Alan W
Well-known member
I know there are several far more tube amp experienced builders on this forum, and am trying to figure out "what went wrong."
I'm using an Allen Encore, which is loosely based on a Vibrolux Reverb. (I built this from a kit about 4 years ago, and it's worked great, more than great!, since day 1.) A few weeks ago, I began to have some issues, where if there was enough loud bass content (typically from my Baritone, but not always) I'll begin to get fading volume and dynamics. This happened about 16-18 months ago, and I decided that the rectifier tube was probably the most likely culprit, and popped a new one in. It's sounded lovely since then—until a few weeks ago. The problem was intermittent, and a visual inspection of the rectifier and 2 6L6es didn't show any obvious issues. So, I just let it ride. But then, during a poor spell (typically no longer than a few minutes), the amp just dies... Well, it blew its fuse.
Biasing is super easy on this amp—there's both external bias check points and an easily reached control. I let it sit for a week, using my old amp instead, and stewing on this. Today I tested the bias, after re-fusing, and one tube was 25~mv and the other 34~mv. (the recommended range for 6L6es is 30 to 35, and the last time I checked bias (maybe a week after I installed that newer rectifier tube) both were very close to 34. In any case, I did not try and play through it, since the bias was so far off (and I don't know if that's a taboo or not, but am cautious, especially about things I don't fully understand..). But I plugged in two replacement 6L6es, and they biased at 32~ and 34~ mv. I just played through it about 10 minutes and it seems fine, right now...
Questions: I know in the past, I can remember hi-fi tube amps spectacularly red-plating, and the sense was, oh, a tube died. My concern with the Encore is that I don't know if the tube that now seems bad (oh, to have a place locally with a tube tester again! It's been years!) just suicided, or if something in the amp helped it along. With my recent retubing, and checking of biases, the bias follows the tube more than the socket it's in, so I'm less worried about at least the bias part of the amp. What causes a tube to just die like that? I probably play the amp 8 to 10 hours a week, at a volume that rarely goes about 95 dB, so I guess I can put 4 years of tube life down to "modern" tubes—but it seems short to me—somehow, having a rectifier go at 2 1/2 years didn't puzzle me as much. Is that considered a reasonable lifetime for recent 6L6 tubes? — they were Sovteks*, which is what Allen used originally. (Well—they were the original tubes.)
I'm going to wait and see if I get more of that "stress" related volume drop happening, and will try a new rectifier then. But, at this point, are there internals (caps or possibly bias resistors) that I should pop her open and look at? When I trouble shoot hi-fi stuff, that sort of fading power usually points to the power supply caps, but this is far more intermittent so I think less likely.
* Editted to correct: the tubes are Svetlana, not Sovtek.
I'm using an Allen Encore, which is loosely based on a Vibrolux Reverb. (I built this from a kit about 4 years ago, and it's worked great, more than great!, since day 1.) A few weeks ago, I began to have some issues, where if there was enough loud bass content (typically from my Baritone, but not always) I'll begin to get fading volume and dynamics. This happened about 16-18 months ago, and I decided that the rectifier tube was probably the most likely culprit, and popped a new one in. It's sounded lovely since then—until a few weeks ago. The problem was intermittent, and a visual inspection of the rectifier and 2 6L6es didn't show any obvious issues. So, I just let it ride. But then, during a poor spell (typically no longer than a few minutes), the amp just dies... Well, it blew its fuse.
Biasing is super easy on this amp—there's both external bias check points and an easily reached control. I let it sit for a week, using my old amp instead, and stewing on this. Today I tested the bias, after re-fusing, and one tube was 25~mv and the other 34~mv. (the recommended range for 6L6es is 30 to 35, and the last time I checked bias (maybe a week after I installed that newer rectifier tube) both were very close to 34. In any case, I did not try and play through it, since the bias was so far off (and I don't know if that's a taboo or not, but am cautious, especially about things I don't fully understand..). But I plugged in two replacement 6L6es, and they biased at 32~ and 34~ mv. I just played through it about 10 minutes and it seems fine, right now...
Questions: I know in the past, I can remember hi-fi tube amps spectacularly red-plating, and the sense was, oh, a tube died. My concern with the Encore is that I don't know if the tube that now seems bad (oh, to have a place locally with a tube tester again! It's been years!) just suicided, or if something in the amp helped it along. With my recent retubing, and checking of biases, the bias follows the tube more than the socket it's in, so I'm less worried about at least the bias part of the amp. What causes a tube to just die like that? I probably play the amp 8 to 10 hours a week, at a volume that rarely goes about 95 dB, so I guess I can put 4 years of tube life down to "modern" tubes—but it seems short to me—somehow, having a rectifier go at 2 1/2 years didn't puzzle me as much. Is that considered a reasonable lifetime for recent 6L6 tubes? — they were Sovteks*, which is what Allen used originally. (Well—they were the original tubes.)
I'm going to wait and see if I get more of that "stress" related volume drop happening, and will try a new rectifier then. But, at this point, are there internals (caps or possibly bias resistors) that I should pop her open and look at? When I trouble shoot hi-fi stuff, that sort of fading power usually points to the power supply caps, but this is far more intermittent so I think less likely.
* Editted to correct: the tubes are Svetlana, not Sovtek.
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