"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

The RCA tube manual publishes some typical operating specs for 6F6 tubes
operating class A in push-pull. With 315 volts on the plate and 285 volts
on the screen, and with a cathode bias resistor of 320 ohms, zero signal
plate current is 62 ma, and zero signal screen current is 12 ma. That would
result in 74 ma cathode current, giving a cathode voltage reading of about
24 ma. Assuming no output, power dissipated by each tube would be 11.5
watts (total of plate and screen power). The 6F6 is rated at 11 watts plate
dissipation and 3.75 watts screen power dissipation, for a total of 14.75
watts.

The SX-25 circuit has 280 volts on the plate and 270 volts on the screen.
The cathode resistor is 350 ohms and it specifies 18 volts on the cathode.
That would mean 51 ma of cathode current. Estimating 10 ma of screen
current and 41 ma of plate current, that results in 7.14 watts dissipated by
each 6F6 plate, 1.4 watts by each screen, and each filament consuming 4.41
watts for a total power dissipation by each tube of 12.95 watts. That is 26
watts of heat in a confined area. Put a 25 watt light bulb in the sme area
and after 10 minutes, try to grab ahold.

73, Colin K7FM

Find a place to get 6-12 volts (rectify the filament voltage) and put in a
little LM386 audio amp.
All your heat problems are instantly gone.
Of course you get 1/4 watt of audio instead of 10 watts.......

Rick K2XT

....and of course you get way away from the Boatanchors philosophy! I think
it's preferable to discreetly unerrun the valve.

"Rick" wrote in message
...
Find a place to get 6-12 volts (rectify the filament voltage) and put in a
little LM386 audio amp.
All your heat problems are instantly gone.
Of course you get 1/4 watt of audio instead of 10 watts.......

Rick K2XT

...and of course you get way away from the Boatanchors philosophy! I think
it's preferable to discreetly unerrun the valve.

Oh, one could leave the filaments lit, and hide the little chip away back in
a corner under some wiring harness.
The receiver might last longer in the process !

Sacrilege! What has this ng come to?

"Rick" wrote in message
...

...and of course you get way away from the Boatanchors philosophy! I
think
it's preferable to discreetly unerrun the valve.

Oh, one could leave the filaments lit, and hide the little chip away back
in a corner under some wiring harness.
The receiver might last longer in the process !

On Jul 24, 3:45*pm, "Rick" wrote:
...and of course you get way away from the Boatanchors philosophy! I think
it's preferable to discreetly unerrun the valve.

Oh, one could leave the filaments lit, and hide the little chip away back in
a corner under some wiring harness.
The receiver might last longer in the process !

But his tubes are metal so why light the filament? Of course, the s/c
solution is sacrilege - no way! I'd just switch the tubes to 6K6G's -
less heat and a nice glow and change the bias to AB (you don't need
class A P-P Hi-Fi quality!)
Cheers,
Roger

It is normal for an Amplifier tube to get verry hot.
RF Amp tubes also get very hot. I have a tube Pullers
from the 1960's just for removing hot tubes.
Stan

I'm working on a Hallicrafters SX-25 Super Defiant. It uses two 6F6
tubes in push pull in the audio
circuit. Boy do they get hot! Too hot to touch. If my finger lingers I'd
have an awful blister in less than a second. Is this normal? I've checked
the tube voltages and they seem to be where they should be...280 VDC on
plate, 6 volts on heater, 18 volts on cathode. Does anyone have any ideas
about how to help these things run cooler?

Steve