ARC VT100 with KT-120 tubes ?

HI,
Power of the KT-120's abilities relative to the 6550's are theoretical differences. The KT-120 is capable of handling more power but only if the power supply voltages are increased. When plugged in as a 6550 replacement and no other changes are made, the amp will put out exactly the same power. No tube will allow ohm's law to be revoked. I'm afraid that the issue of replacement is misunderstood.

Thus, power should not be the reason to replace the 6550's. There may be other reasons such as added lifetime because the KT-120 is working under it's maximum power output that is an advantage. But, not power.

I do not know what Hifigeek1 was hearing. Imagination?

Sparky

I wouldn't expect the tube change to make the amp put out more power. My interest is in longer tube life. And, I'm interested in knowing if there are sonic differences within the power limitations of the amplifier. In other words, will they last longer, and will they sound different (better) ?
thanks for your responses

Karma16. The KT-120 draws a bit more heater current per tube. There are possibly other variables with the KT-120 that I am unaware of... The Ref210's are mono blocks and are rated at 210w across 8 ohms with 6550's @ 120Vac line and that's what I measured. With KT-120's approx. 250w across 8 ohms with the same line voltage. I verified this on my service bench using a Dale 250W 8 ohm non-inductive wirewound resistor. Bias was the same as was plate and screen voltages. You can contact Audio Research and they will give you the before and after wattage outputs for 6550 vs. KT-120, just specify the amp in question. I fix their products in warranty.

HI geek,
OK, but can you explain how this increased power output happens? To get more power you need more power supply voltage and current no matter which tube is being discussed. We can't argue about this. First, did you use a good distortion analyzer to monitor the amps saturation state? Or, how did you monitor distortion? I use a Tektronics analyzer which goes down to .0001% THD and a Tektronix audio frequency sine wave generator which outputs at .00001 THD. Those are real specs. Great test equipment and ideal for this type of testing. They got a lot of use in my repair shop.

Typically, any power amp will use the power available from the supply to establish its limits. But, not always. The exception is if the power supply has extra head room and other circuit factors could then could determine when the amp goes into saturation where distortion increases rapidly. This is not the case with my D250. The power supply is the limiting factor as it is with most power amps including those made by ARC.

I did check with ARC about the KT-120's. They made the point strongy that the KT-120's will not, for example, increase the output power of my D250 Mk II unlesss the power supply voltages and current capacity are increased. I already knew this so it was confirmation. Of course, changing the power supply voltages and current is nearly impossible without a complete redesign. So, for most amps (all that I know of) the advantage of substituting the KT-120's for 6550's is possibly (yet to be proven) increased tube life, not increased power.

BTW, ARC will continue to use 6550's for power supply regulators. I'm sure you know this.

So, please explain. This is important because the 6550 owners of the world are going nuts over this new tube for, I think, the wrong reasons. There are no free Watt's in my world.

Thanks, Sparky

That is correct. You can not use a KT-120 for the regulator tube in ARC amplifiers. ARC is still in the process of determining power output of their amps with the 6550 vs. the KT-120. In some ARC products the KT-120 will not even fit. The KT-120 represents the latest design from the Russians. The tube is capable of running up to a maximum plate voltage of at least 700Vdc. The hope is that tube life will be extended.