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 equip-
ment 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 vari-
ety 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.
