RM100 Bias Information
source: RD/Rich on
MTS Forums
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.
BIAS READINGS:
6L6/5881 28mV to 35mV
EL34/6CA7 30mV to 38mV
E34Ls 35mV to 45mV
6550 35mV to 45mV
6V6 to 21mV
KT88 to 50-52mV (not sure on this one, cause I thought 6550/KT88's were biased
the same?)
Here's an online bias
calculator that's worth checking out if you know your amp/tubes, this may be the
best method to determine your needs.
(The RM100 uses a plate voltage of 500)
Weber Bias
Calculator
To quote Jeff Hilligan (Egnater) on mixing power tubes:
"to hear the difference just pull the fuses of the tubes you dont want to hear.
No need to pull the tubes, just pull the fuse and its fine."
I have also read that if you mix 6L6's and EL34's, that the 6L6's on the inside
sound better.
Other useful info:
(pasted from a thread on the MTS Forum)
RM100 has a 500 plate voltage:
Multiply it by .75
We will use the plate dissipation for a standard 6L6 for this example which is
22 watts. Example: 22 watts divided by 500 plate volts equals .044 or 44
milliamps of plate current draw at idle. The 44mA should never be exceeded! Then
multiply the .044 by .75 or 75% which equals .033 or 33mA. This is where your
power tubes will be out of crossover distortion and will perform very well.
There is an acceptable window of correct bias and if you want a warmer tone or
what some players refer to as a "brown sound" you can use up to 90% of the
maximum current formula which would be 40mA rounded off. Your tube life will be
a little shorter at this setting but only by about 20%. However, I would NOT
recommend this for Chinese tubes or tubes that are sold by the Big guys that
have been remarked unless you know for sure what tubes they really are. The JJ
Electronic 6L6 is a very stout tube and JJ claims that it will dissipate 30
watts. Since I like to run my gear pretty hot I will routinely bias them at 50mA
in an amp that has 485 plate volts. The tone is very warm and the tubes stand up
very well. I tortured a quad in my Fender twin for a year, set at 50mA at 480
plate volts and they were still going strong until I swapped them out for a quad
of KT88’s but that’s another story....
My personal preference for tubes is Doug's Tubes.
-Jim