Power Tube Red Plating

[rick32559]

What would cause power tubes to red plate? I cant remember. Any suggestions?

[Colin_D]

A horribly mismatched pair of power tubes, a miswired/wrong component...
Is it a production amp or a DIY? With 18 watters it's somewhat common for the screens to get red hot when you crank 'em. If you provide more info it will be easier for others to help you trouleshoot it.

All that to say I assume you're troubleshooting an amp, and apologies if you were just asking a general question!

[Alexo]

If the cathode resistor is of too low a value, the tubes will bias too hot and they can redplate. Values around 125 ohms or lower can do it, good to stick around 150 or so.

[CurtissRobin]

A red plate (even a dull orange color qualifies) in an operating tube tells you there's too much current passing through it. When the plate gets hot enough to glow trouble's a-brewin'. It'll have a much shortened life, even if it's fresh out of the box. The most common cause is incorrect bias. If the circuit is self bias (cathode bias) then the cathode resistor is the wrong value. If the circuit is fixed bias (as in Fender blackface amps) then the bias voltage adjustment is wrong. A properly biased tube will never redplate.

HTH
KennyO

[rick32559]

This is a KMD Chassis that was given to me. two 6L6`s for the output section. I`m planning to strip it and build a new amp. I`m trying to decide on the format right now. Four 6v6`s ala Jim Kelley,or 2 EL 34`s etc.... I had a Blackface Pro Reverb that did the same thing. changed the tubes, set the bias and it was good to go. I`ve been working on Tube amps since High School , starting with Seeburg Jukebox Amps,Wurlitzer,Ami-Rowe etc... I have been out of the loop for awhile and need to get back into it. I appreciate all the replies and any help and/or suggestions from everyone here. Thank you All! ;Fred