DRBILL Posted January 2, 2005 Share Posted January 2, 2005 KT90's (Electro-Harmonix) Im really enthusiastic about this tube. Im using a pair in a MARK-III and it is astounding. I listen to a lot of pipe organ music, and the heavy pedal notes (16-32Hz) are often just an expensive draft. Not so with the KT90. Plus, there is no flutter in the highest pitches (5000+Hz) that I have been experiencing with the KT88s when playing together with heavy bass notes. I had almost convinced myself that the power supply was running out of steam. I put the KT90s in the MK-IIIs for a bench test initially. On the scope there wasnt a great deal of difference ---maybe the square wave was a little sharper than on the KT88s in the lower frequencies. But on the Klipschorns, they were something else altogether. The only downside is that they really stink the first time you turn them on! I think it may be the adhesive that holds the base on. I was frantic looking for a component about to fry, then I realized it was the tubes themselves. A half-hour resolved the matter. One of the biggest items is going to be the added headroom for voltages. When you run a lot of the Dynakit stuff on modern house current (mine is often 128!) you can see an occasional red plate or (worse) a glowing grid. I think the KT90 is going to solve a lot of this. I now run my personal stuff through a vintage RCA Isotap. I wish you could still get em. The KT90 is a direct replacement for the 6550 and the KT88. I expect it is going to be around. Try them if you get a chance. DR BILL Quote Link to comment Share on other sites More sharing options...
jorjen Posted January 5, 2005 Share Posted January 5, 2005 Hi Dr Bill, Thanks for the heads up.This is pretty exciting news you bring about the KT90. What bias point have you chosen for your KT90EH? Are they continuing to improve at this point? I just ordered a quad to play with due to your praise. Quote Link to comment Share on other sites More sharing options...
dodger Posted January 5, 2005 Share Posted January 5, 2005 Well Dr. Bill, seeing as you giving them a thumbs up from your experience, I think I may try also. A good post and good to see you on the Forum! Win dodger Quote Link to comment Share on other sites More sharing options...
cjgeraci Posted January 5, 2005 Share Posted January 5, 2005 Dr. Bill, based on your glowing review, I might also have to try this tube in my Mark IIIs. Now, please help educate a tube rookie - put the following statement into novice or layperson's terms: "One of the biggest items is going to be the added headroom for voltages. When you run a lot of the Dynakit stuff on modern house current (mine is often 128!) you can see an occasional red plate or (worse) a glowing grid. I think the KT90 is going to solve a lot of this. I now run my personal stuff through a vintage RCA Isotap. I wish you could still get em." That means the K90 is better in this respect - right? Does that mean that some risk associated with the standard K88 tubes/Mark III combo is alleviated or lessened? What is a vintage RCA Isotap? Thanks, Carl. Quote Link to comment Share on other sites More sharing options...
Ki Choi Posted January 6, 2005 Share Posted January 6, 2005 Dr. Bill, It seems you have confirmed what Jim McShane, the Citation II Guru's been preaching over the years. I will try them on my Cit IIs. Where is the best place to order the KT90s? Ki Quote Link to comment Share on other sites More sharing options...
Coytee Posted January 6, 2005 Share Posted January 6, 2005 I've got a Jolida 502B that came with 6550 tubes, but their website (www.jolida.com) says 6550/KT88....so can I first presume to be able to use a KT88 just as freely as a 6550? Any inherent benefit of doing that? All that said, does this thread then allow me to presume I'd be ok using a KT90 instead? Any benefits of the KT90 over the 6550? Quote Link to comment Share on other sites More sharing options...
Coytee Posted January 6, 2005 Share Posted January 6, 2005 whoops, another thought just hit me... If I am "allowed" to swap out my 6550 for a KT88 or KT90, do I need to give any concerns to the two 12AX7A (preamp) or the two 12AT7 (power drivers) already there? Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 6, 2005 Share Posted January 6, 2005 Carl, I think the RCA Isotap allows him to step down the house voltage a little bit. Basically the same problem I was having with my ST-70. Your power tranny works with ratios. If it was made to have 110 VAC input to get 360-360 out of the secondary, an input of 125-128 VAC will give proportionately higher outputs. Another tranny with output taps (like using a variac, but in steps) allows you to bring down the voltage you feed your amp. And make it the input voltage the parts were designed for. You can buy a 10amp variac for $120, and dial in the voltage you need. Marvel Quote Link to comment Share on other sites More sharing options...
