What is the best way to proceed when replacing power tubes?
The first rule when replacing tubes is safety, as tube devices operate with dangerously high voltages. You should only carry out this work if you are either a qualified specialist, familiar with the subject, and if the manufacturer explains in the operating manual how the tube replacement should be carried out. Therefore, please always observe the operating manual for this topic.
If it is a DIY device or a vintage device, or if no manual is available, please observe the following steps:
Note the bias setting of the old tubes so you know how it was set
Device with bias adjustment – turn the bias to minimum, as you do not know where the new bias point will be
Allow the device to cool down and wait until there is no voltage at the tube socket; this may take several hours
Make absolutely sure that you do not touch the contacts when changing the tubes
Work with one hand; the second hand remains free
Rubber or silicone gloves are helpful, as they can prevent slipping
Pull power tubes with octal bases straight upwards
When inserting new tubes with octal bases, pay attention to the position of the plastic guide key
With 300B or 2A3 tubes, there are two thick and two thinner pins (UX4) to prevent incorrect insertion
Switch on the device and check the bias; either via displays in the device or sockets with a multimeter, usually in the DC volts range
Adjust the bias one after the other to the target value
Check the bias again after 10 minutes, after one hour and after the burn-in period, approx. 50–100 hours, and correct it if necessary
It is normal for the bias value of some tubes to fluctuate during the warm-up phase
The tube replacement should be carried out by a specialist workshop, such as BTB Elektronik, if you have no information about the bias or if the device has to be opened in order to adjust the bias
During the burn-in of the power tubes, you can listen to music as normal