What causes valve to glow red inside ?

[nigelwright7557]

I have an old Simms-Watts 100 watt PA valve amplifier.

It worked fine for a couple of years then I noticed it smelling hot.
I looked inside and one of the valves is completely glowing red inside.
The other s only have the heater glowing.
The amplifier sounds OK.

I have taken out the offending valve and the amp plays ok upto about half power (obviously with a valve missing)

Does anyone know why the valve is over heating ?
I can only guess the valve has reached the end of its life.

 

[M Gregg]

Hi,

The bias is wrong..or the coupling cap is leaking...are the first thoughts..if the coupling cap is damaged the tube will go out of bias and the current will run high..this will make the tube plate glow red hot and could burn out the Op Tx if its not corrected..

The other possible maybe is, is it fixed bias or is it cathode (auto bias)..in this case the cathode cap could be leaking..often valves will be OK even when run out of spec for a while so the tube might not be damaged..unless there is an internal short.

Regards
M. Gregg

 

[M Gregg]

If it was my amp,

I would change the coupling cap with a polypropylene 600V or higher and if it has a cathode cap across a cathode resistor I would change that as well..

If this corrects the fault then the other power tube..s...caps are the same age so change them as well..at least then you should get some reliability.

If it has DC fixed bias then also check that the bias is reaching the tube grid..ie no open circuits on the circuit board or dry joints..if there is one.

The other possible maybe is if its fixed bias have the elect caps in the bias supply gone dry..this will create a low voltage on the bias supply and could send all the power tubes into high current idle.<<<I would think this is Ok because the other tube is not red plating..

eg..
http://www.youtube.com/watch?v=9SfJJ8A8XNM

Regards
M. Gregg

 

[john65b]

Or maybe a spent tube?

 

[JMFahey]

Exchange positions between the redplating tube and a properly working one.

If the problem follows the tube, it's the tube; of not, it's a bias problem at the suspect socket/bad contact/etc.

 

[tomchr]

Exchange positions between the redplating tube and a properly working one.

If the problem follows the tube, it's the tube; of not, it's a bias problem at the suspect socket/bad contact/etc.

Verify the bias conditions, then swap the tubes. Redplating is not good for the tube. If the bias way off (for reasons mentioned above), you'll cause the other tube (the good one) to redplate as well. No reason to torture the tube...

But if the bias currents, voltages are within reasonable tolerances of the design targets, swap the tubes. If the problem follows the tube, change the tube.

~Tom