medium power OTL circlotron amplifier

[midmoe]

as far as the bias circutry goes, I switched back from the pos. ends of the bias batteries going to opposite tube cathodes to going to the same tube's cathodes, eliminating any neg feedback in the circut... I felt it didn't really help and might introduce artifacts from back emf getting introduced into the grid circuts from the speakers, especially since I will be seriesing about 26 8ohm speakers in proposed system...I've heard of a speaker repair company that can re-wind any speaker to a higher impedance.....at least 32 ohms, maybe higher...anyone out there know of a reputable place that will rewind voice coils of my EV-300 18"ers to 32 ohms or higher? If so please advise me of their email....a friend of mine says he has an old japanese stereo from the '60s with no output transformers and two 8", 500 ohm speakers !!! what the figbar ? Ive never heard of anything like that except of the early circlotron speakers from the 50's. Were they wound with resistance wire?.....thanks, midmoe

[cnpope]

Originally Posted by midmoe

as far as the bias circutry goes, I switched back from the pos. ends of the bias batteries going to opposite tube cathodes to going to the same tube's cathodes, eliminating any neg feedback in the circut... I felt it didn't really help and might introduce artifacts from back emf getting introduced into the grid circuts from the speakers, especially since I will be seriesing about 26 8ohm speakers in proposed system...I've heard of a speaker repair company that can re-wind any speaker to a higher impedance.....at least 32 ohms, maybe higher...anyone out there know of a reputable place that will rewind voice coils of my EV-300 18"ers to 32 ohms or higher? If so please advise me of their email....a friend of mine says he has an old japanese stereo from the '60s with no output transformers and two 8", 500 ohm speakers !!! what the figbar ? Ive never heard of anything like that except of the early circlotron speakers from the 50's. Were they wound with resistance wire?.....thanks, midmoe

Very unlikely; there would be no advantage, and many disadvantages, to using resistance wire. (If one were in the game of making a "500 ohm speaker" simply by adding resistance for its own sake, one could more easily just use an external series resistor.)

Chris

[midmoe]

attn: ralph at atmasphere...I have a schematic hard copy I can mail you if requested. The schematics on thread nos. 32 and 35 aren't finalized, not to mention hard to read. I still can't figure out how to load a short video, so for now stills is all I can muster....talked to you this am, thanks for your attention..please email me so I can get correct address. Thanks, midmoe

[midmoe]

am under construction of a stereo version of the circlotron. Gave up on trying to keep the case size small and doubling up the tube count with "candleabras". Am using a double-wide chassis (16"X14"). Simply building two monoblocks in one case...the good part is that with some minor juggling, I can hook up all output tubes in series, with preamp tubes parrelleled, then hooked up in series with the outputs, leaving only a couple 68 ohmers across each preamp trio, to give 6.1 vdc on each tube instead of 6.3 by using a big diode in series, with appropriate crowbar and fuse prtection in case of main diode short or leakage. This will eliminate the bulky, hot fil. xfmrs. saving weight, increasing efficiency, and hopefully ridding the low level hum I think is field induced by the overburdened fil xfmrs. now....my friend brought up the wierd high imedance 8" speakers...only that they are 400 ohms, not 500...still, they look like conventional construction. the cones need repair and are generic, but looking foward to getting them hooked up just to see how they perform after a little work repairs from the holes chewed by mice, parelleling them should be a close match with amp impedance. I will be uploading a completed schematic of the stereo version, with latest updates and a built in meter/test panel, plus all 6 output tubes will be shown. A fellow in columbia has an HP distortion analyzer and can hopefully run the specs in a month or so.midmoe

[midmoe]

will text more in next posting

[midmoe]

trying again to get picture on, sorry

[midmoe]

when first putting together the amp, I had the bias hooked up with ony one supply...not as complex as this schematic, but a rough equivalent. It seemed to work ok, but not taking any chances, as I still had instability problems (before freddie drive), I went ahead and hooked up the bias with two floating supplies of which I'm still using. I worked out this tentative drawing on paper only so far, as an idea to allow one supply to be able to bias both banks of outputs, with a common bias control, and a balance 10K pot even out the small variations, showing up at the output as a DC offset. It would seem to me that the inverting of the two 47Ks back to the respective grid supplies would allow some limited self-corrective behavior, allowing the balance control a less critical setting...comments?

[midmoe]

suddenly both pictures came up.....the first one wasn't there earlier....will now attempt to upload a (single) pic of stereo amp in works

[midmoe]

upon review, I can see now that my bias would be way too negative...but with a lower supply voltage, or perhaps even better, a higher value of resistors in series with the balance control, a correct range of -10 to -20 VDC could be attained with a common bias level contol, with center at the desired -15 VDC between each grid and cathode. This is only preliminary, and more of a question as to how to simplify design, eliminating the extra xfmr, upping efficiency, lower weight, and perhaps a beginning effort of minimizing output DC offsets. Perhaps even a couple transistors could be added into the network to amplify the corrective voltage feedback to allow a stable 0VDC at speaker terms under varying conditions....... midmoe

[midmoe]

perhaps a better starting point for bias idea