Have you pulled the top cover off to see what the actual rating is on the transformer? If you could post a picture of the label that would be great for the thread as we already have a few other models.
here we are, at the end I've done a set of measurement using the original set of tubes from factory...
first of all the picture about the transformer, is somebody is able to traslate from chinese would be nice... well number are self-explaining
and how wires are connected inside the LD2 ... input, those two black-red in between are joined togher to reach the 240Vac, the Green is to OVdc at the output of the bridge whic is connected to ground, so porobaly the transofermer is shielded
and output, heaters ... a is twisted with b... c is twisted with d... the black wire is the connection to elevate the heater voltage... those connetion are all Vac and, in theory, sould be around 6,3V +/- 5% (maximun /-10% but not valid for all tubes models) (sorry... blu mean blue ... and verde means green)
while checking for connection I've seen that the transformer has been quite poorely assembled... so I squeezed the external mtal part to avoid potential vibrations (that can cause electrical noise in the circuits that can damage audio quality)
now the funny part... with the original tubes, so no overload because of tube not considered in the original project, I've measured, with constant voltage from the wall of 220Vac ---> 144Vac at the output instead of 151,25Vac (from (165/240)*220)... not a big gap but 5% lower then target...
then I measured the values at tubes...
so, before somebody will start telling me "take it easy" or "it sounds good so who care" consider that (let's forget for a moment potential problems about shorter tube life)... too low voltage at heaters means lower temperature... that means lower level of electron emitted by katode... that means lower Gm... that means lower dinamic range, lower sound quality
the reason why the transformer has so poor quality is simple, the transformer is the most expensive part inside a tube amplifier... so our chinese friends declared some values and installed something that can't reach those values saving money... oh well, no, for sure it is only my unit that is under-powered
anyway... I'm thinking to remove that transformer and realize an external "brick" with better transformer and two regulated linear power stage to DC supply heaters (from Amazon roughly 10 bucks each), deleting in this way any variation/fluctuation in the 6,3V because of the Vac fluctuations
in the mean time I switched to some other tube well sounding but not so expensive