Beautiful Sound of my VTA-ST120drt1 Part 2

I really meant to simply just post Part 1 a couple of days earlier but was thinking it might be hokey or patronizing. The sound was stunningly beautiful. Magical. Sigh. Then, I had to recover. Is this Karma? Questions to ponder.

Since I was doing the critical listening thing, I was hearing the R channel more the the left at times, and the Reverse Stereo mode on the preamp seemed to balance it out. (This seemed to be a preamp issue but it is easier for me to swap the VTA inputs.) Then I got the goofy idea that I would swap L/R inputs to the ST-120, and then the Reverse mode would then be back to Normal Stereo. All was well. Then, I decided it was actually the recordings, not my hearing or equipment. I switched the Amp selector switch to the SS amp to avoid the hum when removing the plugs. With the ST-120 still on, I proceeded to unplug the L/R RCA plugs. Then in a surreal moment there seemed to be more fireworks emitting from my ST-120 than I have seen since Disney World! As quickly as I could, I pulled the plug on the ST-120! The smell of burned electrical components waffed into the silent air.

"You have to be #@%$* kidding me. The amp's right channel died about a year ago! Maybe the same month! July/August... Again, after a serious listening session while being amazed at its magical sound. What the hell am I doing?" Thinking back to this recent incident, I believe I shorted the input of the amp. I fully blame myself. I've unplugged/plugged hot connections to SS gear my entire life without consequence. I guess it just took me a lifetime to realize the danger of what I have been doing. And, this is my first tube amp. A phrase from the movie Forrest Gump has become an earwig in my mind....   I'm still a little stunned. I just love the sound of this amp.

Naturally, the fuse was blown. The tubes showed no signs of distress so I replaced it and powered on. The power tubes began to glow and then went dark. The fuse blew. So I put it on the bench (kitchen island) and took the photographs. I'm stunned. Wow! What did I do? So I will definitely be ordering a new can capacitor, switch, wiring and post. Odd the other components in the burned area seem unscathed. Test, or just replace them?  Here are the photos I've taken.   https://photos.app.goo.gl/cHG57NFbUgfuKE1NA

Well... Sad!

You have learned to do nothing at all to any audio components, living or dead, with the power connected, much less ON - and that includes components that are otherwise out of the loop. Correct? Nor am I suggesting for even a moment that what happened is due directly to your manipulations. Per your description, if I followed it correctly, your 120 should have been out of that loop.

At the same time - whenever there is a symptom that is not clearly identified, start with the  "Expensive" section of the loop first. Just a suggestion.

Bias?
Hours on the tubes?
Your pictures do not link, by the way.

Oops - the link just tied.

Looks more like a dead-short than a failure. Possibly insulation that just got too hot and the wires too close. Might not be anything that you did recently, but go back to the original assembly.

I'm sure Bob will help you get the magic smoke back in where it all belongs!

From a 3rd grade quiz:
Question #25. If you read the entire directions as instructed, you would have seen this test actually required you to answer only the first 3 questions and then stop.

I know question #25 well because I answered it.

Oh, I'm sure I read Number 7 along with the other warning's, and stupidly ignored it. I probably thought "Wow! VTA is being very cautious; what's the big deal about pulling inputs? Just turn the volume down." I have gotten better at reading instructions; I need to improve heeding them. Sigh.

Thanks for the replies! Hours on the KT-120's...around 200+, bias last checked about two weeks ago at .5.

I will be able to start working on it next week. I will be rechecking all the connections too but this has been seen by two VTA repair technicians both of whom did do some re-soldering of my connections. So, in "our" defense, I'm going with a failed component. After the last repair of a bias resistor I thought "Ahhh, now two knowledgeable technicians have looked at it, made some minor fixes so now she's (ST-120) set for life. Whew. Any worries about my work are over." Seriously, I felt very confident in their abilities and work performed. Absolutely no problems.

As far as heat goes, I use three "silent" PC fans. One in the rear blowing on and around the transformers, and one on each side pulling the heat away. It runs warm, not hot. I do tend to run it 12 - 16 hours each weekend day. Pentode mode. It does not get played more than comfortably loud. It sounds beautiful.

I'll note and photograph the process. It may be of help for others. ??

Ouch... You've got some cleaning to do there!!  

I'm sure if that had happened to my amp I would have let out some of the brown stuff along with the magic smoke

As a suggestion, and I don't want others to shoot me for this, if you don't use the triode mode then dispense with the switches and wiring. After all there can be nearly a kilo-volt differential across adjacent parts of the switch on a high output swing. Once an arc starts then the ionised air and carbon will sustain the arc causing similar damage to that seen.

Looking at the switch I am going with arcing from one side to ground and then you have the full HT so sustain the arc and burning. It would't take much of a slight whisker or protruding lead length from a soldered connection to get close to the switch body.

Also could a small bug have made its way inside and ended up across the switch? A rather expensive bug killer but still same principal.

Good luck with the repairs.

I added a photo (now the last one) that shows Pin 5 on the DPDT switch took most of the charge. I cleaned the area up a bit and you can see that the wire jacket disintegrated. (The wire actually broke at the connection; I placed it there for the picture.) This wire connects to Pin 4 on the V7 power tube. Next most damaged is the wire from the chassis ground lug to the negative terminal of the left speaker. Next is Pin2 wire from the DPDT switch to Pin 4 on the V6 power tube. Next, the wire from the PCB to the triangle lug on the Quad cap. Plus some other wire collateral damage.

https://photos.app.goo.gl/cHG57NFbUgfuKE1NA

Makes me even more convinced that the failure was not due to diddling the patch-cords, but coincidental to it.

