Vox night train redplating issue

[phsyconoodler]

This one is baffling me.
  The amp was dropped off a 4-12 cab and i had to replace broken switches and pots.but now one tube redplates for unknown reason
What I do know is this: plate voltage is good on both tube,screen voltage and screen resistors are intact,heater voltage good,no positive grid voltages(it's cathode biased) has about 18v on both cathodes(measured)
  By all rights this amp should be fine.I had the circuit board out looking for lifted traces,cold solder joints and anything that might be an issue.Nothing.
  One tube redplates and the other one doesn't even get warm,beyond the heater warmth.I can touch the tube.
All voltages are the same on both tubes.
  The pentode/triode switch has 9 wires on it and I did not mistakenly swap any because I did one at a time.I looked at a photo online of the interior and that part looks goo.All grounds are intact too.
  I can't grasp what's making this amp do this.I tried multiple tubes too.And PI tubes as well.

 ????

[VMS]

Did you check grid leak resistors?

[phsyconoodler]

Yes,those are intact They measure 3.3k.

[VMS]

I think those are grid stoppers.

I mean resistors that go from grid stoppers to ground. But I think night train has master volume there. I suggest that you measure the resistance from one end of 3k3 resistor to ground, on both tubes.

[jojokeo]

P, one other thing to test is make sure the socket pins are tight and check the voltages from the top of the sockets. Maybe something got tweaked and the tube's pins aren't getting what you're measuring from underneath?

[phsyconoodler]

all the pins are ok,that's not it.But the master intrigues me.It may indeed where the issue lies.I had to replace 4 out of the 5 pots.Tjhe master is a dual pot and didn't SEEM to be damaged.
  I bet is is.I'm going to replace it and see if that does it.It would throw the grid resistors out of whack if it broke contact.

Nice call!It may just be it!

[DummyLoad]

presume the red plating does not follow the tube if you swap tubes? bad tube?

did you have to replace PPIMV pot? if not, check MV pot. pull EL84 and measure resistance from pin 2 to gnd; rotate pot as you measure you should read anywhere form 3K3 ohms to value of POT + 3K3 ohms.

if you did replace the the MV pot check it anyway. check for cracked traces - bad solder joint - plate-thru on PCB - bad socket.

if all that is in order, perhaps the 1/2 primary of OT on the side that's red-plating has shorted windings.

[phsyconoodler]

It was a hairline crack on the master volume pot.You couldn't see it when the pot was on the PC board but it was very visible when the pot was off.
  VMS you pointed me in the right direction!!!!!!

I owe you a few beers buddy!!!Thank you!!!!

the amp works perfectly now.I am indebted to you guys for always giving me options when blinders are on.I should mention the amp was aprt on the bench when I posed the question and I have a kick-ass desoldering station so the pot was off in seconds.

[VMS]

My pleasure! Most likely I've learned the importance of grid leak resistors from this forum.

If the amp is still open, solder couple of 1M resistors between MV wipers and ground. That way if the pot fails the tubes won't redplate.

Thanks for the beer!   

[Willabe]

Thanks for the beer!   

Hey how many beers did you buy him?         He's still drinkin!


             Brad