Coytee Posted January 6, 2005 Share Posted January 6, 2005 well, interestingly enough, I just called Jolida. The dude I spoke to, said that the 88/90 were both of the KT family and they (Jolida) had gotten away from using the KT family (at least in this amp). He told me they had some experiences of the KT88 "cherrying" up on them and had the potential to do some damage to the tube itself and if not caught quickly enough, to the amp. He told me that if I DID try the KT88, that it would fit and work. He went on to say that I should only listen to it while in the same room, so that if it DID "cherry up" on me, I'd be there to hear the sound degridation and would/could have time to turn it off to save the amp before everything went nuclear . He also said that the KT88 took a lot longer to break in, or perhaps he said burn in (?). Anyways, in his opinion, the KT took 50 hours or more for it to reach that same sound quality as (what I presume he meant) the 6550 which is why they use the 6550. Oh, another thing he said that might be redundant to the above (for you that understand this stuff ) He said they use to put KT88's in, get them all adjusted, get them as far as the shipping dock and yet, when the customer received the unit, it was all out of spec. He made no mention if the customer was easily able to put it BACK INTO spec, he just said way too often, they had service calls about that and that was another reason they got away from the KT88. Quote Link to comment Share on other sites More sharing options...
dodger Posted January 6, 2005 Share Posted January 6, 2005 In a lot of cases the new 6550s are less expensive than kt-88s. If you go to the KT-90, there should be no problem with the ECC-83/12AX7. I've looked and tried tubes (NOS) from the same maker. To me, for all intents and purposes, the KT-88 from that Company and also the new are cream of the crop 6550s. dodger Quote Link to comment Share on other sites More sharing options...
cjgeraci Posted January 6, 2005 Share Posted January 6, 2005 ---------------- On 1/6/2005 2:53:37 PM Marvel wrote: Carl, I think the RCA Isotap allows him to step down the house voltage a little bit. Basically the same problem I was having with my ST-70. Your power tranny works with ratios. If it was made to have 110 VAC input to get 360-360 out of the secondary, an input of 125-128 VAC will give proportionately higher outputs. Another tranny with output taps (like using a variac, but in steps) allows you to bring down the voltage you feed your amp. And make it the input voltage the parts were designed for. You can buy a 10amp variac for $120, and dial in the voltage you need. Marvel ---------------- Marvel, thanks for the tip. That makes sense. I will pick up such a device if I need one, but so far, I have not had any problems with running either my Marantz 8B or my Mark III off of my house current. What alerted you to the problem with your ST-70 and what should I be looking for? Sorry to briefly hijack the thread for that question. Now, back to our topic. Based on the Dr.'s rec, I ordered a set of KT-90s for my Mark IIIs today. I will let everyone know my results after I give them a whirl. Hoo, hoo!! First tube roll. Carl. Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 6, 2005 Share Posted January 6, 2005 Carl, I rebuilt my ST-70 recently. I've had it for 30+ years, and noticed that the PS tranny was getting way too hot. You couldn't keep your hand on it for 20 seconds. It has been a weak spot (among others) on the ST-70. The heater voltages were over seven volts, etc. I hooked up a variac I have, and cranked the line voltage down to where I got the correct outputs. The line was right at 110VAC. It is supposed to be a 117 VAC primary. However, this was a later model of Dynaco that came from Japan. I don't know if it was mislabeled or what. A slight difference would have been okay, but our line is usually 120+. When I lived in central Illinois, it was lower and I never seemed to notice a whole lot. To fix mine, I ordered a replacement transformer from Triode Electronics. It is a beefier tranny, and according to the website the pri. is 12oVAC. The paper that came with it says 117, but it's much better than the old one. I'm waiting on a new set of tubes now, as one of mine (original) has gone south. I can't get it to bias. We'll see if it runs cooler when the new tubes are in. It's way heavier! The laminations are about one inch higher that the old tranny. Marvel Quote Link to comment Share on other sites More sharing options...
jorjen Posted January 6, 2005 Share Posted January 6, 2005 I got the quad of KT90's today, but I still cannot find any bias numbers anywhere. Anyone know where the voltage should be set on these? Dr. Bill, anyone? I have them cookin' at 70 in the VRD's right now, but I cannot help but feel this may be too low for 90's. Maybe I am wrong. Quote Link to comment Share on other sites More sharing options...
