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In Reply to: RE: GRAAF GM 200 posted by 6AS7_6SN7 on January 31, 2012 at 23:54:30
Its been a long time since I've seen the schematic, but if I recall right, the GM 20 uses the output section power supplies to also operate the input section; I don't *think* the GM200 uses that same technique.
Follow Ups:
Hi Ralph, I feel quite honored to get a reply from you, years ago a friend of mine in Oklahoma city built a set of the Novacron kits you used to sell , those were the best amps I had ever heard, I had planned to build a set of the 60's in kit form, I was married at the time and moony was tight, I swore some day I too would have a set of Atma-Sphere OTL amps. Now I have 5 sets of otl mono blocks, 3 sets of nyal otl3's, Fourier panthere's and a set of Trancendents that use the 6c33c's, R.L. Merz 6336 stereo and now this GRAFF GM200... I promise this, my next otl's will be Atma-Sphere's , not because the others have been a bad experience, quite the contrary, I love them all and have no intentions of letting any of them go. The OTL-3's are my favorite. I just hope I can get this Graaf up and going, if it sounds half as good as it looks it will be a great amp. The reviews I have read have been all positive.
Once again I thank you for your correspondence and thanks to Lew too for his input as well.
I'm away and won't get to see the Graff amp til next month, I purchased 70 output tubes today, all matching date code Sylvania's for $5 ea. Plus shipping, that was a good deal, enough to load it up 2 times plus some spares. Can't wait to check it out...
I can't help but wonder what speakers you are using with your OTL3's. As I have written before,
I also have a pair of these driving Stax F83's, (with subwoofers by Roger Modjeski) and I think
the sound is stunning. But I don't listen to other stuff any more at all.
The OTL-3's were most recently running a pair of 1959 Klipsch khorns, 16ohm, they are a fantastic match, when corresponding with Ted Hammond he said that they were near perfect impedance for the otl-3, stating that the amplifier was designed to have full output into a 25 ohm load.
I have also a pair of Von Schweikert vr-4's they are truely full range, flat to 20 hz on the bottom, they are my favorite speaker with the Futterman, I'm using them in a dining room system now being drivin with a 22wpc merz otl.
I briefly looked at the schematic that Luca provided, and I do believe I saw a separate supply for the front end of the GM20. But the output tubes shown on the GM20 schematic are unlabeled triodes, which I believe to have been 6C33Cs. Whereas the GM200 is using something else for outputs that appear to be pentodes. In a circlotron, which the GM20 is, could one run the front end off one or both of the output supplies, successfully? I would not have thought so.
Hi Lew !From my attached link you can download more accurate schematic of Graff GM 20 power Amp .
There is obviously that input differencial stage (6922 tube) is AC bootstraped to output Circlotron power bridge , formed around of two 6c33c-b power tube .
Input differencial stage (6922 tube) DC B+ feed is sourced from the same output power two floating B1/B2+ power supply .
Two separate DC bipolar supply sources is designed to be B+ source for two drivers EF184 tubes (+297V ), differencialy conected .
And - 250 V source is designed to be the B- supply for both differencial input and driver stage , and in the same time is source for negative B- Bias supply for output Circlotron power stage .
It is my personal opinion that model Graff GM200 is designed in simmilar way , more or less .
Best Regards !
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Enlightened Evolution-Astral Projection
Edits: 02/01/12
Yes, I see now that that separate supply at the bottom of the schematic is merely the bias supply for the grids of the output tubes. Interesting.
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Yes I agree with You !More time I spend analyzing this Graff GM20 OTL Circlotron circuit design I find him more and more that is very,very good designed !, and is designed in very elegant way to .
Think beacose that differencial Input gain stage is bootstrapped with Output Circlotron power stage the whole amp circuit is internally very dynamically coupled , and some specific resulting Amp sonic signature exist there for sure to .Best Regards !
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Enlightened Evolution-Astral Projection
Edits: 02/01/12
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