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Roger,Many thanks for a positive response to my request. My questions are not so much asking your response to my detailed design but rather general questions to ensure a design topology that will work well, i.e. design and build a 100W amp. at low distortion and reasonable amount of Damping Factor.1. I agree with you that to design a reliable and long lasting amp, it should be bias at low current and KT88/KT120 is strapped in Pentode mode, at anode set at around 700Vdc and screen at 320Vdc. KT88 in Pentode mode has high output impedance, how would you reduce it? From what I know from Tube design a shunt feedback on the output tube, ie KT88 would reduce it would you recommend this design if not why not? Your opinion please.Not sure what you mean by shunt feedback?2. You have achieved a damping factor of greater than ten from your RM200, it is by no means easy from a tube amplifier, did you achieve this by using large loop gain that is why you use transistor in the input. If so what loop gain can you apply to a tube amplifier at the same time maintain unconditional stability? Is it through special circuitry at the output stage or anywhere else? Any design suggestions to achieve DF=10?I use a matched pair transistor at the input to achieve exceptional common mode rejection of over 100 dB (rejection of cable noise). Rather than look at the loop gain, just look at the final performance under various loads. The feedback in most amplifiers is typically is 10-20 dB to give you a range. 3. The output transformer is of prime importance, obviously I cannot buy your output transformer, however there are a no. of commercial o/p transformer suppliers such as Hammond, Edcor, Magnequest, and a few others, which supplier should I source my o/p transformers from, can you even name a model which I should buy, i.e. to attain 100W/channel the anode to anode impedance should be 5Kohm and at the same achieve low distortion. Is this correct?Actually you can buy any of my transformers and we can make special ones too. I don't recommend any from your list. The high plate voltage is out of their realm. A definitive answer to these questions will help me reduce the design time in experimenting with various design options. Those who read this would like to contribute are most welcomed. I may have another 1-2 questions left, but still formulating them at the back of my mind.The RM-200 is not like any other (typical) power amp and it took a great deal of time to develop it. As you may know I am more interested in making things that have not been made before than just making amplifiers. There are too many variables to deal with for the average builder. Since you already have the voltage parameters why not take a simple driver circuit like the Marantz 8B. I recently worked on an amplifier that was built at the Randall Museum (classes given in San Francisco) and with some tweaking it became very nice. Like the Marantz it has a single triode input stage followed by a long tailed pair phase inverter. A classic circuit which performs well, is easy to manipulate and make stable. You will need some test equipment to do these experiments and that is where the fun begins. You are just the kind of person I am looking for in my school where we intend to build really great stuff. I have all the test equipment and am willing to share what I know with dedicated students. When you consider what amplifiers cost these days a trip to the USA might be a bargain. However if this is your first amplifier project I would advise you to start with lower voltages for your own safety and the safety of the tubes. At 700 Volts things happen very fast and furious. What is your level of experience?Again thanks awaiting your response.
