![]() ![]() That way, you won't fry your new tubes if you make a mistake. While keeping the tubes within specifications.Ī SUGGESTION: You may want to practice taking these readings and making these adjustments with your old tubes still in the amp, or with a spare (used) set. Read the voltage drop across this resistor Replace the ground wire on each power tube Tech support is available from NBS Electronics AFTER 1:30 PM Denver (Mountain) time, Monday through Saturday. If any part of the following instructions doesn't make sense to you, seek help from someone with more experience. ![]() BE AWARE THAT THE ACCURACY OF THE RESULTS YOU OBTAIN WITH *ANY* BIASING METHOD WILL BE DIRECTLY AFFECTED BY THE QUALITY OF YOUR TEST EQUIPMENT, AND YOUR SKILL IN USING IT. Note that these instructions assume that your amplifier is biased by applying a negative voltage to the control grids cathode-biased amplifiers cannot be adjusted other than by changing the value of the cathode resistor(s) so this method does not apply to them. If you feel that the inaccuracy is significant, or if you'd just like to know how large it is for your particular amp, you can always read the voltage drop across each screen resistor, figure the screen current (I = E/R) and then subtract it from the cathode reading. This promotes slightly more conservative operation, which is beneficial to tube life. Since the cathode current will always be higher than the actual plate current, the readings obtained with this method will tend to make you set the tubes a little colder than your calculations will indicate that they are. Plate current can be identical on two tubes (tubes are matched by plate current readings) while one tube is drawing more screen current with this method, the readings will appear to indicate a mismatch when such is not actually the case. The cathode current is composed of the plate current *plus* the screen current. It is performed by reading the cathode current through each power tube. It is far and away the safest of the two methods, and can be successfully done with medium- and even low-quality test equipment. This is the method that is best for hobby techs and do-it-yourselfers. PROPER HIGH-VOLTAGE SAFETY PROCEDURES AND PRECAUTIONS MUST BE PRACTICED AT ALL TIMES WHEN SERVICING VACUUM TUBE AMPLIFIERS!! IF YOU ARE NOT *COMPLETELY* FAMILIAR WITH THESE PRECAUTIONS, SEEK GUIDANCE FROM SOMEONE WHO IS! THE CATHODE RESISTOR METHOD WHITTAKER DBA NBS ELECTRONICS) WILL TAKE NO RESPONSIBILITY FOR ANY INJURIES OR DAMAGE SUFFERED BY ANYONE AS A RESULT OF THE MISINTERPRETATION OR MISAPPLICATION OF THE INFORMATION CONTAINED IN THIS ARTICLE. ![]()
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