.
Now, turn all volume knobs to 0, just to be sure.
3. Measuring electrical values
This is the dangerous part:
1)
Do not touch anything you don't know what it does and be very careful when prodding around the amp.
2)
The amp contains lethal voltage, if you are even a little unsure, take your amp to a pro and follow how s/he does it before trying it on your own.
3) From here on, you'll do everything at your own risk!
Now that I have you scared to death, let's continue:
** check once more that you have a speaker/load attached and that all volumes are at zero **
Don't turn the amp on yet.
Now, with a regular digital multimeter set to measure ohms, measure the speaker outputs (designated "left" and "right") with the multimeter sticks on the outputs and on the center connector (there are five in total). On my amp, the left side had a load of 49 ohms, and the right side had a load of 51 ohms. Write down the values measured from
your amp (they may be different).
Put the new tubes in and turn the amp on like normal with standby mode left on etc, and with the amp still upside down.
Next, measure the plate voltage. Set the multimeter to measure volts. Place the other multimeter stick on Ground (the connector glob where the amp power cable is attached to) and the other multimeter stick to the tube socket connectors (the measurement point is the 5th connector on each tube socket connector). Measure it from each socket. On mine, the values ranged between 430V - 459V, and the final values used were 433V on the left side and 434V on the right side. Write the values down from your left and right side.
And finally, measure the bias voltage from the 7th socket connector of each tube. On mine, the left side was -43 mA and the right side was -43.3 mA. By tweaking the bias, you can set your final tube watts.
4. Calculations
This follows the basic formula of: (Impedance / Voltage) * Current = Power
For example, in my case the left side had a plate voltage of 433V and a load of 49 ohms, and the bias was -43 mA.
Code:
(49 ohms / 433 V) * -43 mA => 4.866 watts per tube (times four for total = 19.464 W)
Now, the right side:
Code:
(51 ohms / 434 V) * -43.3 mA => 5.088 watts per tube (times four for total = 20.353 W)
Now to measure the final result, we multiply each side by two instead of four:
Code:
4.866 W * 2 = 9.732 W
5.088 W * 2 = 10.176 W
9.732 W + 10.176 W = 19.908 W total
From the website of JJ, you can see that their 6L6GC tubes are rated up to 30 W. When you take 70% from the max value for a good operating power, you'll get a max of 30 * 0.7 = 21 W. Keep the values below that. Also, 10.176 W - 9.732 W = 0.444 watts' difference, which means the tubes are reasonably well matched.
5. Adjusting the bias
Now, to the point. What you are trying to do is to get the tubes close to their 70% power rating (in this case, 21 W), or depending on your taste colder/hotter. Do this by turning the bias pot, and then measure the values (plate voltage and bias current) again. The values will roam a little, but try to get a stable reading. If you don't seem to get the left and right side to match reasonably, you can try switching the tubes around.
Now, play through the amp and see if you like the sound, or if you want it hotter or colder. Too hot amp uses the tubes up faster and the sound can be too powerful, almost shrill. Too cold amp uses the tubes slower, but the sound isn't as powerful - perhaps a little lifeless.
Adjust the bias to taste, but stay below 21 W total.
When you're happy, pack the amp up. You need to re-measure the values sometime later (a few days), remember to play through it a bit more often than usual to be sure.
Hopefully this guide makes it a bit clearer. Though it's a bit more complicated than common things, it's not too tough when you do it calmly without hurry and paying attention.
Thanks to Morgathan for helping and showing
P.S.
After remeasuring the amp a week later, the values were:
Code:
Left side plate voltage: 428 V (compared to 433 V)
Left side bias current: -42.3 mA (compared to -43.3 mA)
=> Left side power: 5.064 W (compared to 5.088 W)
Right side plate voltage: 426 V (compared to 434 V)
Right side bias current: -42.5 mA (compared to -43 mA)
=> Right side power: 4.866 W (compared to 4.866 W)
Revision changes:
Rev 1
- First version
Rev 2.0
- Reorganized order and formatting made clearer
Rev 2.1
- Small corrections on measuring procedure
Rev 2.2
- Added pictures
- Cleared up the formulas
- Changed pin naming in text
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Enjoy
