circlotron using 6550s

M

midmoe

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Jul 26, 2013
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hello all, any others out there listening to a circlotron? I built my own with 2 new mods. The first one deals with how the output the stages are coupled to the drivers, and a new bias circuit that needs only one supply per channel with a self-correcting bias stabilizer feature. I use no global feedback. The amp is based on a grommes 260A front end/phase inverter with an additional 12BH7 to drive the 3X3 6550s per side. Using so many 6550s allowed me to series string all the tubes (except the 12AU7 preamps) of which I have a separate DC supply, and now have no filament transformer except the small one for the 2- 12AU7s. I cut the 120VAC in half with a half-wave diode, and get almost the perfect supply. I still have a little diode switching noise, but am going to try some nte 577 diodes, which have a 70 ns switching time, so I'm hopeful that will kill that noise, but it's nearly inaudible as is. Hope to get it as quiet as my Mac 275 I used to use before this this was done. I've been trying to get my new 'puter to upload pics and schematics, so far my new chromebook hasn't been successful in this. Will bring in my 'puter guy if there is any interest in seeing any of that. The output impedance is a little over 200 ohms, so I'm using 3 sets of bose 901s with the 8 ohm speakers wired in series in each system, then all speaker cabinets wired in series giving about 192 ohms per side, which I have turned around and am using the backs as front sides, and having removed the ninth, normally front speaker. Sounds great for an oddball high impedance experiment. In fact, sounds better than my Mac. Still can't believe what a difference not having output transformers makes in imaging. And having no crossover components as I'm using the full range boses, the bass detail is remarkable. Stacking up the boses on top of one another really makes them sound totally different than I would have imagined, those 24 speakers per side really add up to a lot of surface area. Thanks, midmoe
 
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Welcome to the forum midmoe! What a cool experiment. I'd love to hear that system.
 
thanks to everyone for the nice welcoming...I hope my rather rambling entries don't annoy anyone....if you want to see a log of how I developed the circlotron amp I have, go to DIY audio, which I used to be in before I was cyber-bullied off the site. I believe they still have my entries under "medium power circlotron tube amplifier" or something to that effect. I was using the midmoe handle in there too, along with the same in audio karma. A fuzzy simplified amp schematic and a pre-finalized design of the new bias circuit (really just a thought experiment) are my last entries on the DIY site along with a running dialogue of how I figured out the coupling circuit, of which I term the "inverted cathodic floating reference drive" or ICFRD or I see fred or "freddie-drive" for short. The bias circuit finalization never got posted on any site as of yet, as the toshiba hard drive started overheating just as I got all the parameters figured out , and it was the only computer I had that seems to be able to upload to any site. I'm using the finalized version of the bias circuit now, never have to adjust the "bias balance" controls, and only one per side.. I'm using -16volts neg bias on the 6550s with a B+ of 170v. The tubes run cool, you can touch them without fear of being burnt, unlike most high end amps. Still perfecting certain aspects, am dealing with oddities such as the new JJ brand 6550s using a non-standard filament current.(?)..they burn much too bright at the rated 6.3 volts in the tube tester and the amps I've tried them in...haven't put them on the ammeter yet to see precisely what current they take as compared to my old school westinhouses, will report on that later. My friend bought newJJ brand EL34s for his DYNA 70 and he says they seem too bright too...anyone else notice their new JJ brand tubes seem too bright? It almost as if they are putting a hotter filament with over tolerance current just to get their emission up....they sure sound good and seem reliable so far, so I not complaining. But it did make a difference in the two resistor values. I had to put in shunt resistors across the two 12BH7s wired in parallel,per side, to get them to draw the same current as an individual 6550..in the actual circuit, the shunt resistor is about half of what it worked on on paper to be, given that the 6550s are supposedly rated at 1.6 amps...this is why I think they are off toleranced up to almost 2 amps at 6.3v. And to make matters worse, I'm using a half-wave rectified filament voltage, which doesn't really read true on my conventional meters, so I have to guess a little as a pulsating DC voltage is something a tech usually doesn't run into much in normal repair work. I am a retired tv/vcr repair guy, who's always had an interest in music, high end audio and thought I had the ultimate amp when I found a Mac 275 in a flea market for $15.00.....(that's fifteen, people!) Had to do minor repairs, mostly just the tubes weren't with it. Used that for about 12 years with flawless service, exept a 5 watt resistor developed a loose connection in the power about a year ago. Right now, my favorite cd is Lucinda Williams Live at the Fillmore ...a fantastic live recording. There are a few points that you can hear a slight overmodulation in her vocals, but a fantastic presence...almost sounds like a "two mike" recording, but I'm sure it isn't....midmoe
 
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