ARC VT100 with KT-120 tubes ?

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.

I am considering purchasing a VT100MkII. Any preferences on/compairsons between the VT100MkII and the VT100NkIII?

Karma16: please back off Hifigeek. We should all be respectful to each other. Moreover, he has always provided good advice to those of us who are less informed about electronic theory, particularly ARC gear. Oh, my rig is all ARC: PH-7, Ref-3, VS-115 and CD-7. I can personally attest to the very helpfulp advice and information that Hifigeek has always provided.

BTW, the ARCDB website has some info on increased power output for various ARC amps that use the KT-120. See the News tab. I don't know, or frankly care for that matter, where ARCDB got their information.

If you do a broad "KT-120" search on the A'gon discussion forums, I think a number of ARC owners have posted comments about the superior sonic attributes of the KT-120 over the 6550. Is it true, I guess you'll have to spring for the new tube and try it yourself.

Will tube life be extended? We'll just have to wait and see. However, Hifigeek's comment about the KT-120's ability to handle plate current of 700V dc (versus actual bias being set at 65mV dc) is impressive. As a lay person, it would seem (?) that the new tube has a robust build quality that may possibly extend tube life over the 2000 maximum hours suggested for the 6550.

Thanks to everyone for their helpful advice.