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

All OTL amps like high impedance because they have lots of voltage but limited current. Since current is the limit use the forumla

Power = current squared x impedance. The amplifier max current is the same for both speakers but 16 ohms gives you twice the power of 8.

@ramtubes
Roger, I think if you revisit the above comments you will find them to be incorrect. An OTL has to be able to drive real world loudspeakers and so can produce the same currents at the output as any other amplifier. FWIW, the output tubes in most OTLs can easily blow a 10 amp fuse in certain situations without damage to the tubes.

Your use of the formula is not accurate. As an example (I think you have clio9's M-60s on hand) look at the output power at clipping into 4, 8 and 16 ohms. Now this is a smaller OTL, but I think you can see that its output power does not behave as you stated above. The half power of 16 ohms occurs at 4 ohms, not 8.

Two versions of the Futterman circuit. East coast/West coast. Harvey was a hoot. I visited him and his gang around this time. Sadly or not, he folded too. Is there a Futterman curse?

There most certainly is! Every manufacturer that has made a Futterman amplifier has had to go out of business. This is because there is more to the circuit than meets the eye, and no-one was able to do the execution such that a reliable amplifier resulted. The reliability problem is what drove them out. Of course, there are notable exceptions- the Fourier company simply under-rated parts and used sloppy construction, to the point where it would not have mattered what they made- they would have failed and gone out of business anyway. Harvey used surplus and unreliable capacitors in his amps (unreliable because they were not used in the right application).

Since I dont believe wire has a sound I prefer colors. I use all 9 colors in my amplifiers. With colors you can actually start to see the circuit without a schematic.

If you look carefully you will find some long wires repeated so you could simplify things there.

Our  wire is custom-built. Because of that we have to buy a lot of wire at any one time. The long wires repeated is so that all the power tubes have exactly the same series resistance involved (although the cathode resistors dominate that aspect) and transmission line effects are minimized (the output section has bandwidth to several MHz so stability is important). Once you understand how the dielectric behaves you find no need for Teflon.

If my amps are being run at levels that do not bring on distortion, why do they still have that classic "tube-amp" character even at those low listening levels? If it’s not the clipping characteristics that are coming in to play...what is it that produces that classic tube sound as I described it?

@prof
The fact of the matter is that the amps make audible distortion, which is the coloration you hear. Below a certain low power level they can often be making more distortion than at higher levels!

I have a 3 disc LP set of Theodorakis’s “Canto General” that Atmosphere produced, and I found the engineering to be disappointing. It has been muddy, and there has been no bass to speak of.

@unreceivedogma
There is plenty of bass on that recording! We had the biggest bass drum in the state at the time. But it is very deep, and some systems don't play it very well. Since I recorded it, I can use it as a reference and I can tell you that many tone arms don't play that bass right either.

RM

first thanks for the input on Ralphs amp, speakers likely the dead ESL-63 I mentioned in another post. I am away from them but I am assuming with ago all the panels are toast, the limited fault tree look I have done so far indicates EHT power supply issues. I am away from them so do not know on panel color, will check that when home next. Thanks all for inputs on rebuilders.

I am away from my toolbox also but I will get DMM peak voltages measured today, SPL at 1 M

fun, real data will set you free was our mantra at work for 30 years...

Hears what JA had to say in Stereophile.. The Audio Research Reference 75 measures well for a classic tube amplifier design with a single pair of output tubes for each channel and a modest degree of loop negative feedback. Its output transformers are also of good quality, the only proviso being that the amplifier should not be used with loudspeakers whose impedance drops significantly below the nominal value of the output transformer tap
Read more at https://www.stereophile.com/content/audio-research-reference-75-power-amplifier-measurements#jbpCd1H...

I lack 99% of your engineering expertise, but I don't read the measurements as being as bad as you seemingly do. It seems to me that JA has a tendency to disagree with his own reviewer's perceptions as to which output tap is best rather than criticizing the design of this amp and it's output tap measurements per se. In some ways, this entire discussion is premised upon design expectations; should the amp designer account for every possibility or to some degree, is the end-user responsible for matching the amp to a suitable loudspeaker? It seems to me that you adopt the former view, which is somewhat quizzical given your avowed distaste for dynamic coned speakers and your view that ESLs/Planars are the only form of speaker that makes musical sense. For better or worse, at Stereophile high-dollar tubed amps seem to be always matched up with various iterations of Wilsons, none of which present graceful impedance curves. I find it curious in general, Roger, that you defend measurements of tube amps as meritorious in a vacuum (pun intended). You seem to  imply that the measurements may not tell the complete story, but that just the same any mediocrity in the measurements that are utilized is sure to correlate with deficiencies in sound. In other words, I interpret your posts-not just in this thread but in others too-to be consistent with a person who adopts only the first half of the golden oldie phrase, "not all things that count can be measured and not all things that can be measured count". 
This may dismay you quite a bit from an engineering standpoint, but I take measurements of tube amps to be very analogous to measurements of DAC's; the best measuring DAC's don't often sound the best. They don't even necessarily render the so-called "musical truth" the best. Is there a known measurement of tubed gear that can predict a tube amp's ability to render the texture of a violin or the blat of a trumpet, the subtlety of David Rawling's plucking of a Martin acoustic, let alone sound stage width or depth? 
I do agree with you that for the price charged, ARC has no excuse for mounting tube sockets directly onto the PCB. But to my knowledge, that is a reliability issue and not a performance issue. I also agree with you that ARC gear is unnecessarily complex and hard to service. The same could be said for BAT and Lamm and yet they have great reputations. The sheer number of capacitors in both my ARC Ref 150SE and Ref 6 is either alarming or impressive depending upon one's point of view. Again, BAT and Lamm seem to adopt the same approach. 
Your own decision to go hybrid with your higher powered amp and to go true balanced with your higher powered amp but not your lowered power amp is a head-scratcher. If-as you state-monos are more susceptible to hum than stereo amps, why do you implement RCA-only vs. offering true balanced in the opposite direction? And while a solid state input stage may very well offer better measurements, where is the proof that it sounds better? At the end of the day, isn't it indisputable that it is cheaper to produce and less complicated? You on the one hand have little good to say about Rogue and yet when it comes to hybrid tube amps, I think of Rogue (and Musical Fidelity though I don't count their "tubes" as tubes). 

Is it harder to design a amp or preamp?

I would like to hear how each of you figured out how much power you needed to buy in watts?

Never bought by power. Always purchased by sound.
Currently have 100 and 150w SS and 40w tube

84db 1w / 1m. min 3.2Ω, max 16Ω, mean 7Ω

Loud level is 90db C weighted slow measured with REW at seating pos 13ft from LS. Don't have low level. Each selection is listened at optimum for type. Sonic wallpaper is MP3 over internet in MONO, MP3 is intolerable in stereo.

SS amps not rated @ 4Ω