Jimmeq, did you find the problem?

Great job photographing the damage. Based on the blast radius of carbon in the pictures it seems like the center of the arcing was around the switch. That's some seriously bad failure there whatever it was! I am curious to know what you find. Certainly some kind of short.

This has nothing to do with unplugging input cables.

Bob quickly sent me the parts and wire I ordered and I began surgery Monday. As a newbie knowing just barely enough to be dangerous, I'd say it was the switch because it sustained the most damage. And I have no idea why. It has been in Pentode mode for months; I very rarely switch between modes.

The latest photos are here provided the link works. https://photos.app.goo.gl/Lu9nTCxiriRE3o8x6

As I replace a wire I put a label on it so I will be able to get it back together correctly. I have plenty of time so I'm working slow; even forcing myself to stop so I won't get frustrated. This is different than building because I have no instructions to follow. Little by little, with plenty of notes and pictures when I remember to take them....

Measure twice, cut once!

Best of luck!

Well, after around five hours I'm just about finished. The solder is not adhering to the square terminal on the quad cap so I'll desolder and use a little emery cloth. I've put in about a hour a day to keep my sanity as it's tedious for me. I'm just now understanding how as we age, the eyes are not as good as when we are young. Bright lights and check using a magnify glass!


I took some close ups of the switch. My uneducated guess is that high voltage hit the switch and as per the #7 "don't" in the manual. Maybe it was shorted when I unplugged the RCA jacks. I'll take a mock up photo of how I think I caused a short in the right input. As best I can describe the right RCA jack was partially in and the left RCA jack fell on top of it.

The switch https://photos.app.goo.gl/Lu9nTCxiriRE3o8x6

DPDT Pin 5 was connected to Pin 4 on V6.

After completing the repairs I thought performing the initial build tests would be good practice. I was unable to bias the rear tube on the left and after talking with Bob who had me first test the 10 ohm resistors, and a bias check, I replaced one. Also, I replaced both right side tubes.The tubes now all bias to .500. The remaining voltage tests results were on spec! After passing the initial sound test using my iPhone, I put the ST-120 back into the system.

Eureka! What beautiful sound! She's working! (Picture the scene from Young Frankenstein where Gene Wilder exclaims "It's alive...It;s Alive...IT'S ALIVE!) Yes, I'm excited! I'm swapping driver tubes and the rectifier rather than spend the cash to replace them all. Somehow over the last few years I've accumulated almost a dozen 12AU7's or 12AU7A's. It's amazing how great this amplifier sounds despite this.

Well, this incident was nearly the realization of my worst fear. That at some point, there'd be a major failure that I would be unable to repair. But with this forum and Bob's help I was able to forge through and complete the repair! It's one thing to follow the manual's instructions but this time I had to improvise. Now I have expanded my knowledge and confidence in my abilities with ST-120 repairs. No more fear; I CAN repair it. Being a beginner, I am still in awe of my build. And now flabbergasted that I actually repaired it! Whew. I first got the inclination to build a Dynaco when I was about 14 and saw an ad. I was 59 when I assembled my ST-120. Some things just take time. Since no one took the bait with my listing it as ST-120drt1, I joking like to say it means ST-120 Dining Room Table, 1st build.

All is right with the ST-120 again...

jimmeq wrote:After completing the repairs I thought performing the initial build tests would be good practice. I was unable to bias the rear tube on the left and after talking with Bob who had me first test the 10 ohm resistors, and a bias check, I replaced one. Also, I replaced both right side tubes.The tubes now all bias to .500. The remaining voltage tests results were on spec! After passing the initial sound test using my iPhone, I put the ST-120 back into the system.

Eureka! What beautiful sound! She's working! (Picture the scene from Young Frankenstein where Gene Wilder exclaims "It's alive...It;s Alive...IT'S ALIVE!) Yes, I'm excited! I'm swapping driver tubes and the rectifier rather than spend the cash to replace them all. Somehow over the last few years I've accumulated almost a dozen 12AU7's or 12AU7A's. It's amazing how great this amplifier sounds despite this.

Well, this incident was nearly the realization of my worst fear. That at some point, there'd be a major failure that I would be unable to repair. But with this forum and Bob's help I was able to forge through and complete the repair! It's one thing to follow the manual's instructions but this time I had to improvise. Now I have expanded my knowledge and confidence in my abilities with ST-120 repairs. No more fear; I CAN repair it. Being a beginner, I am still in awe of my build. And now flabbergasted that I actually repaired it! Whew. I first got the inclination to build a Dynaco when I was about 14 and saw an ad. I was 59 when I assembled my ST-120. Some things just take time. Since no one took the bait with my listing it as ST-120drt1, I joking like to say it means ST-120 Dining Room Table, 1st build.

All is right with the ST-120 again...

Part of the beauty of these amps is the simplicity of the circuits.

One should probably practice home fire drills when drinking. That way, you'll be properly well practiced for a relaxing music listening session that goes up in flames.

Seriously, sorry to read of your bad experience with the amp, but glad to read of the positive outcome, and all the assistance you received to get you there.