TAD-60 BIAS INSTRUCTIONS Very important :
Tube bias must be set immediately when tubes are first installed. Excessively high tube bias will burn output tubes in a matter of minutes. Output tube glass gets very hot and will burn your skin - be careful. T AD-60 bias adjustments are very precise 25-turn pots. Turning right raises the bias. Please read instructions below.
WITH NEW TUBES INSTALLED:
1. Start with the amplifier off and cold, all tubes properly installed. Turn all 4 bias adjustment screws to the left 3-5 complete turns. Speakers do not have to be connected to set the idle current bias. Make sure music is not playing during the adjustment.
2. Set the supplied digital meter to the marked 2000m V range. Clamp the meter black probe into any speaker terminal post - or simply place the black probe into one of the brass top cover mounts. Place the red meter probe into one of the top chassis gold RCA bias jacks, notice screw adjustments next to each test jack (see picture).
3. Now power up the amplifier NO music playing. Watch the meter - if the meter reading quickly rises above 600 IMMEDIATELY switch the amplifier power off - the bias adjustments are not set to a low initial bias adjustment starting point and need to be turned left a few more turns. Once the amplifier is switched on you should see a relatively low reading:(1 to 300) in 15 seconds. Adjust each corresponding bias screw for this meter reading: EL-34 tubes set to 350, 6550 or KT -88 tubes set to 550. Set the remaining 3 bias adjustments in the same manner by moving the probe to the corresponding bias gold jack and turning the adjustment screw.
4. Let the amp idle for 10 minutes (or longer) at this setting. Watch out, tube glass gets very hot (up to 300 deg. F)
5. After 10-minutes (or longer) of idle time, final adjust all 4 settings: EL-34 set to 350-370. 6550, KT -88 or KT -90 tubes set to 600-620. + or - 20 is accurate enough. Final test and adjust all 4 test points two or more times till all look identical. The bias is now properly set and will not need attention for 6-months.
the correct 4 or 8-0hm + post. Speaker minus - wire goes to the amp 0 ground post. Connect the amp input to your preamp output. The adjustment can be performed with or without speakers, or input connections - just so no music is playing during the adjustments.
Check the bias every 6 months.
Output tubes last 1 to 3 years in most tube amps. Not a bad idea simply replacing output tubes every 18 months to avoid an ugly end. Bad tubes (shorted) will blow the amps easy to change external power fuse. Keep 18-month replaced (used) tubes for spares in case you have one in the future blowout before 18¬months has passed. Tubes are 100% hand made. Hand micro¬welded inside, hand-blown glass outside. Human factors make them delicate from the start. Off the assembly line 500/0 do not function. The remaining 500/0 have ~ test out of usable range. 1 in 4 made are shipped out. A shake test while in transit and full load testing and matching ensures the tubes you receive from Bizzy Bee are the highest quality available. Even with extensive testing there is a 2 % chance you will find a tube that quits prematurely. Handle your tubes carefully. Always set the bias immediately when changing tubes. External test points allow you to easily observe how your tubes are doing. A very high or very low bias reading (off by >30%) on one tube with others looking identical indicates the odd tube has a problem and needs replacement. A simple regular 6-month bias check and complete output tube replacement every 18 months will generally provide non-stop entertainment.
I sell replacement tubes for very reasonable prices. Presently $40 for a matched set of 4 Svetlana EL-34's +$5 shipping. All are heavy load tested then matched under normal operating conditions. Only install matched tubes - this ensures longer life and beter sound.
