As another former PL owner. I like the KT88 power tube rather than the modern production EL34's. I am neither of the opinion that modern EL34's have any kind of rich midrange or bass to speak of. In fact, it is my opinion that they only appear to have midrange, due to their obvious lack of bass. If you look at the specs of the PL Prologue Four (EL34 version) and the Five, which is the KT88 version, you will see it is their more expensive model, though both have now been discontinued in favor of a newer and more expensive model. The power supply in the KT88 Five is 50% larger. That is because the KT88 can deliver more bass. The ability to deliver more bass has to do with a larger power supply. If you look at the design of the KT88 tubes envelope, you will see it is larger and has more surface area than the EL34. This is so it can more effectively eliminate heat, due to its ability to handle more power. In general, 2/3 of your amps power or more goes toward producing bass frequencies. Now, I don't use monkey coffin speakers, (not that I have anything against primates). I do have vintage, restored and upgraded Altec Lansing A7 theater speakers, which are high sensitivity horn speakers. This allows me to use my Rogue 150-Watt M-150 KT88 monoblocks or my Decware 3.9-Watt single ended 6V6GT pentode power amplifier. It is important that my comments are tied to the high sensitivity horn speakers that I am using. The KT120’s do have better low and high frequency extension than the KT88's do and they are more linear in operation. However, this is only true when used with a more powerful amplifier to drive them. They are powerful tubes, but if you have high sensitivity speakers and do not require the power, which is my case, I prefer the sound signature of the KT88. Just substituting a KT120 in an amplifer whose design is for a KT88 or an EL34, won't deliver what the KT120 is capable of. A lot of this depends on the source material you listen to. If I listen to large orchestral arrangements, then I would go with the KT120. But, because my preferences lie with Jazz, Acoustic and Vocals, I prefer the KT88, due to its richer midrange. Mark O'Brian from Rogue, once described the KT88 as having almost the midrange of the EL34, but with better bass. That having been said, I agree with member @unclefred that KT77 are a better choice than the EL34. I am restricting this to modern production EL34's. I used the KT77's in a Line Magnetic 211iA and like them a lot. But, I don't agree that you give up that much modern midrange, in exchange for the nice bass that the KT88's have. KT88's have a beautiful rich, solid bass, which is why McIntosh uses them. Now, it you are after that lush, magical midrange, it does exist only in NOS tubes. Fortunately, while NOS KT88's are unobtanium, NOS EL34 are still available and they do have that magic midrange. I use NOS EL34 RFT's in an Audio Note Kit L4 power amp. Between the amp itself and the RFT's, I think this class A/B amp comes close to the sound quality of a SET. Surprisingly enough, these NOS do have very nice bass, as compared to modern KT88's, though I am somewhat a loss as to why.
Russian Tung Sol 6550 are less expensive vs Russian Gold Lion KT88. The 6550 are very neutral and the KT88 have a little more midrange bloom. It is likely amp dependent upon which tube sounds better.
Thanks for that I'll start looking though those later today during what is fast becoming a serious contender for the most pointless daily Teams meeting of lockdown.
But I love the Harbeths 99% of the time. Seriously though, I do accept that the situation which I would like to improve through tube rolling could also be approached from the opposite end by changing to more efficient speakers. Should I bury or burn the monkey coffins?
For what it's worth.,I like the KT120s in my amps. Got them from Jim McShane. He puts them through a good test and matches them. Jim McShane Home Page
Same for Rogue. OEM's use JJ because they are cheap and consistent with high availability. If you ask Rogue, they will tell you to roll them and and there are much better options, but they are not an option for manufacturing.
There's good odds Luxman is not just using any JJ power tube, meaning theirs are carefully screened for tubes which don't meet standard. And also, Luxman is likely not running them at the higher end of the plate voltage ratings, so being somewhat conservative. And their circuitry designed around that. My experiences with JJ power tube failures are with amplifiers and receivers which run the output tubes hard (and designed that way). Fisher, HH Scott, and Dynakit gear to name some. And also jukeboxes. This was also 7-10 years ago. And good odds JJ may well have improved things. Just some thoughts.
No, not my suggestion at all. I only wanted to point out what speakers I was using and my observations using the different tubes with them. Altec A7's are true horn theater speakers with high sensitivity. They are not dissimilar to heritage Klipsch horn speakers and respond in a different manner than direct radiator designs. They allow you to hear smaller differences in tube designs and by nature are more dynamic. It may well be that with Harbeth's, that you might be better off with KT120's and the added power, dynamics and extended frequency range that they have (in an amp designed for them), over KT88's and smaller power tubes. I have owned Altec's and various Klipsch speakers over the course of fifty years. As such, these are the type of speakers I am familiar with. These are real theater speakers, the size of refrigerators. They are not something that would work in most homes, thus is the domain where monkey coffins rule. The general impressions I get from reading the forums are that Harbeth's like more powerful tube amplifiers. There are a lot of great low powered SET amplifiers. But when you gave only 2-8 watts to work with, then you need to have highly sensitive speakers to be able to appreciate these tubes and the benefits that single ended amplifiers can bring to the table. Just trying to give you my impressions of these various tubes when used with speakers that allow you to discern these differences.
