Just had a 6550 Red Plate on a VTA-120

Hello,
I just had a tube Red Plate on my VTA-120. Music was replaced by a noise; looked and saw a Red Tube. I have a spare 6550 -- what checks should I perform for other damage, either before or after replacing the tube. Was using a Variac, I think the Variac fuse blew ( shows Variac at 0 volts). Killawatt shows 123V on another outlet. Rightside tube fathest from the transformers / closest to the bias test points.

When one of my 6550 did the same thing it turned out to be the 10ohm resistor off of pin 8 to ground.

How many hours on the tubes? On the amp?

Tubes are the original to the Amp. First powered up in September '19. Blew a 10 ohm resistor in the spring -- Bob said to replace the tube with the blown 10 ohm resistor, but I hadn't marked the tube locations on the tubes when I took the tubes off the flip the amp over.... . I marked the locations on the tubes after that & bought a spare 6550. I suspect it is the same tube, and that the 10 ohm is probably fried ( I do have some spare 10 ohm resistors). I will search the forum for other red-plates and see what checks other people have done. Probably wait awhile --- I need to complete a ST-35 for a daughter, want to get that done first. Listen to the ST-35 while examining the 120...

Decided to just check what would happen if I turned all the bias pots to zero bias and replace the tube.
Everything biased nicely , and music is playing. I have an email out to Jim McShane asking for a matched quad of either Tung Sol or Sovtek 6550WE tubes. Not in a rush, will order when he responds. The Sovteks supplied with the VTA ST-120 Amp kit sounded amazing to me ( although I do not have other tubes or tube amps to compare it to. My first Tube Amp. ). Lot's of other things wrong with my stereo that can be fixed before I need to worry about different tubes ( speaker locations, replace cheap input switch box with a preamp, adjust subwoofer level and cutoff frequency ...).

Been using TAD 6550's for many years until one blew a 10 Ohm resistor ..... a few days ago I put Electro Harmonix KT-90 in my ST-120.... sounds GREAT and even more powerful ;-) ;-) ;-)

It sounds like those 10 Ohm 2 Watt resistors are working as fuses in this application and protecting the amp from further damage. Why not add a real fuse in the B+ supply? 2W in 10 Ohms is about 440ma, right? Not sure how much over current it is taking to light up these resistors. Maybe a 1 amp fuse?

I'm guessing there's a real good reason why such a seemingly simple fix isn't done already that you guys will probably tell me about

rjpjnk wrote:It sounds like those 10 Ohm 2 Watt resistors are working as fuses in this application and protecting the amp from further damage. Why not add a real fuse in the B+ supply? 2W in 10 Ohms is about 440ma, right? Not sure how much over current it is taking to light up these resistors. Maybe a 1 amp fuse?

I'm guessing there's a real good reason why such a seemingly simple fix isn't done already that you guys will probably tell me about  

fuses will take longer time to burn off then a surface film resistor.