[Techman]

The large decoupling capacitors used across the supply lines in battery sets either tend to cause instability/motorboating as they lose capacitance or cause more current drain on the battery if they go excessively leaky. These are virtually always duff in battery valve sets where they're permanently connected across the HT line regardless of whether the set is on or off, which is not the case in a transistor set, so a lot less likely to be faulty, but still worth a check, although unlikely to be causing your fault unless they really are leaking badly and killing the battery.