DRBILL Posted September 5, 2006 Share Posted September 5, 2006 I have been a great proponent of the KT90 as a replacement in amplifiers that use KT88s or 6550s. I may have been the first person to comment on these in the Forum. It has been especially helpful in overcoming the higher line voltages when using equipment engineered for 117vac. The KT90 has its own sonic signature in the lower frequencies --an expansiveness not heard in the tubes it replaces, probably because of somewhat greater output.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /> Having said this, I experienced a sudden and destructive failure in a KT90 last evening. It was a classic case of run-away which resulted in a broken envelope. The glass separated from the bottom of the tube as neatly as if it had been scored with a diamond. These are EH and have the so-called "coin" base and were installed in a Mark III. Oddly, the failure of one tube had little effect on the sound at moderate volume. I immediately replaced both KT90s with a fresh set of KT88s to make sure that something screwy wasnt going on with the bias. All was well. It has been my experience that the Russian power tubes have had a tendency to show rosy plates even when properly biased. I have seen this mostly in Mark IVs when the line voltage was >120 even with proper bias. These same tubes, when carried to the bench, would test well within parameters. It had been my hope that these problems would have been solved by the KT90 in the Mark IIIs. This doesnt seem to be the case. This pair has less than two years service with daily use of about two hours. I would have expected heater failure or gradual deterioration of output, but certainly not self-destruction. One such event is too many. I would appreciate hearing from others who may have had troubles with the KT90. DRBILL Quote Link to comment Share on other sites More sharing options...
Coytee Posted September 5, 2006 Share Posted September 5, 2006 I don't have KT 90's anymore but I DID buy 4 of them off of your first publication of them. I had them in a Jolida amp. The difference was easily discernable and I admit I liked it. To me, they clearly seemed to beefen up the bottom end. I included the 90's as well as the 6550's when I sold the Jolida. All of that is a long way of asking this.. and I respect that you may or may not know or be willing to risk giving wrong info... I now have a McIntosh MC-2102 that uses eight KT-88's. Any idea if the KT90's would be a direct replacement in these also? The amp doesn't have any way for ME to change the bias (as best I know). I called the McIntosh factory maybe 2 years ago and asked their technical help guy. He didn't have a clue and wasn't willing to speculate. Upon hearing I was in Knoxville, he DID tell me I should put the question by Terry DeWick as Terry was Da-man! I DID put it past Terry and he's not seen a 2102 yet and said he didn't feel comfortable giving any answer prior to maybe seeing one. (or something like that) That was put to him BEFORE I bought the 2102. I now have one and your post reminded me of that scenario. Any thoughts from you if the 90's would work in a 2102? To answer your question about any troubles, I had no troubles with my 90's at all while I had them. Quote Link to comment Share on other sites More sharing options...
DRBILL Posted September 5, 2006 Author Share Posted September 5, 2006 All of the manufacturer's and vendor's specifications that I have seen have said that the KT90 is a drop-in replacement for KT88 and 6550. Eight KT90s would be a fierce strain on the budget! My conservative answer would be, "if it ain't broke, don't fix it." I wouldn't have used them if it hadn't been for the poor behavior of KT88s and 6550s. That is, with their rosy plates (and sometimes even the grids) when operated on line voltages significantly >117 as is often the case in "modern" times. The bonus was the improved performance in the bass. However, if it was time for the 2102 to be re-tubed, if it was mine, I'd use the KT90s, if for no other reason, the headroom. At least, that would have been my wisdom yesterday. The failure I witnessed could have been an isolated fluke. Unfortunately, it is the kind of fluke that can take out transformers. So, there you have it. I have come down squarely on both sides of the fence, and it cost you nothing. In theology, we called this the via media. DRBILL Quote Link to comment Share on other sites More sharing options...
Coytee Posted September 5, 2006 Share Posted September 5, 2006 So, there you have it. I have come down squarely on both sides of the fence, and it cost you nothing. In theology, we called this the via media. DRBILL Clearly I have just been schooled at the hand of the Master! I didn't know that a blown tube could also take out a transformer. I would expect that a repair cost for that would not be a whole lotta fun. Quote Link to comment Share on other sites More sharing options...
oscarsear Posted September 5, 2006 Share Posted September 5, 2006 I put four KT 90's in a little Cary integrated amp replacing the KT88 looking for greater bass. One went up like the 4th on the very 1st outing. Per Upscale Audio the KT90's are tempermental. Having experienced multiple tube failures (of many types) on the same device I had it checked by the manufacturer who claimed all was well and my tube problems were "unique". I changed tube suppliers and was happier. Ultimately I became tube spooked, sold the Cary with a grand compliment of tubes and went back to high end solid state. Using Accuphase now and am extremely pleased and unafraid to fire it up. Quote Link to comment Share on other sites More sharing options...
DRBILL Posted September 5, 2006 Author Share Posted September 5, 2006 Oscar, Oh, yes. Having just passed the 67th milestone I am all too aware of the bland, risk-free life. I would have wished better for you. DRBILL[] Quote Link to comment Share on other sites More sharing options...
3dzapper Posted September 5, 2006 Share Posted September 5, 2006 Dr Bill, Have you any thoughts on using 5U4GBs to lower the B+ a little in your Mark IIIs? Rick Quote Link to comment Share on other sites More sharing options...
DRBILL Posted September 5, 2006 Author Share Posted September 5, 2006 Rick, It has been my observation that the 5U4s can let things sag a little at high volume, especially pipe organ music with heavy bass. Instead, My whole system operates on three Iostaps which allows me to select a precise line voltage. An added feature is that it lifts my whole system from the power line. With complete isolation, the last measurable bit of hum and power line noise disappears. With Klipschorn efficiency, you know what that means! DRBILL Quote Link to comment Share on other sites More sharing options...
