ARC VT100 with KT-120 tubes ?

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.

HI hifigeek,
Well, you never addressed my questions. I don't know what to think. So, are we to assume that your tests were in some way flawed? That the KT120 was not resulting in more power but you won't admit it?

Since you made the claim of increased power but then won't back it up by defining your test methods, we should be very suspicious. Your good name alone won't validate the tests resultes for me.

If it were me, I would have tested the original 6550's to determine their freshness, run power tests with the 6550's using ARC's THD spec as the control, then, test the amp with the KT120's using the same ARC THD. If you wanted to be really precise the AC line voltage should be monitored. This is the only way to determine relative power outputs.

I think you know all these things. But knowing and doing are two different things. The folks who read this thread may not understand how power tests are run and, thus, have no way to evaluate your results.

Until you prove me wrong, I stand by Ohm's Law and state that simply installing KT120's will not increase an amps power output.

Sparky

Karma16..Have a nice day.