"jaap" wrote in message
l.nl...
jaap wrote:
More so since
this amp is equipped with a regulated PSU: four '2A3' triodes @50mA each,
10% headroom for the regulator gives 220mA and that's quite a lot for
vacuum.
Yours has two 300B's but the story is the same: there's quite some strain
on the rectifier tube, it might be calculated on the edge as voltage at
the output tranny will be over 400V... Without the schematic it is
guessing but probably it would have been wiser to use sand for
rectification.
I would recommend against swapping rectifier tubes if you do not know what
voltage drop to expect. Contact the seller for assistance or buy another
(few) 274 tubes as they come cheap 
I have a pair of RCA MI-12188A amplifiers which each use a Sylvania 5R4GYB.
The power supply consists of a 750-0-750 transformer, into a large choke -
and the current draw of the output stage is on the order of 200mA at 600V.
Six years of problem free daily operation, and I got the tubes used with the
amplifiers! Their emmission has not dropped noticibly on the TV-7, either.
Don't bother with Chinese tubes, if you want reliability, find some NOS
Sylvania. In my limited experience, the best tubes (from a reliability
perspective) are
Sylvania
GE
RCA
in that order.