| Author | Message | ||
     Doug Elick |
While repairing the reverb tank in my HD-130, I noted some odd tube behavior. (Yes, I did a search but didn't find any hard answers). I noticed that the far right tube seemed to be running much hotter and had a much more pronounced blue glow hovering about the inner plate structure than the others. I whipped out the temp. probe for my DMM (touching it to about midpoint up the tube structure) and discovered that while the other tubes were running between 210F and 280F, this one was at a whopping 330 degrees. With the B+ off, all tubes remain within about 10 degrees with eachother, but when they are activated, they increased to very uneven temps (280/245/210/330). After a little research on Usenet, I swapped the offender (which is also the most microphonic of the set,) #4 with #3. Not only did the hot tube remain so in the new socket, but a dimly glowing plate was revealed; it was not visible in the far right position. Touching the probe near the hot spot revealed an envelope temp of over 400 degrees! Cripes! Can old tubes (especially odd brands like Jolida) drift so badly as to exhibit this behavior? What physically has happened to the tube to allow this? Is the grid failing? Sorry for the recent deluge of questions, Doug Elick | ||
| Steve Kennedy |
It is possible that the tubes didn't stand up to the higher than normal plate voltage found in Music Man amplifiers and some of the elements might have internally arced over causing some carbon paths that act like little resistors internally which are in effect "rebiasing" the tube. If the problem follows the tube, its in the Tube! Steve |