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Old 20th February 2013, 04:57 AM   #51
midmoe is offline midmoe  United States
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am contemplating using series string on outputs of a stereo version...6.3 X 14 = 88.2 V......with the supply being 84.4 V, should leave comfortable margin on voltage to outputs. This will allow removal of bulky fil. xfmr, leaving room for a second, parrellel wired-in, 170V B+/B- mains xfmr.....and adding small fil. xfmr for driver tubes....as I figure it , each output fil would have 6.03 v....haven't tried this on the bench, so can make no declarative, table-thumping remarks yet.....midmoe
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Old 20th February 2013, 09:00 PM   #52
midmoe is offline midmoe  United States
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Default short video 001

Click the image to open in full size.the prototype monoblock

Last edited by midmoe; 20th February 2013 at 09:06 PM.
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Old 20th February 2013, 09:44 PM   #53
midmoe is offline midmoe  United States
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sorry, second failed attempt to upload short video will try later. midmoe
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Old 22nd February 2013, 04:16 AM   #54
midmoe is offline midmoe  United States
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stereo version losing appeal, voted down by asectics committe...did notice something kinda odd about the freddie drive. Was seeing if the 1K resistors could be elimenated, so shorted across them and got reduced gain, which I expected...but upon cranking up the preamp about 1db to compensate, noticed a reduction in the high end compared to before changes, so I left in the 1Ks. Tying the cathodes together would add neg feedback and lower impedance (as well as gain) in output stage, but why be frequency selective?
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Old 23rd February 2013, 07:50 PM   #55
midmoe is offline midmoe  United States
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Default second monoblock in works

parts on back order, but should be in soon....looking crude, like the first. Will finish and trim cabinet with logo made from alum. tread plate....
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Old 27th February 2013, 04:01 AM   #56
midmoe is offline midmoe  United States
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I finished second monoblock today, just as power failure stalled first play for an hour or so...I was down to kerosene heat and candles....quite some storm here in midwest. I changed the 82K in the phase inverter plate to grid ckt to 100K, giving a little extra gain, haven't gone in and balanced the phase inverter yet, but it must be pretty close, sounds fine. I'm sure the plate resistors need to be finessed to get a null output from the center of two matched 68Ks bridging the plates....I'm presuming this is the correct way to do this...tubes running not too hot and biases are very stable.....nice to get back to stereo after a week of mono only. As soon as snowdrifts go down a foot or so, will attempt to get local fab shop to cut logo out of aluminum tread plate..and someone has promised me to come in and finally load a short video correctly. Am still looking for two sets of earlier series bose 901s, anybody ? Don't care about equalizer, stands, cabinet appearance, and a few bad or missing drivers, ok. Will drive a reasonable distance within the missouri area to pick up. Thanks, midmoe
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Old 27th February 2013, 02:27 PM   #57
midmoe is offline midmoe  United States
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Default interior shots 001 002

intrior shots of second prototype...still need to add fan and lamp into pilot light jewell.
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Old 27th February 2013, 02:31 PM   #58
midmoe is offline midmoe  United States
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Default interior shots

for some reason, amp didn't come up on last post, only PS
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Old 28th February 2013, 02:11 AM   #59
midmoe is offline midmoe  United States
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Iv'e been thinking about the balancing of the phase splitter....in the original grommes, the grounded grid has less gain so therefore needs a higher plate resistor....as compared to it's mate...since I've introduced so much NF into the mate, I had to switch resistor relationships. I was going to finesse the plate resists to null the bridge, but wouldn't a technically sweeter approach be to equalize the plate resistors, with say 56Ks and then adjust the grid-to-plate resistor, now at 100K (orig. 82K), to a point that brings null at my matched bridge center, placed across plates ? And wouldn't this keeping the gains of each side exactly equal keep them running in the same point of the tube parameters...?..and therfore minimize distortions? midmoe

Last edited by midmoe; 28th February 2013 at 02:13 AM. Reason: clarity
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Old 28th February 2013, 06:13 PM   #60
midmoe is offline midmoe  United States
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equalizing plate resistors didn't work as hoped...I had to add pull-up (330K)from +250 to V1 top plate along with a 150K back to V2 plate(from V1 plate) to get a near balance, but then I'd lost so much neg feedback, overal gain was again too high. And then, using a variable resistor in place of the 150K, to attempt balance, the nul wasn't nearly as complete as with putting things back as they were. Any gains in plate resistances being equal (if any) were outweighed by attendant problems. As it stands, V2 top plate resistor is 56K and lower plate resistor is a 47K, bridged by a 220K in the new monoblock. The older one balanced out with a 150K across the 47K. This achieves a nearly silent nulls. Tom Petty never sounded so good. midmoe
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