KT77 Specs

[fusionbear]

I just saw that JJ KT77's can handle 800V at both the plates at grids? Is this true? Any of you corroborate this?

Any other brand of current KT77's handle this?

Thanks

[gingertube]

It all depends upon if the JJ KT77 is a "faithful reproduction" of the original KT77.

If so then look carefully at this Genelex datasheet from 40 years ago.

http://frank.pocnet.net/sheets/086/k/KT77.pdf

It very specifically says that the tube was designed for Ultralinear Connection (screens connected to 43% of anode taps).

In Ultralinear - as the anode voltage dips due to increased signal current then so does the screen voltage, this helps keep screen dissipation under control.

On page 3? under Tetrode Connection there is a specific warning that screen voltage should not exceed 300V when tubes are connected in normal tetrode connection which is the case for 95+% of all guitar amps.

I have also heard stories (can not comment on accuracy) that they have serious failure rates in your standard Marshall etc.. That 300V screen voltage limit in tetrode mode would certainly explain those stories.

My only experience with them was to fit them into some Dynaco monoblock HiFi amps in place of the original EL34. Those were Ultralinear amps and the KT77 did sound better than the EL34s

Hope this is of some help to you.

Cheers,
Ian

[Firestorm]

No disrespect to any current tube manufacturer out there, but it is exceedingly rare to see published specs that are any different from those done by Genelex/Mullard/Philips et. al. Did they spec the tube or just repost the data? Just saying....

[Firestorm]

No disrespect to any current tube manufacturer out there, but it is exceedingly rare to see published specs that are any different from those done by Genelex/Mullard/Philips et. al. Did they spec the tube or just repost the data? Just saying....

[jaysg]

The original KT77 is the Holy Grail in medium level audio, and has the funny history of the EL-34 spawning the US 6CA7, which in turn spawned the KT77.

Chances are that the geometry of the JJ is approximately accurate, but the tubes don't compare that well in listening lessons. Beyond that, iirc, David Gilmour prefers real KT77s in his Hiwatts, over EL-34s. Makes sense as he uses them as clean power amps.

[leadfootdriver]

Yeah mang, I just put my JJ KT's back in the Express, and they are way tighter and uncompressed compared to an EL34.

[JazzGuitarGimp]

New Sensor Corp (Svetlana) also make a current-production KT77 under the Genalex Gold Lion brand name. Here is he data sheet:

http://svetlana.com/pdf/genalex/kt77-genalex.pdf

[leadfootdriver]

Cool. I think I remember JJ publishing a max watt of around 30-32 watts, but they lower it because people were blowing them up!

[Reeltarded]

I was interested in this tube until I read something about them being just the same as someone else's 34. I was looking for a similar thing to 6550 vs KT88 and never found anything better than the JJ 6ca7 or EL34L in a new tube. I think they are Ls, at least... memory fades after a minute or two these days.

[overtone]

Ian,
I have a few British '70s amps here that were designed around the old MO KT77. They run 450-460V on the screens in tetrode mode. I have been pondering that for a while now. The later versions have an additional dropping resistor shared before the G2 screen stoppers, so maybe they had trouble. Or it was in anticipation of the end of KT77 production and a subsequent concession to run EL34s in the future?

The old MO valves in mine are fine and still threatening, but it is ages since they saw any stage action. They sound fantastic let loose on stage but back at base camp I mostly noodle with the old east german EL34 in their place.
Have to be careful when swapping them out for EL34s, you can easily light up the screens with an E7 and a couple of Lagavulins.

Last year I quizzed the tech of a UK band about their experience of the JJ KT77s in comparison to the old golden lion 77s that they used back in the eighties. He said he was happy, got the Europe tour done on one set, but admitted he had lowered the voltages.

Best, tony

[eddie25]

I just saw that JJ KT77's can handle 800V at both the plates at grids? Is this true? Any of you corroborate this?

Any other brand of current KT77's handle this?

Thanks

I've been curious about this for a while now as well...

One day soon I will order a pair to stress test in my recent amp build that runs about 505V on the screens. Out of three pairs of tubes I tried, only JJ6CA7's work in that thing.

I can't say I believe this 800V rating, but it would be cool to have something that even works consistently with around 500V on screens, which I see a lot of on older EL34 amps.

[Reeltarded]

I like the JJ 6ca7. I like the sound of the 34Ls a little better at regular volumes, but the 6ca7s are better for irregular volumes haha..

Not the sound I expected, but I think they are something else anyhow. They will take a somewhat punishing bias, at least for a 34. They are pseudo 6l6s, aren't they? My experience mirrors your's.

[Matt D]

I went by Eurotubes in here in Portland this morning and asked the guys there about the ratings on the JJ KT77. They had a datasheet in a JJ booklet that I think might have been different from the one JJ posts online. If I remember correctly, the max voltage their sheet gave was 600v.

Also, they pointed out that though the data sheet indicates screen grid and plate dissipations of 6 and 25 Watts respectively, the combined dissipation cannot exceed 28 Watts.

Here is the KT77 sheet from the JJ website. The Eurotubes crew are a nice and helpful bunch.

http://www.jj-electronic.com/pdf/KT77.pdf

[eddie25]

I went by Eurotubes in here in Portland this morning and asked the guys there about the ratings on the JJ KT77. They had a datasheet in a JJ booklet that I think might have been different from the one JJ posts online. If I remember correctly, the max voltage their sheet gave was 600v.

Screens?

[Matt D]

600v on the screens, 800 on the plates.