KT90

[Andy Le Blanc]

I have been using an amp built to handle 6550, KT88, and KT90. And, I must
say that the current production KT90's seem to be really nice. If the amp
they're in has the juice, they put out the watts and have a great tone.
They seem to maintain a consistent character either clean or when naturally
pushed into distortion. So far I've set the static plate dissipation to 35W
per tube, and will try the 42W max as per the 6550-A. Its nice to have the
flexibility of a tube that has a rated max rated dissipation of 50W.
Anybody have experiences good or bad with the current EH KT-90?

[Ears]

Do you run the screens at 300V or up with the plates?

[Andy Le Blanc]

With a effective load of approximately 3.5 K the plates are at 486 V DC.
The screens are at 471 V DC after a 2.2 K screen grid resistor.
And with a fixed bias the tubes have a zero signal plate dissipation of 32.5 W
with 67 ma. measured across a 1 ohm to ground at the cathode. Everything
is well within the maximum figures published for the tube type, the screens
are rated to 650 V, and the plates have a 50 W absolute, although I'll not
exceed the 42W that's published as the max for 6550-A, the power supply
has practical limits too. So far these KT90 are a good tube, to my surprise.
They do what newsensor advertises them to do.

[Ears]

Seems a high screen voltage for 6550.
That figure for effective load makes sense for the high screen voltage.
They recommend 4.5 or 5K for similar plate voltages but screens at 300.
I visited a site the other day that claims KT90 is not a true beam tube. I think the information is BullS but here is the page.
http://www.jacmusic.com/KT88/kt88.htm

[Andy Le Blanc]

The screen voltage is a bit high for a 6550, but its on par with other amps
for guitar. Many amps that use 6L6 look like they'll eat anything other than a
6L6-GC. I am still debating the load. The effective load is flexible, but where
its at sounds very nice. The amp is very usable when played out. Very big
tone with good color. The DC V will drop some as I increase static dissipation,
and at some point I'll try a different loading, which is why I built it.


That is a neat web site. I had read warnings of Chinese clone tubes, its
great to see pictures. I can't verify the development of "KT90",
but it is believable that it's modified from something else. We are the consumers, no
matter what we think, buy more, be happy.

[Ears]

Bump. Someone offered new datasheets in a post that has been lost in recent website crash.
Can they kindly repost, thanks.

[Andy Le Blanc]

Here's a piece.

[Andy Le Blanc]

Here's more, My computer is very patient with me.

[Andy Le Blanc]

More....

[Andy Le Blanc]

Thank you very much, L. Hunt, who graciously E-mailed these.

[Andy Le Blanc]

The amp Ive built with KT90 has become my favorite. BIG and clean in the
right way, dominating power when you want it based on pick attack, pickup
choice, and it has a very smooth natural distortion. I have it in a very simple
head and plan to up grade the power side with Hammond twin replacement.
I can only bias to 35w plate dissipation without taxing the generic twin PT.
The extra 100ma. that the Hammond PT is rated for will help. You need to
mind that the OPT you choose has a sufficient primary DC resistance to keep
these tube from drawing too much current from your PT, they will. The plate
dissipation of the EH product exceeds that of the GE 6550A.

[Jana]

Andy,

What sort of numbers for the primary DC resistance are you looking for?


Have you ever been inside a Sunn Model T? (one of the old ones). If I recall, it had a 47R resistor from the plate of each tube (quad of 6550's) going to the primary. Could this be Sunn's way of increasing the primary DCR?

http://www.schematicheaven.com/newamps/sunn_model_t.pdf

[Andy Le Blanc]

To make use of the parts I had on hand, an OPT with a primary impedance of
1.75k, I purposely miss matched my load to get 3.5k. This works. It is a great
sounding amp. I've seen resistors on the plate leads recommended in several
old publications, but this is a guitar amp. Its simplicity makes its tone. I've
a couple other OPT I'm opting to try. The issue is a result of the design
strategy. Its a wonderful amp as is. Other OPT that are off the shelf 100W
are a twin (2k) and a tone master (2.3K), the hope is to find the optimum for
6550, KT88, and KT90. The KT90 is far end of the range, especially the EH
current production, 54w published static dissipation for the tube. The GE 6550A
had a published of 42w, ordinary 6550 35w, KT88 40w. As is, 35w per tube
seems to be a limit with the KT90 and makes for good common platform for the
other types. Plenty of power. If it still displays a similar limitation with a more
optimal loading choice then I'll most certainly try to push the circuit by
adding resistance to the plate leads, I believe that approach is ment to
limit HF oscillation which may well be the symptom that I'm seeing.
But it sounds sooooo..... good as is.

[tonestack]

The KT90 is a modified EL509/6KG6. The EL509/6KG6 is a sweep tube that is commonly used as a final stage tube in transmitters.

[Andy Le Blanc]

There is quite a bit out there about EI and Manley Labs collaborating to develop
the tube. Its more of an evolved type based on 6kg6, a flyback tube.
The redesign appears to have been done by one engineer at EI.

www.diyaudio.com/forums/showthread/t-24313.htm

www.manleylabs.com

www.tubesrule.com