Re: The 'magic' of the V1 pre amp tube
OK. Electronics is complex so you read things that are not always correct. One post I read, (not here) suggested a lower gain tube in the first position can harm other parts of the circuit and make the power section work harder as it was not designed for something like a 5751 . I doubt it if people have tried it. In my case the amp is a single channel with single volume & gain knob . ECC83 / EL84 tubes. It's actually a Vox AC10 re issue. Is the 5751 tube of higher resistance?
Your correct, what you read is incorrect. It won't harm anything. It may affect the sound or it may not depending on the amp and how it is being used. The 5751 just has lower gain compared to the common 12AX7. In a high gain amp that gets its distortion mostly in the preamp it could reduce the buzzy sound to the distortion that can often be a problem in a high gain amp.
The amps SRV mostly used depended on power amp distortion to get that sound. And he played very very loud, which is how you get power amp distortion going.
The exceptions would be his Soldano SLO100 that he began using shortly before his tragic passing, and/or his Dumble amps. In these such amps the use of lower gain tube in V1 or in V2 will have a profound effect on the gain structure of the amp. It could have been used to help that amp get a bluesier tone and to help tame the high gain nature of that amp, assuming they were speaking about a high gain amp.
Nevertheless, speaking in just general terms, the tube used in V1 of just about any amp is of vital importance, because it sets the tone at the beginning that will be amplified through out the rest the amp. Rolling in and trying different tubes, usually all 12AX7s, there as a trial and error processes, is a way of fine tuning the tone of an amp.
