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

I'd like to echo Roger's comment regarding the use of the term "hybrid" in reference to the RM-200. I also once thought of this amp in the same manner, but given my access to Roger I have the opportunity to not only listen to a lot of his designs but understand how certain designs came to be. I also have the benefit of being the one who tests repaired amps and new designs in my system.

So I was able to borrow an RM-200 for a while and at first I can't say the amp thrilled me, but over time it did grow on me (I prefer 6550s to KT-88s). I certainly don't think of the circuit as hybrid any longer, in fact to me it's a tube amp, and a cleverly designed one at that. As previously mentioned it may be the only amp with a tube output that puts out more power into lower loads. In addition, as I think about it, those amps referred to as hybrids that I have heard of use tubes on the input and transistors on the output. I have not heard of any others with the opposite design configuration.

When it comes to MR amplifiers the RM-10 MkII is my all time favorite. My old friend Paul Rosenthal (RIP Pubul57) and I ran similar systems at one time. An RM-10 with Lightspeed Attenuator. We always commented to each other how such big sound could come from such a small package. As Roger mentioned one of the things he designs for is reliability. I have owned mine for 12 years now, and other than replacing the power tubes have had zero issues with it.

This is consistent, btw, with statements I believe Roger made earlier in the thread to the effect that many listeners require less power than they tend to believe.

Yes Al, in your specific case as indicated.

You have speakers that are significantly more efficient than most AND are also very easy to drive as well if I understand correctly, so relatively less watts needed to do things right.

FWIW I heard the 87 db or so efficient Fritz Carrera speakers to teh amazement of many including me knock it out of the park at fairly decent SPL levels (did not measure) at Capital Audiofest off a 8 watt/ch tube headphone amp. Very impressive! The explanation provided for that was though rated less efficient the Fritz use a "series" crossover approach that helps make them easy to drive allowing a smaller amp to overachieve compared to the norm perhaps.

There are many cases where speakers are not efficient and do not present a near perfect benign load to drive. Expect way more watts needed there, perhaps MORE than people might believe.

It can all happen....

My 60w/ch Bel Canto amp does very well with my more typical load OHM Walsh speakers up to a fairly reasonable SPL in a fairly large area. But my 500w/ch Bel Canto amps knock it out of the park with them at any listenable SPL.

Its like comparing a Toyota which drives perfectly fine within its limits to a Porshe.

Preamps are much easier and to me not so interesting

Roger, IME this statement is false. Many good amplifier designers think that a good preamp is no big deal and then go right ahead and design a poor preamp as a result. This is totally because they really in fact for real don't know what a preamp does! - which is to say, a lot more than just the gain and bandwidth, that sort of thing. If a preamp isn't right, it makes no difference how good the amp or speakers are, the missing information can't be recovered downstream.

I wanted the RM-200 to have good CMRR (hum rejection in simple terms). One cannot do that with a tube at the input.

We get pretty high figures and we do it with a tube.... As a hint, look into 2-stage CCS circuits. You aren't going to get good numbers without a decent CCS, a resistor or a single-stage CCS won't hack it.

It also has something few amps do not have which is the abilty to drive a dipping load with increased power rather than decreased power. Neither CJ, Rogue, ARC or anyone else I can think of has done that.

A good number of 300b SETs can do that. The Wolcott did as well.

Horn speakers tend to have peaks in the response.

Some but not all. CAD has done a lot to improve horn response- if there are problems in the throat where it couples to the horn, all is lost. But I've heard several horn setups where this common problem is overcome, and the result is that they sound very much like ESLs.

I have 6,000 LPs. A lot of them have a good bottom, some have an excellent bottom.

I love the Theodorakis performance, but alas it’s like listening through cotton.

Let’s see what it sounds like in my new room. I’ll let you know.

I have a similar number of titles. The bass really is there. There were only 1000 pressed, so its not likely to be a worn stamper. 

I still find that OTLs at low impedance are current limited. As to 10 amps without damage. When I put a 6AS7 on the curver tracer and go just a bit above the peak rated cathode current I see flakes of cathode coating coming off like sparks from a sparkler at much less that one amp. WIth the grid being so close they can easily fall into the grid wire and POOF. Horizontal output tubes that Futterman and I use are specified for high peak current about 1 amp.

I guess this really depends on what is meant by 'current' (since the word has become a charged term in audio)! And a lot depends on the 6AS7 in question too- the GAs don't hold up; most American tubes have problems in our circuit as well since they really aren't intended for fixed bias operation. We prefer the Russian variant; they hold up the best of any we've seen.

BTW, I like your approach to the whole 'damping' thing and I also appreciate your use the the phrase 'output regulation' (which I see as opposed to 'output impedance') which I see as a more accurate term. You are spot on that far too much attention has been placed on damping factor- have you seen this article by the former head engineer of EV? http://www.dissident-audio.com/Loudspeakers/CriticalLSDamping.pdfIn is we see that no speaker made needs a 'damping factor' of over 20:1 and some need quite a bit less! FWIW the original AR-1 was designed for a 1:1 damping factor.
Also kudos for the comments about too much power. Many amplifiers make excess distortion at lower power levels and when too much power is available in the amp, 90% of the listening will be in this higher distortion region- so not really taking advantage of the amp's capabilities.

BTW I first met Bill Johnson and Robert Fulton at Bob Fredere's house in Minneapolis where the two would meet for listening sessions (at the time the D-150 was ARC's SOTA amp). I wound up running a set of Fulton J's and then Premiers for several years.

RM love your idea about a utility grade ( birch plywood ? ) base for an RM-10 w exchange IF client buys amp. I would be interested in that. I have to say 14# tube amp is attractive as all heck.....

STOP recommending stuff...got the B&K tube tester yesterday, Analog circuit book is due today - you scored yours  for less €
and now a scope? Remember the Mac MPI indicator ? I recall Marantz also has a scope on some gear, as you say fascinating....

so assuming my needs are kit amp building, ESL repair and this and that, which used scope to get ?????

and yes the ESL-63 panels are toast....

Roger, lots of good information as well as memories here.  So much in fact it is hard to keep up!

As one of the older audio hobbyists here I too remember these names from the past.  I recall a visit from William Z Johnson at a local dealer to set up his Tympani/ARC system.  Interestingly that same dealer later took on the Fulton line.  Regarding Bob, "Fulton made good sounding stuff. I don't know how but I always assumed him to be a competent engineer with great listening skills.", did you know his involvement with instrument design?  For example I'd read that he designed trumpet mouthpieces.  That was confirmed by an alumni club buddy who played trumpet with a Fulton mouthpiece.  He was so sensitive to colorations in musical reproduction that he assigned a characteristic color (his interpretation) to the unique tonality of each orchestral instrument.