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Old 20th August 2013, 12:28 AM   #1
GKF is offline GKF  Canada
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Default cary audio rocket 88 tube amp

hi guys
recent purchase of a cary rocket 88 amp. kt88 tubes now.
i'm seeing lots of online posts everywhere about swap to kt120 for a output power increase.
can anyone with tube history set me straight on this subject.
the only reason i ask is the kt88 in a cary rocket are said to produce about 40 watts. this doesn't seem like enough power.
thanks
gordie
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Old 20th August 2013, 01:46 AM   #2
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Cary apparently runs this amp conservatively and rates it 20 watts in triode mode and 40 watts in UL mode. Power out is a function of power in to the tubes as well as operating configuration. Everything is somewhat of a trade-off between distortion, power out & tube life Simply replacing the output tubes with a beefier type will not increase power out.
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Old 20th August 2013, 01:56 AM   #3
GKF is offline GKF  Canada
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Quote:
Originally Posted by HollowState View Post
Cary apparently runs this amp conservatively and rates it 20 watts in triode mode and 40 watts in UL mode. Power out is a function of power in to the tubes as well as operating configuration. Everything is somewhat of a trade-off between distortion, power out & tube life Simply replacing the output tubes with a beefier type will not increase power out.
so i take that you suggest to just leave it alone right? the kt120 will be a waist of money?

why are a lot of people claiming better sonics and 15% more power output from a swap from kt88 to kt120.
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Old 20th August 2013, 02:55 AM   #4
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Quote:
so i take that you suggest to just leave it alone right? the kt120 will be a waist of money?
Yes.

Quote:
why are a lot of people claiming better sonics and 15% more power output from a swap from kt88 to kt120.
1) They spent a lot of money on the new tubes so they must perform better.
2) They replaced poor KT88s that need replacement anyway.
3) Their friends told them they sounded better.
4) They look much "cooler".
5) They somehow believe that bigger/newer really is better.
6) They didn't make true electronic measurements with calibrated instrumentation.
7) They were stoned.

Anyway, all seriousness aside , I have never used KT120s so I am unfamiliar with their actual preformance. If their operating point is sufficiently different along with their gain characteristics then KT88s, I suppose one might hear a difference in "apparent" gain. But I am speaking from basic design theory that dictates there is no free output power increase without increasing input power. What someone thinks they hear, or see, is extremely subjective.

Please, if you know of a post that has credible measured improvment data of KT120s replacing KT88s with nothing else done, I would like to read it.
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Old 20th August 2013, 05:23 AM   #5
rmyauck is offline rmyauck  Canada
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Wouldn't you gain a bit of power if you could increase the bias (if it's fixed or adjustable) as you could increase the dissipation of the those tubes as they could take it due to their higher rating?
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Old 20th August 2013, 07:50 AM   #6
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Power out is not a function of BIAS in PP amps and care should be take about filament...the KT120 current is bigger.

If BIAS is increased the result could be less power as the power supply sags. If 40W isn't enough least 80W is needed...and thats another story.

Last edited by Tube mania; 20th August 2013 at 08:12 AM. Reason: typo :-(
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Old 20th August 2013, 11:58 AM   #7
GKF is offline GKF  Canada
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guys
cary state the rocket 88 has a power supply built into this for 200% duty cycle and stock kt88 bias is 45 to 50 ma and the kt120 are listed to need i think around 68ma.
i've seen on youtube a guy who tests the kt88 then the kt120 . kt 88 produced 38 watts @.9 thd
kt120 when biased right produced 60 watts @ .78 thd.
it is this vid that makes me want to try it.
but like you say if 40 watts is not enough thats another story.
do you mean leave this amp as it is and build something more powerfull?
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Old 20th August 2013, 12:50 PM   #8
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Yes...
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Old 20th August 2013, 01:20 PM   #9
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cary state the rocket 88 has a power supply built into this for 200% duty cycle and stock kt88 bias is 45 to 50 ma and the kt120 are listed to need i think around 68ma.
The high voltage will handle this current. The filament will not.
Do not use KT120's in a Rocket 88

Quote:
i've seen on youtube a guy who tests the kt88 then the kt120 . kt 88 produced 38 watts @.9 thd
kt120 when biased right produced 60 watts @ .78 thd.
Apples and oranges. Are we talking same plate voltage, same output transformers. Or are the test circuits optimized for the different tubes.
Not a valid comparison when subbing into the same amp.

You will not gain any more power and may destroy your amp in the process.

I am Cary's authorized service rep in Canada, and I've seen this before.
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Old 20th August 2013, 02:20 PM   #10
GKF is offline GKF  Canada
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Originally Posted by audiopro View Post
The high voltage will handle this current. The filament will not.
Do not use KT120's in a Rocket 88



Apples and oranges. Are we talking same plate voltage, same output transformers. Or are the test circuits optimized for the different tubes.
Not a valid comparison when subbing into the same amp.

You will not gain any more power and may destroy your amp in the process.

I am Cary's authorized service rep in Canada, and I've seen this before.
thank you so much for jumping in and clearing out this idea. as a cary rep it can't get any better then that. .
so maybe i should just get the best kt88 i can right?
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