Reverb tube redplating

[MickeyB]

Just completed new build. Fender 6G16. Warmed it up, set the bias (36mv) and let it warm up to recheck again. After about 5 minutes i noticed the v4 reverb tube starting to replate. Put it back on standby, then back to hivolts and it took another 3-5 minutes to redplate again. I have a good 12AT7 in it.
Looking for help on what to check for problem.

[keysersoze808]

Just completed new build. Fender 6G16. Warmed it up, set the bias (36mv) and let it warm up to recheck again. After about 5 minutes i noticed the v4 reverb tube starting to replate. Put it back on standby, then back to hivolts and it took another 3-5 minutes to redplate again. I have a good 12AT7 in it.
Looking for help on what to check for problem.

hi just wondering your plate voltage?

[Satamax]

ultrasonic oscilation?

[cbarrow7625]

Shorted cathode cap? remove the cathode cap & see if that helps.

[MickeyB]

Have 480v & 460v on the power tubes.
V6 pin-1 260v pin-3 256v, V5 138v 465v, V4 199v 202v, V3 459v 460v (this is the reverb tube redplating- not V4 as I mentioned earlier), V2 194v 195v, V1 199v 192v.
Would the cathode cap affect just that tube?

[MickeyB]

Sorry Its pin-1 & pin-6, not pin 3.

[MickeyB]

Also its 470v going in the reverb tranny and 465v coming out. Is that right?

[MickeyB]

Or do I need A different rectifier tube. Currently have a 5AR4 in it. Do I need one to lower the voltage down more?. I saw on the original schem where it ran at 440v and I'm at 470v at the reverb tube. Or should I put a resistor there to drop it down?

[MickeyB]

Found it. Loose screen resistor.