Marvel Posted January 6, 2005 Share Posted January 6, 2005 Maybe this will help: KT90 Cheers, Marvel Quote Link to comment Share on other sites More sharing options...
jorjen Posted January 6, 2005 Share Posted January 6, 2005 Thanks Marvel, that will do it alright. Quote Link to comment Share on other sites More sharing options...
artto Posted January 7, 2005 Share Posted January 7, 2005 KT90 and 99 IMO are great tubes and should be a drop-in replacement for KT88 in most circumstances. For a real treat run them in triode if you can. Quote Link to comment Share on other sites More sharing options...
DRBILL Posted January 13, 2005 Author Share Posted January 13, 2005 Fellows, I have been out of town for over a week and upon returning found my desk about to collapse with ASAP nonsense. I think most of your questions have been answered by our colleagues. I'll re-read everything in the next day or two and see if any stones have been left unturned. I got my KT99's from Ned at Triode. From my field-notes I show 560v on the plates and -65v bias (which equals 1.56v across the 11.2ohm shunt). This is for a MARK III. RE: Isotap. Far less expensive than a variac and makes far less a fashion statement if you factor a wife into the equation. As an added bonus, you can choose an output that is isolated from the line voltage, or if that is not deemed necessary, you can choose an output from an autotransformer primary at much greater current. They show up on E-BAY with regularity. I gave $15 for mine, but the shipping was murder. Craig put me on to these. There's a good man! By fooling with it, I can get exactly 110vac with 128vac in. It doesn't do bad things to the shape of the sinewave in either mode. I'm on the edge of my chair waiting to see if any of you guys are going to be as enthusiastic about the KT99 as I. I was in Atlanta inspecting grandchildren. They passed. DR BILL Quote Link to comment Share on other sites More sharing options...
DRBILL Posted January 13, 2005 Author Share Posted January 13, 2005 Looking back over the postings for this subject, I would like to make a blanket correction. The KT90 is a replacement for the KT88. If you get old enough you'll know how this happens. I don't even know if these IS such a thing as a KT99! Sorry! I'll blame it on my fingers (couldn't be my mind)! DR BILL Quote Link to comment Share on other sites More sharing options...
cjgeraci Posted January 14, 2005 Share Posted January 14, 2005 My preliminary results are in - this tube is a winner, especially for the price. The company who rebuilt my Mark IIIs for their prior owner put 6550s in the output stage (Sovtek), so I have benefited from a double-stage upgrade (some have extolled the KT-88 over the 6550s in the output stage). Smooth highs and tight bass at all volumes - including pushing the decibel level. You know you have a winner on your hands after a quick roll when your sound also improves at all low volumes - slightly larger soundstage and creamier midrange. My Mark IIIs are now paired up with my RF-7s (and my Marantz 7T), and those speakers have never sounded better. Looks like I may leave that combo for the 7s, and run the Belles off of the Peach and Marantz 8B. Much of the harshness/distortion present with the Sovteks is now gone. Picked up mine from TubeDepot.com. Competitive prices, and they arrived quickly and safe/sound. Much more listening to do, but preliminary results are very favorable. Carl. P.S. Slight smell on initial start up as Dr. Bill mentioned, but that did not last long. Quote Link to comment Share on other sites More sharing options...
Coytee Posted January 17, 2005 Share Posted January 17, 2005 Kinda related, kinda off topic as a technical question... If someone wanted to do an A/B test, could you put the KT90 on one side and leave the 6550's (I think that's what I have) on the OTHER side and then after biasing, turn it on ??? would that get my name in the paper in a very negative way (obituaries) ?? Quote Link to comment Share on other sites More sharing options...
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