This taken from Randall MTS RM100 amp manual.
MTS Power 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) fast
(hot) enough to keep it from stalling (distorting) but not too fast (hot) to cause excessive wear and overheating. Get it?
Why don?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 and shouldn?t need to.
Why would I want to adjust the bias?
All power tubes are different. They all have unique sonic and electrical characteristics. The MTS 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 MTS 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 of
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) and 45mVDC (45 millivolts DC). Some meters may
read .025 volts to .045 volts. Be sure you understand what your meter is telling you.
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?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 readjust
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.
You should always check the bias readings whenever you replace output tubes and re-adjust if needed. Since we?ve made it so simple,
there is no reason to not do it.
BIAS READINGS:
6L6/5881 28mV to 35mV
EL34/6CA7 30mV to 38mV
E34Ls 35mV to 45mV
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 Blo 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?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.
