CJ LP70S

[franklin18]

Hi all. I have a CJ poweramp LP70S that came with 6550 output tubes. Can I replace them with KT88 tubes (the Genalex GL new issue)? The reason I ask is that I already have quite a few of these KT88 tubes but no new 6550 tubes. Thanks in advance. -Frank

[admin]

First, welcome to CJO. Great to have you with us.

Although similar, they are not exactly the same per specs. I think they should be ok but let's wait to hear what other people have to say. Even if they are compatible, it may change the sound properties so that is something to think about.

Here are the spec sheets for both:

https://tdsl.duncanamps.com/show.php?des=6550

https://tdsl.duncanamps.com/show.php?des=KT88

[AnotherJohnson]

KT88s and 6550s are supposed to be functionally interchangeable (full sets, not mixed sets) in CJ gear without internal mods.

[franklin18]

Thanks. Happy to find and join the forum to connect to so many other CJOs! Thanks also for the quick reply and the information. I also noticed some earlier posts saying for LP65S, KT88 can be used directly. Good news to me as I don't have to stock new 6550 tubes for now, and can have a taste of "tube rolling".

[franklin18]

KT88s and 6550s are supposed to be functionally interchangeable (full sets, not mixed sets) in CJ gear without internal mods.

Full sets for both channels? Would it damage the amp if KT88s for one channel and 6550s for another? Not permanently of course but just to compare them, for example.

[AnotherJohnson]

They sound different.

But it’s your amp and your ears.

If you can mix them and get stable bias, it’s probably safe.

I would not recommend it.

[AnotherJohnson]

But that doesn’t mean it can’t be done. And it also doesn’t mean that the mismatch will bother you.

[roberto]

Also, the 6550 is a great tube, and the KT-120 is a direct replacement. If you can afford both, I would try it too. I have done in several power amps, including ARC power amps, with very good results.

On the other hand, the tubes don't need to be matched pairs. The bias circuit in Conrad Johnson gear allows you to do that. Each tube has its own pot for adjusting the bias at the best working point.

I am in heaven with the KT-120s... they do have a lot of control in the bass and extended wide stage presentation.

Happy listening!

[Big Dog RJ]

Nice one on the LP70S Frank! Very nice amplifier indeed!

I used one with the ACT2 driving my previous Quad ESL 2905's, it was absolutely fantastic! Back then, I did try a few other tubes as well but eventually ventured back to the 6550C type, which are SED (Svetlana Electron Devices) made in Russia, excellent tubes for that particular amplifier.

Other Output tubes safely used: KT88, KT90-ei, 6550- (GE & Tung Sol) and KT120 Tung Sol.

*Note!*
Just make sure before installing new tubes, turn down the bias slightly, not all the way down but about half way, so that the bias circuit is asleep. After installing new tubes then bring the bias back up slowly to the required level, following the LEDs process... which you should be familiar with.

*using different tube types mixed onto the same amp/ channels- definitely NOT recommended! You're messing around with plate current and voltage settings! Just stick with one brand and type please *

BTW, you'll notice with the KT88's, a more tangible midrange, one that wraps around you, an absolute enveloping affect. Well balanced highs and quality bass but not as robust as the 6550C type bass or the all round elevated performance from KT120's. That would be a nice experience from 6550's but only you can determine whether or not you would like the KT120. They are a superior tube though and Tung Sol KT120's are outstanding!

At the end, you may just prefer the KT88's! Whichever Output tubes, as long as you're enjoying those fine tunes!

Cheers, RJ

[franklin18]

Thank you all for the advices and comments. They are very helpful indeed. Glad that I came to this forum. Cheers.

[Puma Cat]

KT88s and 6550s are supposed to be functionally interchangeable (full sets, not mixed sets) in CJ gear without internal mods.

I own an LP70S and I can unequivocally state that KT88s are NOT compatible with the LP70S. Do not use this tube with the LP70S. You can use KT-120s just fine. Not KT88s.

[Puma Cat]

But that doesn’t mean it can’t be done. And it also doesn’t mean that the mismatch will bother you.

No. Do NOT do this. KT88s are NOT compatible with the LP70S.

[Puma Cat]

Also, the 6550 is a great tube, and the KT-120 is a direct replacement. If you can afford both, I would try it too. I have done in several power amps, including ARC power amps, with very good results.

On the other hand, the tubes don't need to be matched pairs. The bias circuit in Conrad Johnson gear allows you to do that. Each tube has its own pot for adjusting the bias at the best working point.

I am in heaven with the KT-120s... they do have a lot of control in the bass and extended wide stage presentation.

Happy listening!

Yes, you can use KT-120s with the LP70S. I have safely and effectively for ~12 years. Not KT88s.

[Big Dog RJ]

Attn: Puma Cat

Why not KT88 in the LP70S? Was there something that went wrong by using KT88's...?

