www.randallamplification.com
31. TUBE BIAS SYSTEM
What is bias? Simply put, it is a circuit inside the power amplifier section that controls the “idle current ” that flows through the power tubes.
Much like the idle speed on a car. There is an optimum setting where the engine (amplifier) is running (idling) (hot) enough to keep it from
stalling (distorting) but not too fast (hot) to cause excessive wear and overheating. Get it? Why don
ʼ
t all amplifiers have bias or idle current
adjustments?.... Most do but typically involve removing the amp chassis from the box, exposing you to very dangerous high voltage.
Special test equipment and knowledge of amp circuits and tubes is also needed. Not a skill most musicians possess.... Why would I want
to adjust the bias? All power tubes are different. They all have unique sonic and electrical characteristics. The RT amplifiers are designed
to accept a variety of different tube types. EL34, 6L6, 5881, 6CA7 and 6550 are among the possible choices. Because they are all
different, they each require different bias settings for safety, reliability and optimum performance.
Please read the following instructions on how to use this cool feature.......
The RT Series of products from Randall takes the danger and mystery out of bias and puts it safely into the users hands. You will need a
decent quality digital voltmeter capable of measuring in the 100 millivolts DC range. This is a very basic type meter available at any
electronic supply house or Radio Shack. They typically cost anywhere from $10 to $25. About the cost of one bias adjustment from your
local amp tech. You will also need a small, flat blade screwdriver to turn the adjustment control.
1. The amp should be on, standby in the
"
“PLAY ” position and all knobs all the way down.
2. Turn the meter on and set for reading DC millivolts. Consult the meter instructions for how to do this properly. Since all meters are
different it is extremely important that you thoroughly understand what you are looking at on the meter display.
3. Insert the Black (Negative) meter lead into the panel hole labeled
"
“COMMON ”.
4. Insert the Red (Positive) meter lead into the panel hole marked
"
“TEST POINT 1
"
”
5. You typically will observe a reading between 25mVDC (25 millivolts DC/.25v) and 45mVDC (45 millivolts DC/.045v).
6. With your screwdriver, turn the
"
“BIAS ADJUST
"
” control to obtain the proper reading for your tube type from the chart below. Write
this number down.
7. Next, move the red meter lead to the hole labeled TEST POINT 2. Write down the value. Both readings from steps 6 and 7 should be
within the range shown on the chart. Readjust if needed to get both tubes into the proper range of readings.
"
8. Lastly, keep the red test lead in the TEST POINT 2 hole and move the Black meter lead to TEST POINT 1. You should read less than
5mVDC (5 millivolts DC) or .005VDC. If the reading is greater, this indicates the tubes are not very well matched. It doesn
ʼ
t necessarily
mean they are defective, just not matched. If the reading is greater than 8mVDC (8 millivolts DC) or .008VDC, we recommend replacing
the pair with a closer matched set.
9. After completing the adjustments, allow at least 5 minutes for the tubes to warm up and stabilize. Then recheck and read- just if
needed. Lower settings will typically sound a bit cleaner and harder while higher settings tend to be a bit more compressed and softer.
Find a range you prefer.
BIAS READINGS:
6L6/5881 28mV to 35mV
--
EL34/6CA7 30mV to 38mV
--
E34Ls 6550 35mV to 45mV
Meter displays differ from one meter to the next. Some may indicate for example, 30.0 for 30 millivolts. Others may show .030 for 30
millivolts. Knowing how you meter works if of the utmost importance.
Additional features of the “POWER TUBE BIAS SECTION”
Fast Blow fuses. One per output tube. In the event of a power tube failure, the corresponding fuse will open protecting the amp from
additional damage. A red LED next to the fuse will indicate that the fuse is blown. If this happened in the past, you would need to take the
amp to a repair shop. They would then hold it for ransom while you figured out how to raise enough money to pay them to fix it. No more.
The amp will now tell you if you have a shorted output tube and which one it is! With the power off, simply remove the power tubes and
replace the indicated fuse with a FAST BLO 250mA (1/4 amp). NEVER EVER use anything but a FAST BLO 250mA (1/4 amp) fuse. If you
do have the misfortune of “blowing” an output tube, we strongly suggest replacing the pair. Remember, your amp will only perform as well
as your weakest tube.
Special Note:
You should always carry a spare pair of power tubes, fuses, screwdriver and your voltmeter with you. If a tube fails at a gig, you could be
back up and running in a matter of minutes. Try that with any other amp.
Advanced theory (for those who care):
Those of you with electronic knowledge may notice we are referring to current draw but are making measurements in millivolts. Ohms law
states that I=E/R or current (I) equals voltage (E) divided by resistance (R). Inside the amp are one ohm resistors in the cathodes of the
output tubes. The external test points allow access to those resistors. When you measure across those resistors at the rear panel test
points, you are reading the DC voltage drop across a one ohm resistor. Referring to ohms law, if R=1 in the formula, then I = E or current
equals voltage. So when you read for example, 30mV you are also seeing the equivalent value of current or 30mA.
WARNING: Do not be tempted to run your tubes hotter than the maximum values in the chart. You may find it sounds really cool as you
destroy your expensive tubes and possibly damage your amp, of course voiding your warranty! Also, in case you haven
ʼ
t found out the
hard way yet, power tubes get extremely hot (as high as 800 degrees)!!!! NEVER touch the tubes while the amp is on. Always allow at
least 5 minutes for the tubes to cool before touching them after turning the amp off.
