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.
richj
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.
richj
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.
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.
Tweeker said:
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.
richj
richwalters said:
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/
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 ?
richj
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 ?
richj
SY said:
More injury to an opened wound...Implying..What tube have we got here with such low ratings ? With such a phantom sized anode, one of the biggest in it's class implying with it's mediocre performance it's hardly worth assembling the blighter in the first place. Coke B'le 807's would perform better and per se more reliably.
My vendor wouldn't risk playing foul with me, I did mention on my previous lot < no foul up's on these and check'm simmering at 500V> (that was 88's).... however there are vendors who would just sell a tube for a cheap buck and say it works and one gets a short lived ticket.. (We get fed up returning phony tubes in the post..)
I'm going out to shovel some snow......blue glowing anode (sky) overhead.
I know there are EH KT90 types around. The question becomes one OF which batch...? 7/10 bad... rest av..
It seems I might have to pay up/ settle for JJ 88's and be content.
richj
Put 150 volts on screen and plate. Do a sweep of gid voltage from, say, -50V on down, monitoring plate current to see if you've got a high perveance tube on your hands.
I understand rich's frustrations 100%, but it may well be a horses-for-courses situation where the tube could prove to be quite useful, just in a different circuit.
I understand rich's frustrations 100%, but it may well be a horses-for-courses situation where the tube could prove to be quite useful, just in a different circuit.
High perveance stuff.......Yes SY... quite right... It entered my mind...umm I honestly wouldn't recon my vendor would risk mucking me about with reflog line output stuff ...he knows I'm up in glass theory & MI & Hi Fi...unless his tester was pizzed drunk 
from weekend hangover.
Hitting the theory..The thought of grid blocking (bouncing of primary and sec electron emission from g1 can oppose grid current flow) can cause thyristor type latch up on certain tubes. I know it's mentioned somewhere in Langford Smith's book.
Ok....If the Jester sold me high perveance rebranded clown KT90's...line output apps wouldn't showup the problem I'm up against. That I'm pretty certain.
but as the grid leak in my amp circuits is 68K and driven from a slam low Z source, the remainder of the amp works pristine with KT88's right up to 40KHz....... the Ei KT90 data mentions direct substitute for KT88 and so on.....so what have
we got here ?
The conditions (quite normal).. 530V B+ UL43% (g2 knat's xck lower) via 220R carbon and quies set at 60mA. This implies g1 at approx -68V.... obviously peak signal swing around that figure.
However, as SY points out, checking at lower electrode voltages, there is the crossover characteristic with neg load lines. I've got a seeking suspicion we may have discovered the problem.
Uncovered a massive crummy manufacturing scandal ?
Any more revelations from the pit ?
richj
from weekend hangover.Hitting the theory..The thought of grid blocking (bouncing of primary and sec electron emission from g1 can oppose grid current flow) can cause thyristor type latch up on certain tubes. I know it's mentioned somewhere in Langford Smith's book.
Ok....If the Jester sold me high perveance rebranded clown KT90's...line output apps wouldn't showup the problem I'm up against. That I'm pretty certain.but as the grid leak in my amp circuits is 68K and driven from a slam low Z source, the remainder of the amp works pristine with KT88's right up to 40KHz....... the Ei KT90 data mentions direct substitute for KT88 and so on.....so what have
we got here ?
The conditions (quite normal).. 530V B+ UL43% (g2 knat's xck lower) via 220R carbon and quies set at 60mA. This implies g1 at approx -68V.... obviously peak signal swing around that figure.
However, as SY points out, checking at lower electrode voltages, there is the crossover characteristic with neg load lines. I've got a seeking suspicion we may have discovered the problem.
Uncovered a massive crummy manufacturing scandal ?
Any more revelations from the pit ?
richj
EI KT90 quality control....
...or lack thereof!
I bought 3 "match quads" of these from a vendor who recommended them over EH he also sold.
When I tested them they looked like they had just been selected at random!
Worse....using all 12 tubes I could not make one reasonably close quad...the best I could manage was two sort of OK pair!
Not counting 2 were duds right out of the box.
Of the working ten the weakest passed 78mA in my tester and the strongest just over 150mA....you could hardly believe they were even the same tube.
After 100 hours of use those two OK pair had drifted apart so far I had to go back to the others to make up a reasonable match again!
I've just had a similar experience with 2 so called matched quads of Chinese KT88s.
I've finally come to the conclusion that NOS tubes aren't overpriced after all because every modern power tube I have ever tried has had poor consistency.
John Corneille
...or lack thereof!
I bought 3 "match quads" of these from a vendor who recommended them over EH he also sold.
When I tested them they looked like they had just been selected at random!
Worse....using all 12 tubes I could not make one reasonably close quad...the best I could manage was two sort of OK pair!
Not counting 2 were duds right out of the box.
Of the working ten the weakest passed 78mA in my tester and the strongest just over 150mA....you could hardly believe they were even the same tube.
After 100 hours of use those two OK pair had drifted apart so far I had to go back to the others to make up a reasonable match again!
I've just had a similar experience with 2 so called matched quads of Chinese KT88s.
I've finally come to the conclusion that NOS tubes aren't overpriced after all because every modern power tube I have ever tried has had poor consistency.
John Corneille
oh yeah....I've also returned several KT90 octet sets to my vendor....there seems alot of duff ones going around.....reselling duff ones? I wonder what's going on behind our backs....
I know the pmcomponents group UK were thinking of a re-interest in KT90 production i.e drastically sorting the tube and it's problems out. I haven't communicated with the company for some time (last year) so I don't know what the up-to-date is.
I'm quite sure there would be a good market for KT90's if somebody could get them to work right. It remains a mystery as the tooling was Philips which was very reliable. It can't have all been bombed by NATO ?
richj.
I know the pmcomponents group UK were thinking of a re-interest in KT90 production i.e drastically sorting the tube and it's problems out. I haven't communicated with the company for some time (last year) so I don't know what the up-to-date is.
I'm quite sure there would be a good market for KT90's if somebody could get them to work right. It remains a mystery as the tooling was Philips which was very reliable. It can't have all been bombed by NATO ?
richj.
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