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One note of caution-- I owned a Melos 200 Triode amp that used 6KG6 horizontal sweep tubes. These tubes were not reliable in this circuit, nor were they readily available. I would hate to see Music Reference build an amp using sweep tubes that was not reliable. There were some novel design ideas in the Melos equipment but I had lots of reliability problems with this amp beyond the tubes and the company is out of business. I know Roger would not market a product that is unreliable so perhaps I am beating a dead horse. But, I spent a lot of time and money replacing 6KG6 tubes, as well their protective screen grid fuses, in that beast. And the only source for the 6KG6 at that time was direct from overseas.My feeling about sweep tubes is perhaps biased by my Ham radio experience with them as used in HF radio transmitter finals. I prefer "transmitting tubes" (e.g., tubes designed for that application such as the 6146) to sweep tubes in radio transmitter final amps because they are more rugged. I suspect tubes like the 6550 are more rugged than sweep tubes in high power audio applications too but I am speculating here, perhaps Roger would like to weigh-in on this subject?
I have not seen the circuit or operating paramaters of the Melos 200 but here are a lot of hits on the net, many of which deal with problems. Is Melos still around? I will say that using sweep tubes in triode mode is a very bad idea and I can't imagine an amplifier having any significant output power per pair of tubes. How many watts per pair is he getting? What is the plate voltage and bias current?The 6KG6 has gone from plentiful to rare in the past year. Up till then it was very cheap and easy to get. So much so that modern ham radio and illegal CB linear amps were made with them. It's the radio guys who used them all up. When the USA decided to block trade with Yugoslavia due to the conflict there were some 18,000 6KG6 tubes impounded on the tarmack at JFK airport. They were locked up for about 8 years. I bid on them but the seller wanted to sell the whole lot to one person. There were a lot of unusable types in the lot because the EI factory dumped their dead inventory on their fellow countrymen doing the importing in NYC. There's a lot more to this story, but that's the part that concerns these tubes. For those needing 6KG6 tubes I have only a few but I do have a good supply of Phillips/Sylvania 40KG6 that can be used in the same socket by changing the filament voltge.
Roger, have you ever given any thought on developing a full-function integrated amplifier, which marries the RM-5 IV and RM-10 II in one box? This would probably not take up much time (a winter project??) to complete given you've got these two proven designs under your belt. In fact, you don't need all that gain in the preamp RM5 ... you can go passive with one of your passive designs except for the phono stage (which is a must IMO).Not sure if you are familiar with a Japanese brand called Leben, but they've had good success selling their (retro looking) CS-300 integrated amp which uses EL84s as output tubes. What would make it sweet still, is if you can add a headphone jack and a bass contour control that you've mentioned too on these forums. I've been tempted by this little Leben package ... though continue to remain loyal to my MR gear. With the way the Yen has shot up of late too, this little amp has become very expensive piece of kit. http://www.6moons.com/audioreviews/leben/cs300x.html
Happy to hear your suggestion for the internal matching transformers. I plan to make them both ways. The matching transformer can be external in a nice box at the speaker allowing a high impedance run to the speaker. It is outside the feedback loop which provides speaker protection and better amplifier stability.For those curious about adding a matching transformer to an OTL, which seems silly, it actually has several advantages. Consider that the matching transformer is designed to present the ideal load to the OTL output stage. This may be as high as 64 ohms, for which no speakers exist. This matching transformer is an autoformer with a low turns ratio of 2 -4 to 1. Because there are no high voltages present and no bias current running in the transformer it can be wound in such a way that it has very wide bandwidth and is smaller than a standard output transformer of the same power rating.
