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Hi Jonny. The idea is that there are some of us who think that the amp has too much gain for our needs. It is designed to be able to cope with High Gain and Metal types as well as the Clean players so there is a lot of potential for overdrive there which means that those of us who only ever play with limited gain in use have to fiddle with our gain controls over the first cramped couple of notches. If you put a valve in the V1 slot which gives that stage lower gain, (that's not the same thing as a "lower gain valve" which is a misnomer), it means that every stage in the amp chain has less drive to it so you need to turn up the Gain control further to get the same original amount of overdrive. It simply spreads the gain we use over a larger section of the Gain control. Personally I would prefer to actually go into the stage and change out resistors to redefine it to a lower gain but that is more awkward.
If you are thinking this might suit your playing then my advice based on understanding circuit design, and that of a few others you may find if you dig around, is to NOT swap out your 12AX7 for any of the usual "equivalents", they plain aren't equivalents at all! 12AT7, 12AU7 and 12AY7 are in no way similar to 12AX7. Techs will generally insist they are. That is based on the number of times they have plugged them into a 12AX7 circuit and it has worked without blowing anything up. There are other parameters for valves which are more important than what people think of as "gain" which are wildly different and will make a mockery of the designer's careful work. The DC levels around the stage will change significantly which means the clipping p[oints will be different so distortion changes. The frequency response of the stage changes too as the loading alters massively. This may make it sound better to you of course but it is no longer a precision designed stage, it is now a random piece of good or bad luck. You pays your money and you takes your choice.
The valve you need to replace the 12AX7 is a 5751 which is very close to actually being a 12AX7 with lower gain, all other parameters are very close too and that matters. You can technically put a 5751 into any of the preamp stages, probably best not to in V3 which is the PI phase inverter stage which feeds the output valves. With 5751s in both the V1 and V2 slots you will drop your gain by about half which expands your Gain control range by a useful amount.
As you asked, you can quite happily put a standard 12AX7 in the V1 slot without any problem. As it is the first valve in the chain, any noise here will be amplified by the following stages so it is often of benefit to use a designated Low Noise 12AX7. That will be a little more expensive but be careful you don't get stung paying a silly "premium" price for something which is simply selected from the batch in a few seconds test. It also can help to have a "Balanced" 12AX7 in the V3 slot as each triode in the 12AX7 package drives one of the output valves, so balanced triodes mean a balanced signal on both sides of the output. That said the assymetric distortion that an unbalanced PI valve produces may sound better to you. Keep the metal valve cover on V1 too of course, it reduces noise pickup.
If you are thinking this might suit your playing then my advice based on understanding circuit design, and that of a few others you may find if you dig around, is to NOT swap out your 12AX7 for any of the usual "equivalents", they plain aren't equivalents at all! 12AT7, 12AU7 and 12AY7 are in no way similar to 12AX7. Techs will generally insist they are. That is based on the number of times they have plugged them into a 12AX7 circuit and it has worked without blowing anything up. There are other parameters for valves which are more important than what people think of as "gain" which are wildly different and will make a mockery of the designer's careful work. The DC levels around the stage will change significantly which means the clipping p[oints will be different so distortion changes. The frequency response of the stage changes too as the loading alters massively. This may make it sound better to you of course but it is no longer a precision designed stage, it is now a random piece of good or bad luck. You pays your money and you takes your choice.
The valve you need to replace the 12AX7 is a 5751 which is very close to actually being a 12AX7 with lower gain, all other parameters are very close too and that matters. You can technically put a 5751 into any of the preamp stages, probably best not to in V3 which is the PI phase inverter stage which feeds the output valves. With 5751s in both the V1 and V2 slots you will drop your gain by about half which expands your Gain control range by a useful amount.
As you asked, you can quite happily put a standard 12AX7 in the V1 slot without any problem. As it is the first valve in the chain, any noise here will be amplified by the following stages so it is often of benefit to use a designated Low Noise 12AX7. That will be a little more expensive but be careful you don't get stung paying a silly "premium" price for something which is simply selected from the batch in a few seconds test. It also can help to have a "Balanced" 12AX7 in the V3 slot as each triode in the 12AX7 package drives one of the output valves, so balanced triodes mean a balanced signal on both sides of the output. That said the assymetric distortion that an unbalanced PI valve produces may sound better to you. Keep the metal valve cover on V1 too of course, it reduces noise pickup.
