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KT120 back to 6550

Seen a ton of threads praising the KT120 over the 6550 or KT88. I get the new tube's durability and design but given that your amp was originally designed around one of the older tubes has anybody gone back.

I recently purchased and ARC Ref 110 that came with a set of KT120s. The output tubes test and match well so they don't seem to be at the end of their life (same for the signal tubes). 

This was a change from a VT100 equipped with 6550s. The VT100 had good midrange but seemed a little lean otherwise. Percussion was fantastic and voices just right. Just deep bass a little thin. The REF 110 definitely fills in the deep bass but seem a little congested in the midrange. Both top and bottom are an improvement.

Both these amps were sandwiched between an LS25Mk1 and a pair of JBL 4430s. Could be a synergy thing, these speakers work very well with high power solid state (Adcom GFA 5802). I've been trying to work up a good tube amp for these but so far I'm not feeling the love.

So the question is could it just be this system would work better with 6550s instead of KT120s?

Thanks
monoogan

Showing 4 responses by monoogan

monoogan OP
21 posts
 
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.
monoogan OP
21 posts
 
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 OP
21 posts
 
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.



monoogan OP
21 posts
 
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.
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