Here’s the amp. It's a stereo power amp with a standard valve lineup (Each channel contains two EL34 output valves and a 6SN7 and an ECC83 in the driver and preamp sections respectively). The valves normally live under a mesh cover, but I’ve removed that for the photo. The reported problem was that in both channels, only the EL34s were lighting up.

The valve heaters check OK for continuity (never neglect the obvious!), but the 6SN7s and ECC83s aren't getting any heater power. No doubt these valves have a special, DC heater supply and this has failed. However, there are also some wacky voltages present on some of the other valve pins, and this suggests that the heater supply isn't the only problem. Time to get the back off, metaphorically speaking.