CJ LP70S

[Big Dog RJ]

Yep that's correct. Infact, Jeff, and prior to that, both Ed & LJ, and my trusted CJ tech have all confirmed the use of KT88's, as a direct replacement of 6550's, and when I did try them, there were absolutely no issues! The highs were nice and smooth, not as extended as the KT120's though, the bass was quite fast and tight but not as solid as the KT120's, that leaves the midrange. This is where the KT88's excel. Their midrange is more palpable, you can feel a lot more of that enveloping affect, and this is one of the main reasons JF uses the KT88 series in the top line ART series amplifiers that are designed for Class A operation.

Having said that, and looking at other brands, like ARC, Jadis, VAC, VTL and so on, they've all moved onto the KT120's and KT150's. With the only exception of Manley Labs, still using EL34's on their Neo Classic 250's and the KT90's on their top of the line Neo Classic 500's. I've tried all these tubes, and can confidently confirm that I do prefer the KT120 in my particular rig but that's a personal preference, nothing else.

The KT150's are now widely used in ARC tube gear, and CJ also offers the KT150's in their other ART amps, which are designed on Class AB, and I've heard then extensively and they are superb! (the ART150 & ART300's).

So, when Puma Cat said not to use KT88's in the LP70S, I would like to learn as to why? He hasn't specified why, just says not to... doesn't help much.
There's probably a reason why he says this and to why the KT88 didn't work on his LP70S... but what were the issues? This is something new and would be helpful to others. I just checked with my CJ tech yesterday, and he still says no issues there, as long as they're all the same tube type. Obviously don't mix & match due to gain & plate voltage variations... even minor variations can cause issues in the circuit over a period of time, plus it messes up the bias circuit, and when idle current can't be set properly, the Output tubes will not operate within their specified parameters.

Still waiting to hear from Puma Cat...

RJ

[AnotherJohnson]

PC’s response was to ask Jeff.

Roberto pointed out a possible bias issue which I noticed after I’d also speculated about bias being the possible fly in the ointment.

My WAG is that the KT88s sound lousy in the LP70S.

In any event, my sense is that this thread was started to avoid the expense of a complete retubing, and if complete retubing was necessary, then to do it at minimal cost.

PC is right in the sense that KT88s aren’t a CJ approved substitution… but when did that ever stop someone who wanted to try different tubes?

[Big Dog RJ]

Yes, that could well be the case, such that the KT88's didn't bring out the optimal performance of the LP70S.

Also in every CJ tube amp owner's manual, it's clearly stated that they know of no other tube brands that will enhance the performance / sound of their amp... and they highly recommend purchasing tube replacements from CJ Design!

Having said that, if you were actually looking at purchasing any of their newer range, such as the Classic series or ART series amps (the ART amps that Admin has), CJ will actually ask the owner which tube choice they prefer! You can order these amps with either KT88's, 6550's, KT120's or EL34's.

I remember when taking the Classic 120 and ARTsa for a short while, my CJ dealer offered a choice of Output tubes. I was quite surprised and asked them why the sudden choice... Their reply was this choice is coming from CJ HQ. So, it seems that JF is offering choices now.

May be that's better coming from CJ HQ itself, rather than owners trying to tube roll and usually end up overdoing it!
I remember having the ARTsa fitted with EL34's and it was fantastic!

No news from Puma Cat though... so we're still guessing.
Best, RJ

[SolderSlinger]

As an FYI CJ amps that come from the factory with 6550, KT88, and KT120 output tubes have the bias led reference voltage "ledrefv" set to 1.2V which corresponds to 60ma of plate current.

Amps that come from the factory with EL34 have the "ldrefv" set to 1.0V which corresponds to 50ma of plate current.

Rolling between the EL34 and 6550 derivatives, requires either measuring the voltage drop across the cathode resistors to set correct current, or adjusting "ledrefv" to the correct current. If not then if EL34 tubes are installed in a 6550 amp, and the LED bias indicator are used to adjust current, the plate current will be 60ma instead of 50ma. Vice versa if installing 6550 tubes in an EL34 amp; LED bias indicators light at 50ma.