This is also my impression. Before there were more powerful tubes like KT120's, someone might use 6550's for a symphony orchestra because they are more neutral and linear. While someone else might opt for KT88's for Jazz and Vocals due to their increased midrange presence and noticable bass. Because of my listening preferences, I opt for KT88's.
My impressions of the JJ power tubes are similar and go back these years also. When JJ that are run hard, such as the ones I ran in my original Rogue M-120 monoblocks, go out, they tend to take other components out with them. I have also had them blow and break the glass envelope at the base of the tube. I will add that I thought the JJ's did sound nice. Rogue preferred to use JJ's as their smaller signal tubes and Electro Harmonic KT88's as their power tubes because they were inexpensive, fairly reliable, easy to replace but not as destructive as the JJ's.
My Primaluna can output the folowing: With EL34 it's 38 W in TR / 70 W in UL With KT88 it's 40 W in TR / 72 W in UL With KT120 it's 45 W in TR / 85 W in UL With KT150 it's 50 W in TR / 88 W in UL So KT120 probably give best bang for the buck on output increase (from EL34s) based on current pricing of the Tung Sol 120 and 150.
I’ve had Tung Sols in mine. I won’t do it again. There were always weird popping sounds on power up- but the PL Autobias board always gave the green light. One night after I went to bed, my wife was watching a movie and one channel went dead. She said there were red lights lit on the amp. She turned the amp off and used the TV speakers. She forgot to tell me. So the next night I came home from work and flipped the amp on and went to the record room to make a selection. From the other room I heard a loud pop and one channel was totally dead. No bad tube lights either. I called Upscale and they said the fuse on the autobias board had likely blown. They were right. I replaced the fuse and the output tubes and have been perfectly fine since.
I originally bought a pair of Rogue KT88 M-120 monoblocks. As they gad some age on them and the JJ's took out parts, I used Rogue's upgrade program to upgrade them to the KT'88 M-150's. I bought a Cronus Magnum I, with KT120's to use as a back up amp. I found that for half the retail price of the M150's, the CM came close to the sound quality and power of the monoblocks, running in the ultralinear mode. The M-150's required a quad of KT88's in each monoblock. So the KT120's do provide the greatest bang for the buck.
Dude, I thought I was getting somewhere then you lay this on me. Were they KT120? I assume the bias switch was selected correctly for higher bias tubes / no WAF contribution? Anybody else had issues with Tung Sol in the Primaluna? My big takeaway from this thread so far is that a few new production power tubes can have underlying QC problems in use. My stock Primaluna EL34 (which I understand are Shuguang) are still going fine after around 500 hours. Only had my PL red-light once, and that was due to the wife cycling the power on/off/on too fast. Was all good after a cool down. Tubes and wife.
New production tubes definitely are not built to the standards of the vintage stuff. But this doesn't mean that a many of these tubes don't sound great, because many do. IMO fusing of the output tubes has become critical when one is using modern output tubes today. Traditionally the cathode resistors acted as the fuses inside of classic tube amps. But I don't think that this is sufficient with todays tubes, lest you seriously risk damaging your output transformers. This is why I have begun adding fuses to output transformers on my amps in the last few years. By doing this, I can use whatever tubes sound best in an amp, and not have to worry that QC is gong to result in an output tube turning into a roman candle and potentially destroying an irreplaceable output transformer.
I told my wife and son there's a danger of electrocution should any tube equipment be touch by anyone other than myself.
I'm assuming that this was just to keep them away from your gear, not because you actually believe this...
No, they were 7581A’s- approved by PL and everything was set correctly. The amp had been running for hours when it shut down. This happened after 5 months of use.
That is one of my favorite current production tubes. Not the most powerful, but I absolutely love their sound. However, I'm not certain if the 7581A screens have high enough voltage ratings to be safely run in an UL amp which was designed for EL34s and higher voltage tubes. So that may have been the source of your problem.
Those tubes get good reviews on other forums / sellers on the Web. Maybe you were unlucky and took delivery of a hidden fault. Not read of any similar issues.
What voltage does the B+ supply run at in that amp? I'm guessing its more than the maximum 440V screen rating for a 7591A. If so, then the manufacturer is telling you to run this tube beyond its maximum ratings. And that is a good way to get a tube to arc...
That JACMusic article was a good read, very informative. Will dig into the others over coming days. Nice one
Regarding Brimar, I thought even during the golden years of British valves, they were often made at various factories. I run a U-52 rectifier made by GEC in the power supply to my phono stage and it was branded Brimar (not that this saved me much money). You are aware that the acronym meant to describe a British tube meeting a US spec. I have only one amp here now that uses a 6550-it's an ARC dual 75a that I bought new and periodically refreshed over the years. Before ARC changed hands I bought a full complement of parts, including glass, to refurbish it, but that project remains undone. It's a nice looking amp though and I rarely had problems with the tubes ARC supplied for it. The more I learn about valves, the less I know. Stuff branded on the glass as made in Great Britain actually manufactured by Amperex in the U.S. Those odd Cyrillic tubes which can be used in a MIG fighter, an ICBM, or a Lamm SET. And the "real" DR Reflektor 6 H 30--- I'm happy just to find Tele ribbed plate 12ax7s that play nice, forget about NIB/NOS. No question is stupid when it comes to tubes.
