Randall RM series tube bias readings

Randall RM20 and RM22

Tube type
Bias voltage reading range
EL84/6BQ515mV to 20mV

Randall RM50, RM80, RM100 and RT2/50

Tube type
Bias voltage reading range
6L6/5881 28mV to 35mV
EL34/6CA7 30mV to 38mV
E34Ls 35mV to 45mV
6550 35mV to 45mV
6V6 18mV to 21mV

How to guide

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.

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