Re: 12AU7 preamp tubes
just put the 12ay7 tube into v1 and see how you go. The fist tube slot has the biggest role in determining the basic amount of gain and sound that will be sent through all the rest of the stages of your amp. It will decrease your preamp gain significantly, but it wont necessarily increase your maximum clean headroom because your power tubes and PI will still be able to reach saturation levels. What you will notice is that your volume knob on the amp will be less sensitive, so you will have less gain at the same spots on the dial as when you have a 12ax7 in there. The highest possible saturation when you have the volume on 10 will be slightly decreased too. However, your amp can still get power tube overdrive, and at that stage there will not be much more volume it can give - and certainly no more clean headroom. Given that your amp was dsigned for 12ax7s in the preamp im not really sure that you will get a "better" clean sound, or even if you will get much more clean headroom due to the fact that you have a 15 watt cathode biased amp. Cathode biasing gives a superb harmonically rich breakup from the power tubes, but it is not a particularly efficient way of getting headroom and volume from tubes. No amps can do everything for everybody. So try it out by all means, but i'd wager that the thing sounds its best and warmest with the stock tube configuration.
The other tube change that will make a difference (better or worse is up to you) is to put a 12at7 into the PI slot (usually the last preamp tube slot). 12at7s have only 80% the gain of a 12ax7, but can handle roughly 10 times the voltage. They are very effective in the phase inverter role because they tend not to overdrive in the way that ax7s will, and run cooler and more reliably. They also work in v1, but i find that they seem to thin the sound a little. Anyways, putting a 12aT7 into your PI slot could be a good experiment and should stiffen up the feel of the amp and give a little more punch in the low end.
This all comes with a caveat: nearly all my experience is in experimenting with fender amps, so although these tube swap recommendations will be safe enough, they might sound like total crap in the context of your nice hand wired vox.
