"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