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Old 8th January 2013, 07:10 PM   #61
Alexsk is offline Alexsk  Canada
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One more about C3M oper. parameters. 300b biases usually in around -70v In self-bias it works by keeping G at 0v and Cathode at +70. In order to get maximum output from 300b
driver needs to swing +/- 70v with more then +70v 300b goes in A2. In order to do that, anode of C3M needs to go to Ua - 70v, but should not go below Ug2 (in A09s G2 is 66v). Another works, C3M anode voltage should be > then 66+70 >133v . In A09s I believe it is about 140v. What you have to do is change c3m current to get ~140v on the anode by adjusting value of the cathode resistor to dive a bit less current.
HTH
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Old 9th January 2013, 09:56 PM   #62
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Hi Alexsk

In post #33 you write that you preferred the c3m running at low current 2,5mA or am I reading your post wrong?
I have checked the old article from glassaudio issue 198, and in that amp the c3m is run at much higher anode current, but in a similar circuit, but with a voltage devider instead of the Zener reference on g2.
Which do you prefere ? Have you tried both low current and high current ?

Thanks
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Old 10th January 2013, 12:24 AM   #63
Alexsk is offline Alexsk  Canada
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hi Fonnesbek,
yes, i did change my opinion back and forth... i was running c3m at 14ma and 150v g2,
later i tried a09 driver and initial impression was very favorable. A few month later i've switched back to original 14 ma and recognized that it is more musical, especially with complex orchestra. My current preference is to run C3M at high current, that driver measures obviously better and as of today i would say sounds better as well.
The good part is that it is very easy to change driver from "a09" to 16ma/150v and form your own opinion. From experience, i'd recommend to add greed stoppers on both g1 and g2 of c3m, anything from 39 to 120R will do, also wont hurt to add greed stopper to 300b.
whatever i ended currently is the following: anode on the c3m has (from b+) 1k5/10u630v to the ground- 1k5/another 10u/630v to the ground - 12k - anode. On g2 30k from b+ and 22k/10u200v to the ground. G2 resistors run at almost 10ma current, acting also as HT bleeders. Values allow safely use 200v capacitor.
Another thing, i've mentioned before, i am a bit skeptical about e-choke. In such simple amplifier power supply has huge impact on the sound quality.
Thanks,
Alex
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Old 10th January 2013, 08:46 PM   #64
Jogi is offline Jogi  Germany
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I second the findings of Alex, as I tried quite some drivers for the 300B and ended up with C3o as the best for my liking. C3o is the 6.3V heater variant of C3m, so I didn't had to change transformers in an existing amp.
Driving the 300B with just 2mA is inviting distortions at big signal level, although it still sounds better than the usual 6sn7 driver. If you want to listen to big orchestra or other complex music, the higher current is clearly more capable to deliver the goods.
But, sometimes people prefer distortions
I used a 150V zener clamped to the cathode to get the voltage for the screen, stoppers at the grid, g2 and g3. Maybe at these low currents as in Yamamoto amp the stoppers are not needed, with 16mA they are for sure. And 39R ist too low, I would go for minimum 220R, I chose 1K.
I upload my schematic as PDF, its a sad try with MS paint but I hope it gets clear...

Cheers, Juergen
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File Type: pdf c30 300b pdf.pdf (78.2 KB, 118 views)
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Old 10th January 2013, 09:00 PM   #65
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Thanks, I'm almost finished with my second amp :-) will try the low current Yamamoto circuit first.
And then after some time I will give the high current a try.
Thanks for the drawing it is very readable ;-)
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Old 10th January 2013, 09:14 PM   #66
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One thing, you have written to 300b anode it should be to 300b grid ?
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Old 10th January 2013, 10:51 PM   #67
Jogi is offline Jogi  Germany
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Yes, of course the grid of the 300B, thanks.
Will be interesting to get your opinion about high/low current...
The circuit I attached is pretty simple, no big voltage dividers for g2, just a 24K resistor for setting the current. The capacitor parallel to the 150V Zener shuts down the noise of the diode.
I used an additional choke from B+ at 440V in my case to lower it to the 420V for C3o, and to decouple it further from the power supply for the 300B. Heater for the driver is just AC, but for 300B I used some kind of voltage/current regulator, which improved the sound very nicely.
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Old 11th January 2013, 12:30 AM   #68
Alexsk is offline Alexsk  Canada
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Nice design, a bit different from my approach- i was trying to use as less electrolytic caps as possible, ending with only one in the cathode of c3m.
For your entertainment some photos:
1. Distortions at 1khz 3v rms into 8 ohm at -45dB
2 Distortions at 1khz 8v rms into 8 ohm at -35.5 dB
3. in/out phase at 50hz.
4. in/ out phase at 20hz
all measured with c3m at "high" current. OPT transf. Hashimoto 30-3.5
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File Type: jpg image.jpg (54.1 KB, 167 views)
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Old 11th January 2013, 12:32 AM   #69
Alexsk is offline Alexsk  Canada
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hmmm,...only one attachment...
2. 8v rms
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Old 11th January 2013, 12:33 AM   #70
Alexsk is offline Alexsk  Canada
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50hz phase
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