KT120 back to 6550

Out of curiosity I swopped in a matched set of Genelex Gold Lion KT88s replacing the T-S KT120s.  Perhaps a hint more of sweetness, but definitely a loss of authority, that I liked so much to begin with.  Went back to the 120s soon after.

I believe that if an amp can work well with 6550's and can also accommodate KT120's...then the KT120's would be the superior tube in that amp. However, the big question is whether the amp can actually support KT120's without damage...as most that were designed for 6550's cannot. I went from an amp with 6550's to a mono block with KT150's...very big difference. 

Monoogan ;

I had the same experience when I tried KT 120's in my Prima Luna . 
Pretty good top and bottom frequency output but the all important midrange left me wanting . 

Good luck

Thanks every one. This is actually pretty helpful.

So for Saki and Twoleft it sounds like the tradeoff is maybe midrange definition versus extension at both ends? I think that's what I'm hearing.

As a side note towards the end of Ref 110 production Audio Research started shipping the amps with KT120s instead of the 6550s. As far as I know there was no other change to circuit or transformers. I assume they liked the way they sounded in the amp.

Of course they may have just been running out of good 6550s.

I recently compared Genalex KT-88 and Tung Sol KT-120 in my Emotive Audio Vita amps.  I have been using a set of KT-120s for the last 5 years but recently one of them was making some suspicious noises, so I swapped in my well-broken-in set of KT-88s while I waited for a new set of KT-120s to arrive.  Bottom line: in my amps the 88s lack the bass weight, power and authority of the 120s.  They don’t sound bad but the 120s sound a lot better. 

@salectric that was pretty much my experience too.  In a tube integrated, you can take care of the sweetness factor with the pre-amp tube(s).

I have been on fence with tube rolling in my ARC VT-80. With its versatility with different types of output tubes, including 6550, KT88, KT90, KT120 or KT150, I just can’t decide which one’s to try.

One thing worth pointing out, I am using VT-80 for my mid’s and high’s only in a bi-amp setup. The LF’s are being fed through ATI 543NC Class D amp. As is, everything sounds pretty amazing. I am using LS28 pre and speakers are B&W 800D2’s.

@saki70,
@twoleftears,

After reading your comments, I am now anxious to try Genelex Gold Lion KT-88’s over T-S KT150’s. What do you guys think?

http://audioresearch.com/en-us/products/power-amplifiers/vt80

That VT-80 is so user friendly (a really good direction for ARC) I could see buying both and rolling between them. It's only 4 tubes.

You're in a really good place. It sounds like you really don't need the bass extension of the KT120. You just need a tube with good midrange and top end performance.

The only downside I see is that in a 75 wpc amp the KT120s would last a lot longer. Again it's only 4 tubes.

@monoogan, 

Good point, it’s only 4 tubes :-) 

I think I should try the Gold Lion KT-88’s since I am already familiar with the T-S sound of KT-120’s. Granted, KT-150 has more power and might be slightly more extended overall. 

Thanks!

@saki70 : the 120s may seem to make the mids thin, but not if you have a preAmp that can compensate for that. 
E.g. I have the PrimaLuna Dialogue HP integrated tube amp, and swapped the stock 12AU7s for NOS Brimars and Mullard’s. The Mullards in particular lived up to their rep with midrange beauty with the KT88s.

In fact, it was too much midrange. So I think the Mullards with KT120s would then have better balance in all highs, mids, lows while retaining the glory of each. Just my experience. 

OP, I worked with Kevin and Jarrod at Upscale Audio on Matching tubes with my PRimaLuna HP Integrated (Kevin is part owner of PL) 

Through talking to them and reading the Upscale Audio website and watching Kevin’s short videos on many types of tubes , not only KT120s, KT150s, etc, but specifically tubes for ARC amps. 

Kevin has a video specifically for tubes for ARC owners. I don’t recall much of it because I don’t have arc gear, but he talked a lot about current, electricity matching with the electronics to not only get good sound but good tube longevity in ARC gear

upscaleaudio.com
-no affiliation-

I think PrimaLuna and Rogue have pushed Audio Research to come up with amps like the VT80. ARC power amps normally are seriously fussy about tubes which I'm sure was no end of frustration for Upscale. They've sold enough tubes over time to figure out what will work and what won't.

I'm sure the VT100 scares the crap out of them.

What Audio Research has been doing since like the mid 90s is direct coupling the driver tube (the last small tube) to the output tubes. So when you bias an ARC power amp you're biasing the driver tube which then biases the output tubes. If the driver tube goes weak or fails or is just way off from the tube you swapped out you lose bias and then it's all smoke and burned boards.

No tube fuses, we die like real men!

Meanwhile everybody else is going from EL34s to KT150s with a single credit card swipe, so you get the VT80 or a Rogue or PrimaLuna.

The plus side is the coupling capacitor moves back to between the last gain stage and the driver which I'm guessing helps with the sound they are after. But now there is more direct interplay between the driver tube and outputs which is why ARC power amps are so darn fussy about tubes.

If the driver tube goes weak or fails or is just way off from the tube you swapped out you lose bias and then it's all smoke and burned boards.

@monoogan This statement is false. If the driver tube dies, the power tubes are held in cutoff and are not damaged. If the driver tube is weak, you may have troubles getting enough bias current on the power tubes. So in this *particular* regard, the amp is fail safe.

lalitk, I would suggest checking with ARC first before rolling in the KT120's and particularly the KT150's. I would be surprised if they told you that your VT-80 was compatible with these tubes. Best to get ARC's blessing before you damage your amp.

According to ARC, both the VT-80 and VT-80SE can run KT150s. Both amps ship with KT120s now. And all four tubes are fused.

Atmasphere

Bad assumption on my part. I'm guessing if the driver fails the output grids get pulled down by the negative supply cutting off the tubes. instead of just losing bias.

Sorry for the misinformation.

Tommuch, I see that the VT80 can apparently run with KT150’s. What is odd is that the newer VT80SE includes the KT150’s, and so I’m not sure what is the difference between the 80 and the 80 SE??

@daveyf,

The VT80 was originally designed with the KT120, the predecessor to the KT150, but these two tubes are electrically interchangeable and the demand for delivering the VT80 with the KT150 could not be ignored. So ARC decided to introduce VT-80 with KT-150 tubes and dubbed the amp as SE version.

Lalitk, I believe the KT150 requires a higher draw, as such it can damage the transformer if that is not up to the task. Presumably, the transformer in the VT80 can handle the draw like in the VT80SE?