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Old 29th March 2006, 02:15 PM   #1
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Default KT90's anyone using'em ?

I'm getting alot of trouble with internal flashovers using KT90 tubes in standard UL circuits in lieu of KT88's. . Anyone had sim experience ?

I'm running these tubes in parallel pairs UL-PP in a 35W guitar amp 265V B+ and constantly blowing B+ fuse. Swapping with KT88's is abs troublefree. Is there something about KT90's I should know about or missed ? It's often stated that this tube is a drop-in replacement for 88's and can take more B+. I daren't turn B+ up to 550V. I know the internal res of the KT90 is a touch lower, but I wonder if this tube is a born looser.

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Old 29th March 2006, 03:00 PM   #2
Tweeker is offline Tweeker  United States
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I've read that some runs of 'KT90s' have had qc issues, your particular tubes may be losers. Are you sure they are biased correctly etc etc.?

If a pair of KT88s work and you only need 35 watts, you might consider just sticking with them.

KT90s seem to be popular triode strapped, so I dont see why UL should be an issue.
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Old 29th March 2006, 04:48 PM   #3
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I have KT90s - older series 2, I think, running in PP UL at ~430V without problems - actually much nicer sound than KT88/6550's in this stereo amp. Couldn't be happier.
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Old 29th March 2006, 05:32 PM   #4
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Quote:
Originally posted by Tweeker


If a pair of KT88s work and you only need 35 watts, you might consider just sticking with them.

Yeah, I know but I have a tempting high power switch eg, B+ at 530V and any tube not up to par.......ZAP. I do like to run MI to 175W per ch.
Biassing fine....80mA each quite modest at 265V and 55mA with 88's on high B+. well within specs and no sign of oscillation.
Clearly KT90 Qc looks suspect (yet again in my case) ....considering Manley used them en-masse in their amps. I will have to get mine rechecked and the truth known.

I've always considered KT90's as a "hard tube" electrically, that when driven into clipping as in MI, perhaps there is a more drastic impedance change perhaps a "latch-up" when compared to other tube classes ....OMO. I've never had these problems with the 88 class. ANy o/p tube should survive a slam drive test at full power, i.e square wave into hard clipping them flipped into sine wave without snags. I use a fuse in centre tap o/p tranny and to blow a 1A with quad KT90s is a sure warning signal. The power supply has more poke than a hospital defibrilator.... so respect is needed.

The tubes I find most consistent on power uppage and well tolerant is the JJ KT88's (a pity about the bulb style) ;in comparison theSvetlana KT88 is overpriced and gives lower poke.


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Old 29th March 2006, 06:06 PM   #5
edl is offline edl  Hungary
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The later (during and after the NATO-bombing) manufactured Ei KT90's are made in poor quality.

Anyway I suggest to give JJ-KT77 a try. They giving 70W clean power in PP-AB, with low distortion and low power. Their price is good too.

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Old 29th March 2006, 07:46 PM   #6
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Quote:
Originally posted by edl
The later (during and after the NATO-bombing) manufactured Ei KT90's are made in poor quality.
BR,

Aah...... shattered my confidence....you would say that.....irritated by the 90' problems I put in my reference set of KT88 GL replica by Tesla and by golly they deliver exactly what I'm after. But theyr'e not staying in... Thanks for KT77 tip....will check up B+.

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Old 29th March 2006, 08:39 PM   #7
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally posted by richwalters



Aah...... shattered my confidence....you would say that.....irritated by the 90' problems I put in my reference set of KT88 GL replica by Tesla and by golly they deliver exactly what I'm after. But theyr'e not staying in... Thanks for KT77 tip....will check up B+.

richj

ask Jim McShane for KT90
I don't have exact URL,but little googlin' must be helpful

he was always source for reliable KT90

edit:

http://pages.prodigy.net/jimmcshane/
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Old 29th March 2006, 09:22 PM   #8
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I have some EI brand KT90's and have used them in my Quicksilver mono block amps with no problems.
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Old 30th March 2006, 01:42 PM   #9
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http://pages.prodigy.net/jimmcshane/

Interesting commentary and site but I detect not all is well with KT90 Qc for some time and remains hesitant.
Anyway I've shaken my vendor and they are willing to swap etc....

I've got a sneaking suspicion that some KT90's are also made in the F.East and rebranded.

Are there any recognisable etch code marks on a EI KT90, if so where would they be ?

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Old 30th March 2006, 01:53 PM   #10
SY is offline SY  United States
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One other possibility is that these really are rebranded sweep tubes, and the particular beam plate/screen really shouldn't be run over its (probable) 150-200V rating.
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