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

"Even Steven" wrote in message
m...
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
Steve,
Check the control grid voltages to make sure the bias is right. Sometimes a
leaky capacitor from the previous stage's plate will turn on the following
stage hard. Besides overheating the tubes, this can also burn out the power
transformer. If you haven't replaced all the electrolytic and paper
capacitors, I would suggest you do so before proceeding further.
Regards,
Bob AD3K

"Even Steven" wrote in message
m...
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

While I agee with Bob Spooner that you should check the
grid voltage (should be at ground potential) my experience
with 6F6 and other metal power tubes is that they run very
hot and its pretty much normal for them to be hotter than
you can touch.
If the grid coupling caps are leaky they will put some
positive volage on the grid. This will make the tube run hot
but will also usually produce very noticable distortion. Use
a very high impedance meter to make this measurement.
Again, hot power tubes are normal.



--
Richard Knoppow
Los Angeles
WB6KBL

"Richard Knoppow" wrote:


"Even Steven" wrote in message
om...
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

While I agee with Bob Spooner that you should check the
grid voltage (should be at ground potential) my experience
with 6F6 and other metal power tubes is that they run very
hot and its pretty much normal for them to be hotter than
you can touch.
If the grid coupling caps are leaky they will put some
positive volage on the grid. This will make the tube run hot
but will also usually produce very noticable distortion. Use
a very high impedance meter to make this measurement.
Again, hot power tubes are normal.

I'll agree with both the previous posters - check the caps on the
control grids of the 6F6s. BUT that having been said, as I remember it
the 6F6s in my old SX-25 got hot enough I could keep my coffee cup
warm on them. Those old metal beam power pentodes like the 6F6 and 6L6
would burn your finger very quickly.

I do not remember the company for sure (possibly Millen?) but back in
the 50s and 60s a company made slide-on heat dissipators for the metal
shell octal tubes. I had a piar of them installed on the 6F6s in my
old SX-25. Touch base with any "old timers" you know and see if they
have a couple of them stashed in their junk box.

Best Regards,

Bill Bowen
Sacramento, CA

Even Steven wrote:
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?

If the quiescent cathode voltage is good, you can rule out a bias problem
although it might be worth replacing the coupling cap pre-emptively. The
heat tends to bake 'em.

About all you can do is remove the 6F6 and put in a tube with a higher
dissipation rating.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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 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

On Jul 21, 11:48*am, "Bob Spooner" wrote:
"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- Hide quoted text -

- Show quoted text -

Good question. Also if there is 18 volts across the cathode resistor
and it measures within limits (immediately after switching off i.e.
hot) that will allow calculation of the cathode current being drawn by
the tube or tubes.

"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