Does anyone care to ask an amplifier designer a technical question? My door is open.

"As I tell everyone. Light loading can be done on any tube amplifier. If you dont play above 80 dB go to the lowest tap. If you play 90 go one higher, if you play more than 90 you may have to use the high tap to get the volume you desire. This change should be far more apparent than a cable change. 7 ohms on the 8 ohm tap is no problem for the amp but lower taps always perform better technically though how it sounds to you is more important.

"4 ohms on the 4 ohm tap produces 100 watts. 8 ohms on the 4 ohm tap around 60 watts. As you go down in taps you spend more time in the class A region. The distortion and damping are much improved.

"Would be nice if you had a way to measure the peak voltage you require, then much better advice can be given. You can also get at that with a SPL meter at one meter at listening level. Then with speaker sensitivity we can compute it

"Everyone needs an SPL meter. For $50 its an execellent investment. People spend more than that on a poorly designed high end fuse. Please do not buy premium fuses, TuningFuses are the worst and the others I have not dissected but the people who sell them should be dissected."

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Just an anecdotal experience.  I own an ARC Ref 150SE.  It has 3 output taps: 4, 8 and 16.  I settled on using the 4 ohm tap for many of the reasons you mentioned above.  John Atkinson reviewed an earlier version of the a few years ago and made the following comments:

"As expected, the Ref150's output impedance varied according to the transformer tap selected. The 16 ohm tap measured 1.4 ohms at low and middle frequencies, rising to 1.9 ohms at the top of the audioband. The figures for the 8 ohm tap were 1 and 1.4 ohms; for the 4 ohm tap, they were 0.55 and 0.87 ohm. All three taps offer quite a low source impedance for a transformer-coupled design; as a result, the modulation of the amplifier's frequency response, due to the Ohm's Law action between that impedance and that of our standard simulated loudspeaker, was relatively mild." See Atkinson report here:

 https://www.stereophile.com/content/audio-research-reference-150-power-amplifier-measurements

So, in my experience, the Ref 150 SE, on balance, sounds best when light loaded off the 4 ohm tap. At times, I thought the 8 ohm taps sounded better, but after a while, I found the sound bright, probably because there is a 20++ ohm peak at the midrange cross over point.  And as you predicted, bass is tighter when played off the 4 ohm taps, although I think speaker impedance in the bass region bounces around quite a bit but has a 3.8 ohm saddle in the 50 to 200 Hz range.

As far as loudness is concerned, my speakers are rated at a 92db sensitivity and the amp has plenty of power to drive the music to uncomfortable loudness levels, albeit with little perceive distortion or signal scrambling.

So the bottom line is that there seems to be something to be said for light loading.

Bruce

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Hello Roger,

I'd like to come back to tube testers.  I'm not sure I understand correctly about your recommendation for a simpler device, to measure emissions.  Is this for the basic purpose of monitoring for tube failure?  Would I need a tester with more capacity in order to check prospectively for tube matching?

Thanks,

Mark

I would like to know the capacitance of the panel itself, the electrode spacing and the polarizing voltage.

Direct Drive is the best way to go. I have found many cases where half the power goes into the step up transformer and other EQ components. How does he EQ the speaker these days? I just started reading the
speaker description. Sounds like its an entire system.