I've been rolling tubes thru my 74 Silverface Princeton Reverb. It came to me with what looks like all the original RCA tubes. The amp is in nice shape and doesn't look abused. Edit; It has a Weber replacement speaker, original long gone. I have recapped it (it had all the original caps) and the HT voltages check almost spot on with the schematic. All within 2 or 3 volts. Fluke meter. The first HT node measures almost exactly 420 volts as per the schematic. This kinda surprised me, as I thought with the higher wall voltages these days it would be a little high. Wall voltage measures 121.8 at this moment. In my stash of tubes there is this International branded 12AX7/ECC83. I put it in the V1 socket and was surprised to find the amp much quieter. Not just a little quiet, a lot quieter with that tube. By my ear, about half the volume as any other tube. The amp actually sounds quite nice with this tube, just very quiet. At full volume there is just a hint of break-up. With any other 12AX7 in V1, the amp is quite loud at full volume and has lots of break-up. Is this a sign of the tube getting weak, or is there such a thing as a "low gain" 12AX7?