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Questions about tubes and gear that glows. FAQ

Re: Power ratings...

Thanks to all of you who read my manual, for quoting it and defending the amp to those who out of hand disclaim groundbreaking research into new circuits. There is far more to me done. Just because the tube development stopped in the 60's, the circuit designs don't have to. I've got dozens more up my sleeve, but they won't get out of my lab unless I get some help in the production and marketing end. How many of you know of the published 100 Watt/pair design for El-34's run at 800V plate, 400V screen? That came out with the tube in the 60's.

Let me go through the numbers and tell you how I get more than the specified 35 watts. Actually at the plates of the tubes, where all the tube manuals (bibles) take the data, I am getting about 45 watts. Some of you have misquoted the bias, dissipation and done some poor math, so sharpen your pencils. First the bias is indeed 30 ma. but for the pair of tubes and measured across 10 ohms, hense the 300 mV set point. Though 25 ma will double the tube life, (under light loading or soft playing) it will increase the distortion. Since the tubes are cheap, I let people decide for themselves. At the 700V B+ (yes, 700) the idle dissipation is 21 watts/pair or 10.5 per tube, well within the 12 watt rating. The screens run at 350 V and very cool, further enhansing the tube life. (The opposite of many designs like David Manley and early CJ). It's not the plates that mind the HV, it's the screens.

First to fat-bottle, this is no power game and the specs are real. The rest of your comments are not clear enough for reply at this time. I do welcome clear statements and clear questions based on something technical. I am a degreed engineer (UVA and Stanford) with a good ear, a necessary combination for good designs.

Now to 7N7, (by the way that's a cool moniker based on a very old loctal tube). I always do dissipation studies of my amplifiers, others don't and make amps that hurt tubes. I never hurt tubes, because I respect them. At 70% efficiency (which I get with my HV and others won't with 300B+), I have about 64 watts input (IxVB+), 45 watts output (before transformer losses) and 19.2 watts of dissipation PER PAIR!! So what's the problem?

I was about 6 years ahead of the electrical generation crisis making a 70 watt amplifier that drew only 70 watts at idle and low volume. I see 7N7's amp in a hungry beast expecially when you add the 100W it takes to light the 813's. With supply losses I'd bet we hit 400 watts at idle for about the same power of the RM-10 at several times my 0.5% thd.

Also thanks for noting the low noise. The RM-10 is the lowest noise tube amp out there that I know of (with decent sensitivity) and it was no accident. I intended that the amp would get hooked up to highly sensitive speakers. Hum and hiss are 300uV max, and hiss alone is 150uV typically. That is 10-20 dB better than most tube amps out there. Years ago the noise tests by Stereophile were overquoting the noise of power amps by about 20 DB or more due to a error in the test setup. I alerted Tom Norton and they fixed it but never said anything about it. The RM-10 is 80 dB re. 1 watt and 96 dB re. full power. As you see, S/N is reference dependent. So if you see a power amp with over 100dB S/N re. 1 watt it's likely not true nor necessary. This is because the noise of the preamp will override the amp noise. By the way, the only way to judge an amp alone is with shorting plugs in the input. If the preamp is connected you have the line stage noise present all the time regardless of the volume control setting as the pot is before the line amp. A good preamp has about 15 uV of output noise. When amplified by the power amp this will result in 250-500 uV of noise at the speaker, more than the noise of the RM-10 alone. Often the preamp is the limiting factor. That is why Music Reference preamps have gain switching that lowers the noise to mate the power amp and system needs.

I am happy to provide the 19 page manual with schematic to those interested for a small charge. Contact ramlabs@silcom.com. I do not mind quotes from the manual, but please do not post it as it is a copywrighted work.

If there is enough interest, I would consider offering a kit or selling the special transformers necessary for those of you who want to experiment with these kinds of designs.

For those of you who want to get involved with me at this level of engineering, there are employment and education opportunities at www.ramlabs-musicreference.com. Keep in mind this is hard work but a lifetime labor of love for myself and those of you who care enough to have good sound in your homes.

Thanks,

Roger A. Modjeski
Owner/Designer
RAM LABS/Music Reference


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