I have an intergrated Luxman 38FD in for repairs since it has a couple
of stuffed tubes,
and what looks to be a stuffed OPT, a type OY15, which is 5k to 4+8+16,
which appears to have
shorted turns and the power out is only 6 watts max instead of about 25.

I happened to have a quad of 50C-A10 out of an amp I was given years ago
with crook OPT
which had been removed from the chassis.
So I was able to test this amp with the right tubes.
But both OPTs seem to have problems and even with no load, the anode
voltages at each
of the 50C-A10 beam triode tubes are 12% different on one channel which
does manage 15 watts,
and 15% different on the low power channel.
I have further checks to test inductance to confirm my fears about the
OPTs,
before I give the bad news to the owner.

Anyone have any of these OPTs laying around? Its a long shot, but one
never knows unless one asks.
And I'd reckon the Luxman amps have disgustingly thin wire for the
primary coils.
Its one reason why I refuse to use less than 0.3 mm dia copper wire.
The idle current might only be 50 mA, but when a tube decides to die,
there may be 500 mA flowing, and its this much current that the OPT
should be designed for,
not the piddly 50 mA.

The 50C-A10 is an interesting tube in that is appears to have beam
forming plates and a
screen, which seems internally connected to the anode.
Luxman had this valve made for them specially and with 440v Ea
it allows 31 watts power output in class AB1 with 2.5% thd and
with about 5 watts of class A.
Ra is about 750 ohms and U is about 8, so for a triode,
its compares favourably with an EL34.

I guess that tetrodes or pentodes connected as triodes can make
technically better triodes than a real triode of the same rated Pd,
since the screen
acts to place the electrostatic effect into the tube at a close distance
to the grid and cathode.
Thus the anode can be a larger diameter structure, and thus handle more
heat
dissipation, that if it was kept at the same dia as the screen
structure.

The 50CA10 has nice curves, and would be a darling if connected in class
A,
but using such a tube in class A with 20 watts of idle power is baking
them a bit,
and a larger octal tube would fare better for longer.
The 50CA10 isn't made any more, needs a 12 pin TV tube socket,
and has 50 volt heaters.
Just what Luxman were thinking about when they decided on such a tube
is beyound me when there were already plenty of fine output tubes
around. But there is nothing like making all your customers come back
for new tubes from the one and only source.

Finally, anyone with any 50C-A10?
No doubt the price of NOS would be expensive?

My customer may be prepared to spend on some spares,
but if over a certain figure, I would advise him to
rewind the power tranny which has gone noisy, and fit
6CA7/EL34/6CM5/EL36/5881/6L6 instead of the 50C.

Maybe he parks the amp.
That would be a pity because the rest of the preamp and driver amps are
in perfect
working order, even the phono stages have the right RIAA eq, which is
amazing, and all the myriad of switches
and small tubes are all still OK after what must be 40 years.
It even has what look very old dusty russian made 6267 special quality
pentodes wired as triodes for the power
amp inputs, and they both measure 90 volts at the anode on each channel.

It has the dreaded oiler caps the japs liked to use, and which so often
leaked their oil all over the chassis and then become quite leaky, but
not one seems to have gone leaky yet after all
these years.
All the resistors look fine.

There are four small ( carbon? ) bias trim pots, two to set the average
bias level and two to balance the bias for each channel,
and none appear to have degraded.

Patrick Turner.