"Bob Spooner" wrote in message
...
"COLIN LAMB" wrote in message
m...
If the cathode resistor is the correct value, and the
voltage across it is correct, then the power drawn by the
tube is correct. The tubes are running Class A and so
over half of the power consumed is dissipated by the
tubes (divide by 2 of course). You can increase the
cathode resistance or increase the screen resistor, to
reduce power. This could cause a mismatch in the output
transformer ratio, but being Class A, it should still
sound fine.

Generally, what I would expect you would be seeing
(actually, feeling) is a combination of small factors.
The filament voltage is probably above 6.5 and the plate
and screen voltage may be a little above specified. This
is because we have a higher line voltage. Often, you
have to think about incremental improvement, rather than
night and day. If you drop the filament voltage about .5
volts, you will decrease heat from the tubes by 1 watt.

Also, adding a small muffin fan can do wonders to remove
heat from those old metl boxes.

73, Colin K7FM
Colin,
Do they actually run class A even though the output stage
is push-pull?
73, Bob AD3K
Push-pull stages do not have to be Class-A but often
are. There are advantages in stability and lower distortion.
The reason for biasing toward Class-B is to increase
efficiency. A pair of 6F6 tubes running Class-A will put out
about 10 Watts while in Class-B fixed bias it will be nearly
double this. About the RX I know of that ran the output in
Class-B is the Super-Pro series and there the tubes were run
as triodes.
The dissipation under no signal conditions will be
lower for Class-B but the tubes will still run hot. From the
voltages in the SX-25 schematic the tubes seem to be run in
Class-A or close to it.
Its typical for any power tube, metal or glass, to run
much to hot to touch. In fact, most tubes will be too hot to
touch.
If the measured voltages are correct the thing is
running correctly.
Again, while the coupling caps could increase plate
current that would show up in the measured cathode bias
value plus it would cause very noticable distortion. I think
your RX is working fine.



--
Richard Knoppow
Los Angeles
WB6KBL