[QUOTE=I've measured the plate voltage on the reverb recovery tube (V4) in my SF Deluxe Reverb:
- 237 and 241 VDC instead of 170-180 on the schematic! [/QUOTE]
I assume the voltages at the other preamp tubes are close to the schematic? This means that the tube in question isn't pulling enough current through its it's plate resistor(s). Since both sides of this tube share a common 820 ohm cathode resistor I would start by checking its value & make sure it has good continuity to ground. Also check the plate resistors themselves and make sure the grids have a DC return path (resistance) to ground so that they are being held near 0VDC at idle. If all this looks good it might be a bad tube.
***Note - All the usual precautions apply here - if you aren't familiar with tube amp servicing then you should take it to a professional. There are dangerous voltages inside your amp that can hurt or even kill you. Nuff said?