Output tubes and their impact on output power

[ronenash]

This post follows an explanation that I PM'd Ray (aka rlw3). Ray suggested that I post it here as some other might find it useful. I finally found the time to do so. Hope you will find this useful and thank you Ray for suggesting this.

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Ron hi,
how much power difference do you think there is when subbing 120s for 6550s [in my cj LP275]? 10-20%? I read somewhere on aa the 6550 draws 35w and the 120 draws 60w might have referred to plate current which I don't know anything about. Ray

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Hi Ray,

The number you quoted are the maximum power a tube can dissipate without suffering damage. Most circuit designs will not get close to this number as it will result in rapid tube failure and severely shortened tube life. As the LP275 and all other CJ tube amps for that mater were designed for the 6550 tube they are biased at around 26-28w. Putting another tube in (KT120) will not change this. The bias point will determine how much of the amplifier's power is delivered in class A but it has no influence on the power of the amp.
Theoretically you can say "now that I have a tube that can dissipate more power I can bias the amp more into class A and as a result have the amp operate in class A up to a higher power level". This would be true if the amplifiers power supply and mechanical structure can support the higher current required and can dissipate the considerable additional heat. BUT, as the amplifier was designed with other operating point in mind, this will can clearly lead to amplifier failure.

To achieve more power you need a higher B+ voltage and a larger power supply. This is the voltage on the tube anodes.

As an example, my Beard P100 amplifier delivers 100w of power from 4 KT88 tubes. CJ amps with 4 output tubes (6500/KT88) will typically deliver 60-70w as CJ design their amps more conservatively. In order for the Beard to support such high power levels from two output tubes per channel it need a beefy power supply which results in a large and heavy amp. The Beard weighs as much as my two LP125 mono-blocks put together.

So you ask, "why does CJ not design their amps for higher output out of a given set of tubes?". The answer is tube life. Running the tubes close to their limits results in shortening of tube life. In the Beard the output tube will typically last for two years whereas in CJ amps tube can last 3-4 years without a problem.

Hope I managed to make some sense of all this. If you are interested in understanding how a tube amp operates and want to get some more information follow the following link. It does require a higher level of understanding in electronics.

Valve Amps

Cheers,
Ron

[GaryProtein]

I'm a solid state fan, but even so, knowing this about tube amplifier design is interesting to me.

[rlw3]

Ron, thank you again. When I sub kt120s for 6550s I always have to turn down the bias pots. Does this mean the tube is loafing in a cj circuit? Do the 120s burn out faster or slower than 6550s? Thanks again.

[ronenash]

Originally Posted by rlw3

Ron, thank you again. When I sub kt120s for 6550s I always have to turn down the bias pots. Does this mean the tube is loafing in a cj circuit? Do the 120s burn out faster or slower than 6550s? Thanks again.

Ray, all this means is that the bias point for the tube is a bit different. If you sub the KT120 between them selves (between channels or within one channel) you will also have to re-bias. The bias poi t for every tube is slightly different.

No worries.