Manley Labs 300B owner manual Tube Replacement, Fuses

5.) The correct bias reading in "Single-Ended"mode is 550 mVDC. A reading of

540-560mVDC will be within specs. The bias in single-ended mode was set at the factory.

If, however, the S-E bias reading is out of spec after warm up, the bias may be adjusted as follows: 6.) Carefully turn the amp on it's left side, Be sure to brace the amp so as to prevent roll over.

7.) Locate the 1/4'' round hole in the bottom cover, near the front group of ventilation slots.

8.) Make sure the amp is in S-E mode, warmed up, muted and with voltemeter attached. Then insert a 1/8'' - 3/16'' slot screwdriver through the hole. Rotate the screwdriver until properly seated. The control is about 2 inches below the cover plane. A screwdriver guard is provided to prevent accidental contact with other components.

9.) Slowly rotate the screwdriver while watching the meter reading. The meter may be in either "read bias" tip jack. Clockwise increases S-E standing current. Adjust standing current until meter reads 550 mVDC.

TUBE REPLACEMENT:

Should the 300B tubes require replacement (or the originals are not returned to their proper sockets), make sure both bias adjust trimmers are turned fully clockwise before applying power to the amp. Then follow the bias procedure outlined above. The bias should be checked daily on a new set of tubes for the first week of operation. Remember to let the amp warm up for 20-30 minutes before making final adjusments. The 5U4G, 6SN7 and 6SL7 require no adjusments.

FUSES

There are two fuses in the amplifier. One is the mains fuse located in the AC input module on the rear panel. The other is a P-C board mounted B+ fuse inside the amplifier. The fuses used in your amplifier are standard 1/4" x 1 1/4" SLO-BLO types. The correctly rated fuse has been installed at the factory for your country's voltage. If replacing a fuse, always unplug the amplifier's power cord from the wall outlet and always use the exact same type and ampere rating fuse as the one you are replacing. Failure to do so will void your warranty and can be a dangerous fire hazard. For 117 volts mains correct fuse size is 3A slow/blow 3AG. For 240 volts the correct fuse is a 1.5A slow/blow MDL 1.5A for the mains.

The internal B+ fuse is a 400mA 3AG ceramic body type unit. If this fuse blows, the tube filaments and badge lamp will ligth up, but no sound will be heard since the high voltage will be absent. If this fuse fails, it indicates trouble which will generally require technical attention. Contact your dealer or Manley Laboratories if this problem should develop.

Caution ! High voltages and high energy storage makes opening the amplifier enclosure hazardous for any uninformed person ! Call us or your dealer for specific instructions or for answers to any questions.