Adjusting the "ledrefv" is quick. Much quicker than measuring the cathode resistor voltage.

[smodtactical]

Hey Solder slinger. So just to confirm changing the LED reference will allow me to use el34s with the lp70S ?

Any tips on getting the lp70S open? I removed the side plate screws the top screws on the circular area and it wouldnt budge. I took the bottom off fine though but top just won't come off.

Thanks.

[roberto]

The 6550s pin array is different to the EL34s. They are NOT direct interchangeable. The difference in not only the plate bias current. Is the pin array too.

Look for the pin out for each tube. You will see the difference. the third grid on the 6550 is connected to the cathode, pin no. 8 and the third grid of the EL34 is connected to the pin No. 1. They are not similar.

[smodtactical]

Ya thats what I thought but my friend said its interchangeable. I think rather than struggling this hard i should just get an MV60 or classic 62

[smodtactical]

Its annoying the classic 60SE has kt120. Id love the SE with teflons + vishays with the el34s.

[admin]

Perhaps this is just personal preference, but I love the sound of KT120's. I run them in the my ART monoblocks and couldn't be happier. Maybe it's just me but I don't view them as a deficiency.

[smodtactical]

Perhaps this is just personal preference, but I love the sound of KT120's. I run them in the my ART monoblocks and couldn't be happier. Maybe it's just me but I don't view them as a deficiency.

I actually totally agree with you. I prefer the kt120 over the 6550. But first point is I want less power since im using headphones so i drop the noise floor. And also I loved el34 with my headphones on the prima luna evo 300. So I want to try to get that kinda sound.

[roberto]

Tube liking...that's a matter of taste, like who likes garlic and others hate garlic.

I am with Admin. The KT-120 is the 6550 newer design. I has more power dissipation, and has an incredible bass control. The bass is robust and tight! The definition is truly live like. Also, the midrange is outstanding. I like a lot what my Classic 120SE. It comes with 8 KT120s, and believe me when I say, it is powerful. I think the CJ rated power is very, very conservative.

I know that CJ offers better power amps than my 120SE, but I am truly super happy with the quality sound that comes from it. It is dynamic, tons of dynamics, crystal clear, and really offers what my ears want. In my system, I never felt that it was weak in any section. I really don't want to listen the CJ top models, because I probably will became hooked as I am with this one. The power supply of this MONSTER is stable and has a lot of juice. I never felt that in a very difficult passage of the music, it was not supplying the energy needed. The Teflon Caps and the Vishay metal foil resistors are a wonder to use and listen to. The signal to noise ratio is like a good SS, there is no hiss or hum, or any unwanted noise in my system.

The ET-7V2 matches great with it, offering the best sound system that I ever own, and believe me, I have been over 50 years in hi-end audio. Also, I service here in Costa Rica, the most hi end brands, so I know what is inside of many well known brands. I know their electronics design and the kind of electronic components that they use. So, right now, being older enough I can be very demanding and very picky. And with CJ, I am very happy. I know that there are other good brands, all I can say is: Conrad Johnson sounds right!

Happy listening!

[smodtactical]

Ya CJ lp70s beat many amps. R800i, pass x350.8, don sachs kt88, prima luna evo 300 + 400, an old sim audio amp.

Granted I havent tried any ultra pricey amps but I bet even then CJ will put up a fight. I think the parts quality is very high.

[admin]

Perhaps this is just personal preference, but I love the sound of KT120's. I run them in the my ART monoblocks and couldn't be happier. Maybe it's just me but I don't view them as a deficiency.

I actually totally agree with you. I prefer the kt120 over the 6550. But first point is I want less power since im using headphones so i drop the noise floor. And also I loved el34 with my headphones on the prima luna evo 300. So I want to try to get that kinda sound.

Perhaps I missed it in an earlier conversation, but how do you wire your headphones directly into the amp? Even 2 watts of power in a typical headphone would blow out your ear drums.

[smodtactical]

Speaker tap cable, its spades to xlr female. Then I have a pre going into the lp70S. Not a lot of play with the pre though.