JJ KT-77's

[jaysg]

I know it's too soon for anyone to have samples, so I'm asking if anyone has experience with the originals? They're supposed to be the Holy Grail for the Dynaco Stereo 70. General expectations for JJ's ability to produce it accurately? Opinions on tone vs. EL-34's?

[fightingzero]

I can't wait to get my hands on one to try out in my HO. I've heard good things about them that they are more rounded and articulate. Think 6l6 meets EL34. At least that's the impression I'm under from the JJ site...

[jaysg]

I can't wait to get my hands on one to try out in my HO. I've heard good things about them that they are more rounded and articulate. Think 6l6 meets EL34. At least that's the impression I'm under from the JJ site...

One of the chief characteristics, is that they can not only take a high plate voltage, they can take a high screen voltage -- 800V each.

[Mr_dB]

Dunno about the JJs, but in a Stereo-70 I'd take Tesla EL34s over M-OV KT77s any day. Or real Mullard EL-34s over either.

[fightingzero]

I can't wait to get my hands on one to try out in my HO. I've heard good things about them that they are more rounded and articulate. Think 6l6 meets EL34. At least that's the impression I'm under from the JJ site...

One of the chief characteristics, is that they can not only take a high plate voltage, they can take a high screen voltage -- 800V each.

good call....i missed that

[markd]

I can't wait to get my hands on one to try out in my HO. I've heard good things about them that they are more rounded and articulate. Think 6l6 meets EL34. At least that's the impression I'm under from the JJ site...

One of the chief characteristics, is that they can not only take a high plate voltage, they can take a high screen voltage -- 800V each.

I have a question about this. I have never used the KT77, but the GEC spec sheet here:

http://www.mif.pg.gda.pl/homepages/fran ... k/KT77.pdf

says on page 3 that the KT77 was designed primarily for ultra-linear operation(like the Dynaco Stereo 70), and that if it is connected as a tetrode in class A/B that the screen voltage must not exceed 300v. I don't know if JJ is making them better than the original GEC's but for most EL34 guitar amps that won't work. Is this just another maximum rating that we ignore? It's not mentioned in the JJ specs here:

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

[markh]

...the KT77 was designed primarily for ultra-linear operation(like the Dynaco Stereo 70), and that if it is connected as a tetrode in class A/B that the screen voltage must not exceed 300v.

Yeah, I have that 1977 GEC book, and that's a weird limitation, because the design max for the anode and screen combined is 600v. In the UL connection at rest, the screen and anode are at essentially the same high voltage, so I don't understand why it would make a difference.

The JJ KT77s were cloned from a set of original GECs, so they must be very similar in terms of internal structure.

Also, some Hiwatt amps shipped with KT77s, and none of these had UL OTs (or 300 volt screen supplies!)

--mark h

[KT77]

there's a post over at Metroamp forum where a guy eval'd in his amp (I assumed it was a Superbass, but???)

http://www.metroamp.com/forum/viewtopic.php?t=4315

I have a Burman Pro-502 that is spec designed for KT77s (as all the Burman's were)...I'm running E34Ls in it at the moment for obvious reasons

Screen voltage is around 450-460V
Plates are at 475
Uses Partridge OT...NOT UL mode
I've been in touch w/ Burman thru email in the last few days to get more info on the OT spec
I'm waiting for tubegetter give me all his KT77's or I'll nuke Lost Vegas
anyway.....in the right circuit (Burman) I'm sure they'll roar
The Burman pre and output circuit is amazing even w/ the '34Ls

where can anyone get the JJ's for eval? I don't want to keep them...they'll die

[jaysg]

where can anyone get the JJ's for eval? I don't want to keep them...they'll die

I asked LV about them. He has 100 on order. I found a thread on another board where a couple guys had samples. Apparently, the bias is wrong, i.e., in the range of a 6550. Guess it should be more like an EL-34. They expect JJ to fix that in the next batch. It would be very nice if that 800V screen rating isn't a mistake.

[zaphod_phil]

UL is a funny thing. On the downswing, the screen voltage ends up higher than the anode voltage. I like to use current limiting resistors with UL, typically 470 ohms.

I don't see why there should be a problem with screen voltages, as long as screen dissipation/current is carefully kept under control. If high screen voltages can be handled in UL, we should then be fine with the same high voltages in regular tetrode mode.