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RE: Moving From 845 tubes back To KT120/KT150 based amps ???

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In Reply to: RE: Moving From 845 tubes back To KT120/KT150 based amps ??? posted by decooney on December 18, 2024 at 19:55:16

Got it. Thanks for the responses. This was helpful and comparing to other forums - No further questions from me on this.
Similar to what member @Mick Wolfe shared here - The other answers and supporting details I was looking for was found over on AK from amp designer Ralph at Atmasphere. Its my 93db/6ohm speakers as the culprit. While they do well with my PP Ultralinear amps at 100wpc or 50wpc Class A SS amp (at low or model level volume) the lower midrange and upper bass and continued dynamics i'm looking for is NOT going to be found with even the best 845 triode mono amps (with my speakers). Similar to Mike Wolfes comments - Atmashpere Ralph e covers many reasons for this being the case and agrees with my own concerns using 845s with my speakers. Also Luke at Manley said something similar to me a while bck about me trying this with amps lacking sufficient bandwidth for my speakers. Got it better now,
This helped, Thanks.

Edits: 12/19/24

Follow Ups:

I can add a bit to my comments - Ralph 14:09:45 12/20/24 (0)
Let's say you got the bass out of an 845 SET so you don't have to deal with the various problems that make SETs problematic with bass.

You still have to deal with the highs. Anyone that plays with SETs (and there are are fair number of DIYers on this site) knows that the more power you design the amp for, the harder it is to get full bandwidth. This applies to the highs as well. I've no doubt that your KT150 amps have more high end bandwidth than an 845 SET does.

SETs are usually run without feedback. So they cannot do anything about the phase shift that results from either a low frequency rolloff or one at the other end. Applying filter theory, if the slope is 6dB/octave you'll get phase shift up to 10x the low frequency cutoff, and down to 1/10th the frequency of the high end cutoff.

And this is why bandwidth has been important in amplifiers with no feedback or moderate feedback. If you have no feedback, to not have phase shift in the audio band you need 2Hz to 200KHz. An 845 SET is going to have troubles getting to 30KHz and might be challenged just to do 20KHz. I suspect your KT150 amps can do better.

If you have enough feedback (+30dB) then you can control the phase shift caused by a nearby-to-the-audio band cutoff. You usually can't put that sort of feedback in a tube amp because of the output transformer so bandwidth is important.

Phase shift over a band of frequencies is interpreted by the ear as a tonality- a rolloff can cause darkness in the highs, in the bass a lack of impact.

Now you might do well with a smaller PP amp that doesn't make as much power as the KT150 amp, simply because smaller transformer coupled tube amps usually have more bandwidth on account of the output transformer. The Harmon Kardon Citation 2 had full power well past 50KHz for example; a little 5 Watt tube amp we make has full power bandwidth past 100KHz.

But I suspect you'll want to keep the power you have with your speakers. Most of the time you won't use it but the idea is that the amp is mostly loafing. Its the mark of the best systems that they never sound 'loud' even when they are (+95dB); always sounding relaxed.

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