Hello everyone,
I've been enjoying my VTA ST-120 for couple years now, with Gold Lion KT88 power tubes. While listening the other day, I noticed some "thumping" sounds, then a sudden drop in volume. Looked over to see the right rear power tube redplating. Switched the amp off immediately, my friend says he saw a flash in the tubes when I switched it off. I didn't see it myself, but I don't doubt him.
This event blew the fuse. May have also taken my vintage Amperex 5ar4 with it, though I haven't tested that yet.
To see what's up, I took out all 4 power tubes and the rectifier, and I installed four Sovtek 6550 power tubes (the ones that came with the amp) and a cheap GrooveTubes GZ34 rectifier.
Powered on, tested with some music. Sounded OK. Checked the bias after a minute or two. Right side bias was reading fairly normal, about where I had it set. But the LEFT side bias is very weird.
When testing the lower right test point on the left side, the reading is fairly normal. But the upper left test point (the one with the "Biaset" marking) I'm getting 24 Volts DC!!!!
That can't be good!
I didn't bother trying to adjust the bias with a screwdriver, I just double checked my reading to make sure I wasn't going crazy, then switched the amp off. Came straight here.
This seems particularly odd. A power tube on the RIGHT side was redplating. But now I'm getting weird readings on the LEFT side.
What does this reading mean? How screwed am I? How can I get my amp back in working order?
EDIT: It seems that I may have a bad resistor. According to this thread here, I measured resistance of those bias pins with the amp turned off. All measured ~10 ohms, save for the culprit which read zero. So now my question is: where is that resistor, and what do I replace it with? And a follow-up question would be, in what way could the right side redplate event be related to this left side bias resistor failure?
I've been enjoying my VTA ST-120 for couple years now, with Gold Lion KT88 power tubes. While listening the other day, I noticed some "thumping" sounds, then a sudden drop in volume. Looked over to see the right rear power tube redplating. Switched the amp off immediately, my friend says he saw a flash in the tubes when I switched it off. I didn't see it myself, but I don't doubt him.
This event blew the fuse. May have also taken my vintage Amperex 5ar4 with it, though I haven't tested that yet.
To see what's up, I took out all 4 power tubes and the rectifier, and I installed four Sovtek 6550 power tubes (the ones that came with the amp) and a cheap GrooveTubes GZ34 rectifier.
Powered on, tested with some music. Sounded OK. Checked the bias after a minute or two. Right side bias was reading fairly normal, about where I had it set. But the LEFT side bias is very weird.
When testing the lower right test point on the left side, the reading is fairly normal. But the upper left test point (the one with the "Biaset" marking) I'm getting 24 Volts DC!!!!
That can't be good!
I didn't bother trying to adjust the bias with a screwdriver, I just double checked my reading to make sure I wasn't going crazy, then switched the amp off. Came straight here.
This seems particularly odd. A power tube on the RIGHT side was redplating. But now I'm getting weird readings on the LEFT side.
What does this reading mean? How screwed am I? How can I get my amp back in working order?
EDIT: It seems that I may have a bad resistor. According to this thread here, I measured resistance of those bias pins with the amp turned off. All measured ~10 ohms, save for the culprit which read zero. So now my question is: where is that resistor, and what do I replace it with? And a follow-up question would be, in what way could the right side redplate event be related to this left side bias resistor failure?
