Amplifier valves get hot and there are very high voltages present, so make sure the amp is switched off and and the valves are cold. Hot amp valves can give you a bad burn ! The output valves , the rectifier, and ECC99’s all get especially hot ! My most recent burn was off the little output valve in the picture above, the EL84 output pentode
The first basic question is how often to replace valves? Marshall recommends every 2- 4 years. Sometimes the the pre-amp valves become “microphonic”, especially, if the amp is moved around a lot. It is easy to test. Just tap each valve in turn when the amp is switched on and if one is really loud from the speakers it is time to change it.
When changing the output valves it is best to chang them as a set and then have the bias checked to make sure it is set in the right parameters. If the bias current is too low , the amp will sound “thin2 and with lower volume. If set too high it will sound great and loud but the output valves will not last long.
Whern changing output valves a word of caution on 6L6’s and 5881’s. You can replace 5881’s with 6L6’s because the 6L6 has higher voltage ratings . You cannot automatically replace 6L6’s with 5881’s until an engineer verifies that the amp is not using voltages on the output that will damage the 5881’s. I am being cautious here. I had a set of Fender branded 6L6’s that were actually Sovtek 5881’s and the Fender branding was on the top of the Sovtek branding but still legible. There is a picture of these valves on this site on a discussion on repairing a Fender Hot Rod series amp.
It is MANDATORY that you re-bias the output valves when switching from 6L6 to 5881 and vice versa. You risk damaging the valves and the amp if not done. YOU HAVE BEEN WARNED !!
So now you are ready to swop over the valves. The pre-amp valves might have metal cans over them Push them down and twist ant-clockwise about an 1/8 of a turn and they will pop off. Just pull straight out. They have 9 pins and you will see a gap. They will only go in one way.
EL84 output valves have 9 pin bases too, and you can pull them out in the same way. The larger valves have 8 pins and they have a large case pin in the centre with a key on it. Remove the valves pulling from the base and not on the glass. Older valves can split at the base when removing and the glass can break. When inserting the new valves also hold them by the base and push in. It is a good idea to put them in the holder and do not pus till you feel the click of the centre pin clicking with the matching groove on the valve . Do this carefully. I have seen valve holders that have been ground to pieces with folks trying to force the valve in.
And that is about it. I do not apologise for saying it again. Always check the bias once you replace the output valves !!!
Steps to follow when changing your valves in your amp