Please specify your findings as this is the first time I've ever come across any issues using the KT88 series.

There are other tubes that obviously can't be used in that circuit, such as EL34, 300B and so on... but KT88 should be fine. As far as I know, it's part of the same family of KT series tubes, didn't have any issues on our LP70S amps.

Would be interesting to know as to why not, hence we learn something new!
RJ

[AnotherJohnson]

Yes, I’m curious to know why PC would be adamantly opposed to this swap.

The bases and pin assignments, including heater voltage and current, are identical.

They are widely viewed as harmlessly interchangeable.

I couldn’t care less myself … the whole idea of tube rolling is contrary to everything I hold dear. But that just means that it’s not for me. There are plenty who want to hear these substitutions.

Has CJ definitively canned the KT88 as a 6550 compatible swap? If yes, is it because of some hidden danger? Or a life issue? Or does it just sound lousy.

Another post today claims that the LP70S is indistinguishable from the ART150 in the real world, and that power tube life is likely to be 8 or 9 years … so we all have “much to learn.”

[roberto]

Hola chicos,
Something about tubes. And same happens with transistors. You can replace a KT88 with a KT120. But it is not wise to do it backwards. You can replace a SS transistor with a specs with 10A, 75 Watts power dissipation, and 60V Collector voltage with a 15A 100 Watts power dissipation 100V Collector voltage but not backwards.

In other words, the KT120 is a bigger tube. I do like a lot the bass control and the clean midrange with the wide stage that is granted with it. On the other side, it is more sterile, not too tube warmth as the other tube like the 6CA7/EL34. This tube is not compatible for a direct exchange for the KT120, but it has a warmth, a richness in the sound quality that I truly miss in the KT120.

The KT88 has this kind of richness and warmth sound quality. If your amp is designed with it, you are allow to use both. But if your amp is designed to use the KT120, you must do an internal changes for the right and proper bias current for the KT88.

Happy listening!

[Puma Cat]

Yes, I’m curious to know why PC would be adamantly opposed to this swap.

The bases and pin assignments, including heater voltage and current, are identical.

They are widely viewed as harmlessly interchangeable.

I couldn’t care less myself … the whole idea of tube rolling is contrary to everything I hold dear. But that just means that it’s not for me. There are plenty who want to hear these substitutions.

Has CJ definitively canned the KT88 as a 6550 compatible swap? If yes, is it because of some hidden danger? Or a life issue? Or does it just sound lousy.

Another post today claims that the LP70S is indistinguishable from the ART150 in the real world, and that power tube life is likely to be 8 or 9 years … so we all have “much to learn.”

Because Jeff at Conrad-Johnson explicity told me NOT use KT-88s with the LP70S. He made it very clear that tube is not suitable for use with that amplifier.

[Puma Cat]

Attn: Puma Cat

Why not KT88 in the LP70S? Was there something that went wrong by using KT88's...?

Please specify your findings as this is the first time I've ever come across any issues using the KT88 series.

There are other tubes that obviously can't be used in that circuit, such as EL34, 300B and so on... but KT88 should be fine. As far as I know, it's part of the same family of KT series tubes, didn't have any issues on our LP70S amps.

Would be interesting to know as to why not, hence we learn something new!
RJ

Talk to Jeff at CJ about it.

[AnotherJohnson]

I wonder if it is an issue of harming something, or just the general idea that they’re not recommended. No unauthorized tube replacements are ever recommended by manufacturers, and I agree with this.

But boys will be boys. Tube rollers are never convinced.
Maybe the issue is that the KT88 won’t properly bias in this application. It’s a simple matter to try them and find out. Unless JF says they won’t bias, and then there’s no reason for anyone to try them.

In any event, if it were mine, I would not mix tube types, and I would choose a recommended tube.

I love the KT120s … the only downside to using them in the LP70S is the issue of the tube cages not being tall enough to accommodate them. JF once told me in passing that most customers leave the tube cages off anyways.

This tube substitute issue is not an issue that is likely to be one that CJ would deem worthy of much consideration. The recommended tubes are the ones they picked, and as we know “CJ knows of no other tubes that would lead to better performance.” In my opinion, this is a very reasonable position for CJ to take.

The LP series does not get as much love as it deserves. My LP275Ms were great amps once I got the hum issues sorted out. But JF told me that the ART series were even better … and I believe him.

[AnotherJohnson]

Here’s what Svetlana said about their KT88.

SVETLANA TECHNICAL DATA
KT88
High Performance Audio
Beam Power Tetrode
T he SvetlanaTM KT88 is a glass envelope beam power tetrode. It is intended for audio frequency power amplification service. High plate dissipation rating, close manufacturing specification tolerances and thorough processing pro-
vides improved reliability and superior sonic performance.
The Svetlana KT88 is manufactured in the Svetlana factory in St. Petersburg, Russia, and is designed to be a direct replacement for any KT88, KT90, KT99 or 6550.