Brenell 2 track teething problem

[hapbeat]

I recently got a lovely old Brenell all tube 2 track (1968). All works well, except for input levels going in; whether it's mic/line or just using the input stage as an amp, levels are quiet..and getting quieter! Tape playback is fine and very loud. I suspect it's a tube or two in the input amp stage. Am I right or is it likely a cap or something else...Hopefully it is a tube, so I can pop a replacement in myself!