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PostPosted: Sat Jul 29, 2017 2:57 am 
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Location: Calgary, AB, CA
Hello,
I need to replace a pair of power-supply regulator, 6550c, for ARC Ref 210.
Can you suggest what should I get? KT88, KT120 or the Winged C 6550 NOS? The power and driver tube had already changed from 6550C to KT120.
I don't have enough experience for tube rolling. Please help.
thanks
Vic


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PostPosted: Sat Jul 29, 2017 6:06 am 
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I can't imagine why you would change that tube to a different tube than the original 6550. I definitely wouldn't do so without speaking to ARC customer service first. Given that you have mono blocks, I would change the tube in question in both amps at the same time.

The power section of that amp originally came with 6550 tubes, but that is simply because the KT-120 wasn't available at the time. Audio Research has never used KT-88 tubes in any of their products even though they are electrically comparable with 6550.


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PostPosted: Sat Jul 29, 2017 6:51 am 
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Location: Repentigny, QC, CA
Yes you may check with ARC.
But in tube regulation stage, gain and circuit topology makes a regulation very important for
your driver stages.( noise, voltage stability, etc)

Characteristics of 6550 tube in this regulator circuit need to be as ARC designed it, I would not roll tubes there.
Small driver and output tubes yes.


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PostPosted: Sat Jul 29, 2017 9:22 am 
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as another has mentioned, changing the regulator tube to anything other than a 6550 as arc has designed could be catastrophic.
you may confirm with arc but I will bet my cup o coffee you cannot run the 120 in that position.

ime, tube rolling refers to changing like tubes for different brands... rolling a Svet 6550 to an SED 6550 to a Tung Sol 6550...

Going from a 6550 to a KT88 to a KT90 to a KT120 is a tube substitution not rolling and that is where one needs to know the electrical changes or to best have arc on speed dial.

now, you've already subbed the power tubes to kt120's. were the amps sent back for power supply mods? you need increased power supply voltage and current to properly run these. sure they may work but not to their potential.

imo, the 120, unless the amp in question is properly set up for it, has been overrated. Running the tube under spec may improve longevity but don't expect a big increase in power output and be aware these draw higher plate current. some amps may not be up to the task.

there are those who say the 120 is sonically superior but again ime when compared to a kt88 I noticed little difference and what I did notice I chalked up to used tube vs new.

bring on the wrath lol!!


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PostPosted: Sat Jul 29, 2017 10:59 am 
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BinkyTheCat wrote:
now, you've already subbed the power tubes to kt120's. were the amps sent back for power supply mods? you need increased power supply voltage and current to properly run these. sure they may work but not to their potential.

there are those who say the 120 is sonically superior but again ime when compared to a kt88 I noticed little difference and what I did notice I chalked up to used tube vs new.

I have the little brother to these amps, the ARC Ref 110. ARC switched from 6550's to KT-120's for this series of amps in 2010, and no circuit changes were required. ARC has since given the OK to use KT150's in my amp as well.

I have (strong testing) SED Winged C 6550's, Tung Sol KT-120's, and KT-150's; I found the 120's better than the 6550's, and the 150's are better again. I briefly tried Gold Lion KT-88's, but they didn't seem to offer anything over the 6550's in my system. Of course tube rolling can be a little subjective, and the results may be very different in other equipment.


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PostPosted: Sat Jul 29, 2017 12:02 pm 
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Location: Toronto, ON, CA
Yes, confirm with ARC before subsituting the regulator tubes with anything other than 6550. If you want something stable, reliable and longer(er) life get the NOS GE 6550.

But for power tubes go for the KT120.


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PostPosted: Sat Jul 29, 2017 12:13 pm 
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they may have given the ok to run the tube but that's referring to the increased heated current to my understanding. with that in mind yes, its a drop in replacement.

however, like running snow tires in the summer, you aren't experiencing their full potential.

in another thread questioning the kt120 in the ref 110 and 210, a Kalvin from ARC replied:


"Yes, I would recommend retubing with the new KT 120s instead of the 6550s. The KT 120 is more dynamic and plays better bass. However, this is not a direct replacement. The Ref 210s must be updated for correct operation. The update can be performed in the field by a qualified electronic technician or at the factory. The cost of the factory update is $250.00 plus shipping."

I assume a tweak in both bias and power circuits to meet the 120's requirements.

the answer I'm not aware of tho is if the mod is done can you go back to a 6550 or is the amp now running beyond that tubes parameters? a literal tube eater per se?


either way, waaay off topic.... no 120 in the regulator circuit.


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PostPosted: Sun Jul 30, 2017 10:41 am 
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BinkyTheCat wrote:
in another thread questioning the kt120 in the ref 110 and 210, a Kalvin from ARC replied:

"Yes, I would recommend retubing with the new KT 120s instead of the 6550s. The KT 120 is more dynamic and plays better bass. However, this is not a direct replacement. The Ref 210s must be updated for correct operation. The update can be performed in the field by a qualified electronic technician or at the factory. The cost of the factory update is $250.00 plus shipping."

I assume a tweak in both bias and power circuits to meet the 120's requirements.

the answer I'm not aware of tho is if the mod is done can you go back to a 6550 or is the amp now running beyond that tubes parameters? a literal tube eater per se?
either way, waaay off topic.... no 120 in the regulator circuit.

Thanks, I didn't realize there was anything to be changed to run KT-120's. My Ref 110 came to me with KT-120's, and it seems happy right now with 6550's. I have new KT-150's on order as they sound the best, my last ones drifted a bit and don't bias a precisely as I'd like.

Totally agree we are off topic with power tubes; the OP should stick with 6550's in the regulator circuit. Winged C SED brand are no longer available, but there's lots of other suitable choices from Sovtek, EH, or Tung Sol.


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PostPosted: Sun Jul 30, 2017 7:04 pm 
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I own Audio Research's VS 115. Purchased this amp used with KT 120's installed. Contacted Audio Research and was told in no uncertain terms to replace with 6550's. Before replacing with KT 120's or even KT 88's, I recommend to contact AR.


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