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Tungsol kt-120 and Class A amps

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Posted on December 20, 2012 at 16:56:36
starchild
Audiophile

Posts: 161
Joined: August 21, 2001
 I've read a couple of posts on other websites that indicated a kt-120 used in a Class A amp (my amp is a class A amp) may generate more power than a kt-88 in the same amp. I'm not technically savy enough to evaluate this. What are your thoughts?

BTW, I recently obtained an octet of the kt-120. I find them to be impressive even though I only have about 5-6 hours on the tubes.

Thanks,
Mike

 

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RE: Tungsol kt-120 and Class A amps, posted on December 21, 2012 at 09:40:54
Mechans
Audiophile

Posts: 1804
Location: East Coast
Joined: May 23, 2004
 That is indeed what is claimed. The KT-120 has higher plate dissipation =Watts than a KT-88. I don't know if your amp will automatically make more power without adjusting the bias current in the least or whether a KT-88 tranny will provide the needed current etc.

 

RE: Tungsol kt-120 and Class A amps, posted on December 21, 2012 at 11:47:28
xti16
Audiophile

Posts: 82
Location: Streamwood Il
Joined: February 25, 2012
 I was under the impression to get all the power benefits from the KT120's you would need higher plate and grid voltages. It's OK - correct me if I'm wrong :)

 

RE: Tungsol kt-120 and Class A amps, posted on December 21, 2012 at 17:22:17
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 With all else equal, unless you increase the voltage swing on the OPT primary (which will also require additional current) the voltage swing on the secondary won't change. And that means no more power. Since power = voltage squared divided by resistance, no more secondary voltage means no more power.

Let me give an example... the Citation II has a power rating of 60 watts per channel into an 8 Ohm load. So using Ohm's Law, we know that Power (watts) = the voltage squared divided by the resistance. So

60 watts = voltage squared divided by 8.

Solving for the voltage gives us 21.9 volts. So to produce 60 watts into 8 Ohms the OPT secondary has to produce 21.9 volts.

The Cit II OPT's primary impedance (resistance) is 3200 Ohms. So with 8 Ohms on the secondary and 3200 on the primary, the turns ratio (I'll skip this math for now) is 20 to 1, primary to secondary. Now that I know the turns ratio I can calculate the voltage swing required on the primary by multiplying 21.9 by 20 (the turns ratio). This gives me a required primary swing of 438 volts.

So to get more than 21.9 volts across the 8 Ohm load would require a voltage swing of more than 438 volts. And Ohm's Law says that 21.9 volts across an 8 Ohm load requires 2.74 amps of current in the secondary.

So even if the KT-120 is capable of more current than the KT-88 Ohm's Law says unless the voltage swing is more than 21.9 volts on the secondary all it takes is 2.74 amps. It's sort of like a car - no matter how much horsepower I have under the hood, the speed limit is still 45 MPH!

Also, keep in mind it take TWICE the amplifier power to raise the volume by 3db. It takes a 25% increase to get even a 1db increase, too small for many people to even hear. So even if there was a small power increase most people wouldn't even know it.

This is a very simple explanation of what can be quite a complex subject. If others want to jump in feel free!

 

Not sure how Iron fits into the math, posted on December 21, 2012 at 17:36:43
FenderLover
 But, different amps with same topology of output tubes sure do sound different with various sized output trannies. It's partially what gives a blackface Bandmaster or Bassman the solid bass versus a Tremolux or Vibrolux.

Then, there's the speaker efficiency...

 

RE: Not sure how Iron fits into the math, posted on December 21, 2012 at 18:55:37
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 There are a lot of variables besides power - those variables account for much/most of the tonal differences.

 

RE: Tungsol kt-120 and Class A amps, posted on December 21, 2012 at 21:08:48
wheezer
Audiophile

Posts: 4309
Joined: January 24, 2001
 The KT120's increased heater current needs to be considered as well.

 

RE: Tungsol kt-120 and Class A amps, posted on December 22, 2012 at 12:11:45
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 You are correct..The kt120 has more plate dissipation and its like putting a larger displacement engine in a car.You can do that but to realize its potential,you have to increase fuel pressure as well as letting more air in thru the intake..For the tube to realize its power potential,you have to increase the plate voltage,which is much like adding a bigger fuel supply to an engine.The drive voltage at the grid, also has to increase which is basically the same as putting more air into a bigger engine.
If you starve a bigger engine for fuel and air,its the same as starving a tube for voltage and signal.It will work,but not as its potential.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

Thanks for the responses..., posted on December 22, 2012 at 16:26:34
starchild
Audiophile

Posts: 161
Joined: August 21, 2001
 They were very informative.

 

RE: Tungsol kt-120 and Class A amps, posted on December 22, 2012 at 22:37:27
danlaudionut
Audiophile

Posts: 5480
Location: Schenectady
Joined: June 6, 2002

Mike

This one has me intrigued.
Looks great for my woofer.

DanL



 

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