SilverSport Posted September 5, 2006 Share Posted September 5, 2006 ...using Ei Yugoslavian KT90s in VRDs (thanks robster []) and thanks DRBILL...they sound great... Bill Quote Link to comment Share on other sites More sharing options...
Coytee Posted September 5, 2006 Share Posted September 5, 2006 My whole system operates on three Iostaps which allows me to select a precise line voltage. An added feature is that it lifts my whole system from the power line. With complete isolation, the last measurable bit of hum and power line noise disappears. With Klipschorn efficiency, you know what that means! (in English...) Exactly what does that mean and how does one achieve it? I've got a basement room in open studs right now and it will be next on my list after the bathroom is done. I like the idea of no hum... Quote Link to comment Share on other sites More sharing options...
3dzapper Posted September 5, 2006 Share Posted September 5, 2006 Rick, It has been my observation that the 5U4s can let things sag a little at high volume, especially pipe organ music with heavy bass. Instead, My whole system operates on three Iostaps which allows me to select a precise line voltage. An added feature is that it lifts my whole system from the power line. With complete isolation, the last measurable bit of hum and power line noise disappears. With Klipschorn efficiency, you know what that means! DRBILL That makes sense. I guess I just like them because they are tough and cheap. Then too, I built a very stiff power supply that might mitigate the bass sag you refered to. I run the power amps directly from the wall through a capacitor bridged power strip and the pre through a voltage thingamajiggy. With no signal, I can tell the amps are on from a faint 120Hz buzz. I think it is radiation from the unshielded 2uF input cap to the power supply. When I get a "round tuit", I will try eliminating that cap and go choke input. Sorry just thinking out loud. Rick Quote Link to comment Share on other sites More sharing options...
mike stehr Posted September 6, 2006 Share Posted September 6, 2006 I have a Magnatek isolation transformer with a 230/120 volt primary and secondary that's been sitting for a couple of years now. Dr. Bill's comment about isolation transformers has me curious. I suppose I should find a box and a recepticle and put it together. I can find out if it does quiet things down a bit. I can't see how it would make much difference with a directly heated AC filament, try and see I suppose...... I have a Admiral 6BQ5 push-pull console amp that I rebuilt and put a different driver circuit in. The PS is still pretty much stock, with the exception of a higher resistance dropping resistor, a re-formed NOS cap can, and a 5Y3 instead of the 5U4. (for more voltage drop....) It's a CRC supply. On the Cornwalls, it's one of the quietest amps for some reason, on par with the Magnavox SE 6BQ5 amp (CRC PS), and a bit more quiet than the Magnavox 6BQ5 PP amp, which is a CLC supply. The Magnavox PP 6BQ5 amp seems to have a issue, and I'm thinkin' it's that cap can. It should be really quiet. Pondering by keyboard.......now that's constructive....... Quote Link to comment Share on other sites More sharing options...
DRBILL Posted September 6, 2006 Author Share Posted September 6, 2006 Coytee, Look in e-bay under Isotap. They are almost always available. It is a combination isolation transformer and voltage selector. It was popular back in the days of b&w tube TVs. The issues were "blooming," sync., picture not filling screen, and power line noise. Today they go for little or nothing, but beware shipping cost! They are HEAVY for their size. The Klipschorns are so efficient that they make hum and line noise audible --stuff you would never hear in regular "hi-fi" speakers. Our friend at NOS VALVES put me onto them. I use three on my rig because the isolated output is only 275va. each. WARNING: They are not pretty! If you are married and would like to stay that way, you'll want to hide these on the floor behind your rig. DRBILL Quote Link to comment Share on other sites More sharing options...
Coytee Posted September 6, 2006 Share Posted September 6, 2006 I saw one on ebay... It plugs into the wall and has 6 outlets on it. I see the voltage selector. I presume there's more to it than that. I guess the "direct" outlets are hard wired throughput to the house current and as such, might pass on any noise? I further guess the 'isolated' outlets are somehow connected via transformer and through that process, the noise in the household can't cross the same bridge. If you use a transformer, don't you lose something in the translation? So, do these pass the volts across at the expense of losing some amps or something like that? In other words, (ya gotta remember the non-technophyle you're dealing with)... ok, so in other words, on the direct side, you have 110 volts and say, 15 amps, where as on the indirect side, you have 110 volts and maybe 7 amps? (I just made that up to get point across) Regardless of any of that being accurate (since I haven't a clue if it is), why does it have different outputs, high, med, low?? What do those refer to? We need a Isotaps for Dummies book!! Would you use one for one amp, or can one handle three (tube) amps?? As for hiding them... will they fit in a bass bin? [6] Quote Link to comment Share on other sites More sharing options...
mark1101 Posted September 6, 2006 Share Posted September 6, 2006 Richard, Since you have the basement walls open I assume you are adding wiring. Why not just create a couple of dedicated circuits with their own breakers. When I built my house I specified specific cicuits like that all over the place. My pet peave is lights that dim and blink. Just trying to help. Quote Link to comment Share on other sites More sharing options...
Coytee Posted September 6, 2006 Share Posted September 6, 2006 Seems I'm hijacking this thread, I'll start another one for the Isotap. You all carry on your discussions about your self destructing tubes! [8-|] Quote Link to comment Share on other sites More sharing options...
DRBILL Posted September 6, 2006 Author Share Posted September 6, 2006 Coytee, Thanks for putting me out of my misery. DRBILL Quote Link to comment Share on other sites More sharing options...
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