KT-120 bias problems in my VTA ST120 Amp

My ST120 has been working flawlessly for about 18 months with great sound and no tube problems. Being incapable of leaving well-enough alone, I wanted to try a matched set of KT-120s (from a reliable source) to replace the Tungsol KT-88s in my amp. I installed the KT-120s and then let the amp warm up (while listening to Solar by Miles Davis) so I could bias the tubes. The sound was fine. After about 5 minutes I heard a rumbling, then I turned the volume all the way down to bias the amp. The heaters were on but there was no bias voltage (I didn't notice a rectifier flash). Not being able to get bias voltage, I replaced the rectifier, then went to bias again and I saw fireworks in the rectifier tube, then the fuse blew. I put in the first rectifier, hoping that it was still good, and the fuse blew. After being down two rectifiers and two fuses I decided to put things back the way they were, so I re-installed the original KT-88s, put in my last spare rectifier and another fuse. Everything was fine and I reset the bias to 0.55v.

I'm glad that the ST120 amp is a nice, solid design and nothing under the hood blew up in spite of my abuse (in this case not referring to my music tastes).

So before I sacrifice any more rectifier tubes, is the best procedure to install the new KT-120s on a cold amp and immediately bias to 0.55v? Or could it just be that one of the KT-120s is bad?

Im no expert but it sounds like a shorted tube. Wonder if you could do a comparison between all the kt120's with an ohm meter to check for shorts. I think i read that the bias current is similar to the 88's. If anything they are higher! Good luck.

I have been running KT-120's in my VTA ST120 for about 6 months with no issues. Sounds like you have a tube that failed.

My dad's old Heathkit TC-2 tube tester works, but there is no setting data for a KT-120 tube. Anyone have suggestions for settings that would be appropriate?

bcinnamon wrote:My ST120 has been working flawlessly for about 18 months with great sound and no tube problems. Being incapable of leaving well-enough alone, I wanted to try a matched set of KT-120s (from a reliable source) to replace the Tungsol KT-88s in my amp. I installed the KT-120s and then let the amp warm up (while listening to Solar by Miles Davis) so I could bias the tubes. The sound was fine. After about 5 minutes I heard a rumbling, then I turned the volume all the way down to bias the amp. The heaters were on but there was no bias voltage (I didn't notice a rectifier flash). Not being able to get bias voltage, I replaced the rectifier, then went to bias again and I saw fireworks in the rectifier tube, then the fuse blew. I put in the first rectifier, hoping that it was still good, and the fuse blew. After being down two rectifiers and two fuses I decided to put things back the way they were, so I re-installed the original KT-88s, put in my last spare rectifier and another fuse. Everything was fine and I reset the bias to 0.55v.

I'm glad that the ST120 amp is a nice, solid design and nothing under the hood blew up in spite of my abuse (in this case not referring to my music tastes).

So before I sacrifice any more rectifier tubes, is the best procedure to install the new KT-120s on a cold amp and immediately bias to 0.55v? Or could it just be that one of the KT-120s is bad?

Did you turn the bias down when you installed the KT-120s? ALWAYS turn the bias down to the minimum current (or very close to it) before changing any power tubes. If the old tubes were set at 55 ma. (.55 volts) and the new tubes bias hotter - meaning they pass more current under a given set of conditions - the new tubes could have been passing a LOT of current and damaged themselves. Always turn the bias way down first (before the amp is fired up) then bring the bias current up to the spec in a couple steps. Otherwise you risk what appears to have happened.

If you want you can send me the tubes back and I'll examine and re-test them. Those tubes were run at 465 volts with current over 100 ma. and behaved flawlessly on my test rig. I'm not saying a tube couldn't have failed, but I suspect they may have been passing a lot of current (that's what likely blew the rectifiers). They could have easily exceeded what the rectifier can handle causing it to melt down and pop a fuse. Four KT-120s running hard can eat a 5AR4 alive in almost no time.

Putting the cooler biasing KT-88s back in reduced the current draw and the rectifier survived. I'll bet those KT-120s are in fine shape - but it appears they took out a couple rectifiers and fuses on you! Biased properly, however, I think they'll be fine.

I'm with Jim on this, the 5AR4 tube has a max rating of 250ma and I'd never run it continuously over 200ma if you want it to last six months or more.
The driver board uses about 20ma leaving you about 45ma per output tube.
For the VTA120 amp the Weber solid state rectifier is the smart choice, and if using kT120's it's going to be a must !!

Honestly I wouldn't try using a 5AR4 with the KT120's. I've even had issues with cheap 5AR4 tubes blowing in my st-70 that has KT88 tubes. If you keep the bias way down you can probably get away with using the KT120's for a while, but ultimately you'll probably need a rectifier upgrade. Personally I've used gz37 rectifiers in my M125 monoblocks with KT120 tubes for almost a year now without any issues. If you don't wanna spend $100+ for rectifiers that might ultimately blow anyway just get the weber rectifier.

At the risk of taking the thread off topic, what are the possibilities for beefing up tube rectification without going to exotic/expensive NOS tubes? For instance, given space on the chassis could someone modify a ST120 for dual 5AR4's in parallel... would they share the load evenly? Would it be better to run separate 5AR4's for left & right power tubes, plus a smaller [EZ81 maybe] for the driver board? I've been wanting an ST120 but the idea of a solid state rectifier bums me out a little, and I'm hesitant to use any tubes that aren't still in production.

If anyone has ideas on how to make this work, I'd love to hear it.

I'd do the build on a custom chassis with a 5AR4 for each channel, simple.