[Brett Buck]

I did the series diode trick on my U70 a few years ago when it was first mentioned. Today I finally had my first "failure" of the rectifier tube in my Ultimate 70 since then. In this case, the tube did not arc over and fail outright like all the others had, it manifested itself as a turn-on/warmup thump  and some static, and a mechanical hum from the tube itself. There was no hum through the speakers. It still played mostly OK aside from that.  Replaced it with a new one, and all of this went away, back to normal.

     I was wondering if this was an issue where the series diode had failed short on one plate, thus putting have the regulation back on the tube plate for half the swing, and leaving the other with a diode, but I checked the diodes and they seemed fine. I tested the tube on a tester, and it still showed OK, but I put it back and all the same symptoms returned. I didn't put a scope on the rectified output, at least not yet, so I am not sure what the issue might have been.

    This isn't all that bad. I estimate it lasted about 2000 hours, which is by far the best it has ever gotten with any new-manufacture 5AR4/GZ-34 I have tried. I think the diodes helped it last this long.

    Brett

[rlee8394]

I'm sure the diodes helped. Since they are cheap, you could double up on them by using two in series. That would provide twice the reverse voltage capability as well as a measure of redundancy in case one shorts out. I have been using Chinese Shuguang 5AR4 rectifiers and find them quite reliable.

Here's something else you can try. Once you install the diodes, single or double, you can short the cathodes together at the tube. This would allow the diodes to handle the rectification duties while the tube rectifier just acts as a slow turn-on device. Doing this also connects the tube rectifier plates in parallel, effectively cutting the resistance in half, thus lowering the voltage drop across the rectifier tube and providing a bit more voltage, and power. I'd use two 1n4007 diodes in series if you chose this configuration.

Ron

[Brett Buck]

I'm sure the diodes helped. Since they are cheap, you could double up on them by using two in series. That would provide twice the reverse voltage capability as well as a measure of redundancy in case one shorts out. I have been using Chinese Shuguang 5AR4 rectifiers and find them quite reliable.

   That's the beauty of this arrangement - even if one of the diodes does short out, it reverts to the original design with the tube rectifying the signal again. If you bypass the tube entirely (as previously discussed, a bad idea) and just use diodes, you need to double up on them to guard against them shorting out.

       I haven't had that much luck with the Shuguang tubes, they didn't fail as quickly as the JJs, but didn't last that long compared to the Sovtek. My temporary spare is an RCA/Japan 5AR4 that has maybe 10,000 hours on it, still works. I have found most of the discussion of "NOS tube sound better" to be untrue, but the old ones sure seemed to last longer.

    Brett