Note that the cathode resistors are 10 ohm 1W, don’t use a higher wattage rating. It is intended that they blow in the event of a problem. It wouldn’t hurt to check D1, D2, D3, and D4, the 1N4007 flyback diodes. The tubes may have tested good but inspect the sockets for cracked or bad solder joints, etc.
Ampeg posted this bias procedure on TB. Check the bias with the 6L6GC tubes installed.
V4BR BIAS INSTRUCTION:
1. CONNECT LOAD TO AMPLIFIER {this can be a speaker cabinet or a dummy load}
2. TURN R16 (BIAS ADJUSTMENT POT) FULLY CCW (MINIMUM SETTING)
3. TURN MASTER CONTROL FULLY CW
4. TURN POWER SWITCH TO ON AND WAIT 2 MINUTES
5. TURN STANDBY SWITCH ON
6. ENSURE THERE IS NOTHING PLUGGED INTO INPUT CHANNELS
7. WAIT 10 MINUTES {I like to wait 30 minutes}
8. MEASURE VDC ACROSS R12, THE CATHODE RESISTOR OF V5
9. ADJUST R16 UNTIL YOU MEASURE 450mVDC ACROSS R12
10. MEASURE VDC ACROSS R13 (V6). SHOULD FALL BETWEEN 400mVDC - 500mVDC
11. REPEAT STEP #10 FOR R14 (V8), THEN R15 (V7)
12. ALL MEASUREMENTS FOR V5, V6, V8, AND V7 SHOULD BE WITHIN 400mVDC -500mVDC
12A)
IF #12 IS TRUE, ENJOY 100W OF V-4B
IF #12 IS FALSE, THE TUBES ARE NOT PROPERLY MATCHED FOR YOUR PRECIOUS V-4B, SO GET NEW ONES.