I've been following these recent posts and have learned a lot. I appreciate your efforts with using the proper vocabulary. I understand different amps create different variables with respect to idle current. That said there still seems to be a consistent range of idle current for each power tube.
I understand that some manufacturers run their idle current high because it imparts a particular sonic quality. Your amps conversely run with a lower idle current for the purpose of longer life of the tubes.
My amp kit is essentially an oversized ST70 built to use 6550/KT88 tubes. (thus named the ST120). It uses 10ohm resistors on each tube for calculating idle current. In the kit instructions the idle current should measure .550VDC for each tube. Does that seem high to you?
Thanks for your compliments on teaching this topic. It is one of the most mis-understood of the popular discussions I see on many forums.
I looked up the ST-120 and found the tubes4hifi amp and I assume that is the one you have.
Your idle current works out to 55 mA.450-500V. Converting 55 mA to amps we have 485 V x 0.055 amps = 26.7 watts, well within the 35-40 watt rating of the original Genelex KT-88. You should be just fine with that operating point. What brand tubes are you using? Some modern KT-88s are not reliable above 30 watts and should be down-rated. I have done in depth research and re-rated several tubes to where I think they are safe. A 30 watt KT-88 should really be run at 15 watts if you want the long life. Not everyone cares about long life, we should just keep that in mind. Many modern KT-88s don't deserve the 40 watt rating. Some manufacturers say "if it looks like a KT-88 then it must also have the rating of a KT-88" However they don't consider the fact that the originator of that tube did some very special things and used some special materials to get that performance.
One of the things I would like to do in my school, if I could find a student to do it, is to do the research on a sample of modern tubes and publish the results. There are so many factors that influence life that warrant a study. Although Sylvania, RCA, and GE did life studies I doubt any of the current manufactures does. In the heyday of tubes, which was entirely driven by color TV (re my discussion with Sylvania) the tube makers were dealing with the TV manufacturers not the end user. The TV maker wanted a quality TV (Zenith and RCA leading that) that needed only occasional servicing. That's why those companies really worked on tube life. When the transistor took over color TV that goal was lost.
A few things about ratings. Very few, if any, tube manufacturers stated the life of the tubes they made at full dissipation. When I visited the Sylvania factory I asked the question "how long will your tubes run at full dissipation" the chief engineer of 25 years said "Well you aren't supposed to run them at full dissipation because they will only last about 1000 hours. If you run them at half dissipation they will run 10,000 hours." Now Sylvania made some of the best American tubes and he also told me that their goal was to make a color TV run 10 years at 3 hours a day with their tubes and that they had pretty much achieved that if the designer of the set ran things properly. Unfortunately life vs dissipation is never given in any data book I have seen. I was lucky to have the Sylvania enginieer share that information early in my career so I didnt make the mistake some designers make and run the tubes full out.
This is just a thought but many modern tube amplifiers are designed by people who have not thoroughly investigated tubes. Perhaps they think that tubes are like transistors. In essence transistors, properly heat sunk, can be run at full dissipation and not impact their much longer life. Without mentioning names, there are several modern amps of all nature (Transformer coupled, OTL, Single Ended) by many first time makers that have run tubes at full dissipation and learned their lesson, at the owner's expense in many cases. that full dissipation is a short road to unreliability. I have worked on many amps where only the strongest power tubes will make it past burn in on my bench.
This is why I run my tubes around 1/2 rated dissipation. I recently replaced one driver tube in an amp I had sold locally and serviced through its entire 25 year owner ship by a friend. In that period we only replaced one of the eight Siemens EL-34's and this driver tube only because it became noisy. Those remaining tubes 10 out of 12 in the amp have at least 20,000 hours on them. Compare that to ARC who recommends replacing output tubes every 1000-2000 hours.
