Why don't preamp tubes redplate?

[EtherealWidow]

I'm just wondering why I never hear about preamp tubes redplating. It seems to be only with power tubes. From my understanding preamp tubes have all the necessary components to redplate. Is it how the power tubes are wired? Any technical knowledge appreciated.

 

[Koreth]

Tubes redplate when their plates are dissipating more energy in Watts (a unit of heat FWIW), than what they are rated to handle. The preamp circuits simply don't run the tubes hard enough for this to happen.

An example: The most common preamp tube, the 12AX7 has a maximum plate dissipation of 1W. Into a 100k? load resistor, at a voltage supply of say, 400V, the peak dissipation through the plate would be .75W. That's the peak dissipation. Average dissipation is going to be lower.

 

[EtherealWidow]

AH! Thank you! Alright. That makes sense.

 

[94Tremoverb]

Preamp tubes used as reverb drivers can redplate - the plate resistance is much lower (just the drive transformer primary) and the voltage is usually higher too. Fender SF amps are especially prone to it, I've even seen a couple of 12AT7s melt in these - literally, the glass bottle softened to the point the vacuum failed - because they got the bias a bit wrong! You don't normally notice because the tube is under the metal shield, but if you take it off and look closely you can see the redplating.

It's probably possible for a preamp tube used as a cathode follower to do it too, if the cathode-to-filament insulation breaks down. Some Russian 12AX7s seem prone to this, but I've never seen one actually redplate, they usually just fail.

 

[EtherealWidow]

Alright. It's just that I never hear of preamp tubes redplating. Thanks for all the answers though, you two.