The phase splitter valve while it is a pre-amp type valve is really not part of the pre-amp chain at all. It is actually part of the output stage. When an amp has two or more valves then it operates in a mode called push pull. Its job is to invert the signal so that one of your two output valves is driven at any one time. This is a an oversimplified description but I hope good enough for most folks!
Its key job is to drive the output stage with all the amplified signals from the previous valves and it does have an impact on the sound.
If you want loads of headroom and lower distortion then the phase splitter needs to have the ability to handle the large voltage swings. If you want a more crunchy sound then phase splitter with less headroom will do that.
The schmematic on the front page of this article is called a long tail pair and you can see that both halves of the valve are used. It is a good idea to have both of these sections matched and balanced so that your output valves are driven evenly = better sound.
The alternative less common set up is still to use a pre-amp valve and in this case the first triode in the valve is just used as an amplifier and the second triode is the actual phase splitter. The advantage of this set up is that you do not need a balanced valve to drive the output stage.
You can substitute an ECC83 in this position with a 12AY7 or the 12AX7LPS
ECC83 ‘s or 12AX7’s are used in many amps as the phase splitter. they have less headroom and so will produce more harmonics in the output stage = richer and a much crunchier tone. It is no surprise that Marshall amps have an ECC83 phase splitter. Fender with its cleaner tones uses the 12AT7 ( ECC81) as the phase splitter. If you want to go really cruchy use a Sovtek 12AX7WB as it does not have a lot of headroom
You cannot substitute an ECC81 / 12AT7 type with an ECC83 /12AX7 type
Hifi amps and Juke boxes should replace an ECC83 phase splitter with either a JJ 12AY7 or a Sovtek 12Ax7 LPS to ensure that the warmness coms come the out put valves and not the distortion of the phase splitter.
If you are not sure about what type of phase splitter you have in your amp. Just email us ( contact info on the homepage) letting us know the make and model and we will advise if a balanced valve is needed.