Yet another 12B4 line stage, or is the 12B4 better than the Grounded Grid..... - Page 21 - diyAudio
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Old 4th April 2006, 11:49 PM   #201
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which schematic did you use? the original, the one with ccs?
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Old 5th April 2006, 12:17 AM   #202
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This one:
http://www.diyaudio.com/forums/showt...926#post776926
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Old 5th April 2006, 12:35 AM   #203
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Remember, that post was for a CD output stage. I said it could be ADAPTED for line stage use. You wouldn't use the resistor divider and LCR filter before the grid in a line stage. Did you put all those parts in? You would just connect the grid to a volume control wiper (I'd add a 1MEG safety resistor from grid to ground and maybe a grid stopper of 100 ohms).

You should measure gain of 5X to 6X (around 15 dB). And yes, a 10 W resistor will be very hot with 5 watts dissipated in it. That's normal. I would make sure that it has plenty of air flow and is not right next to other components that it could cook.
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Old 5th April 2006, 12:46 AM   #204
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If it's not too much hassle, may I suggest you try this- http://www.diyaudio.com/forums/showt...678#post821678

That's what I use and sounds just right and gain just right for my 45, 2A3 and 300B amps.
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Old 5th April 2006, 01:15 AM   #205
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Thanks for the replies guys.

Brian, I did remove all of the filtering components, I have connected a 220k resistor from grid to ground, thats it nothing more.

arnoldc, if I use the schematic that you showed, how would I incorporate the CCS?


BTW what is the ouput impedance of the linestage?

This just hit me, I am presently using headphones (Senn HD 580) with the circuit, is that why I have low gain?


Thanks
Lawrence
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Old 5th April 2006, 01:38 AM   #206
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Hi Lawrence,

replace the cathode resistor with the CCS from Brian. But my 12B4 is on the low side- 17mA.

Quote:
This just hit me, I am presently using headphones (Senn HD 580) with the circuit, is that why I have low gain?
ooops! i think you need more drive
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Old 5th April 2006, 04:07 AM   #207
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Quote:
This just hit me, I am presently using headphones (Senn HD 580) with the circuit, is that why I have low gain?
As Arnold said, that would be your problem! IIRC, those phones have an impedance of about 300 ohms. Most linestages will not drive them at all. The output Z of my circuit will be about 900 ohms. That makes a divider ratio of 300/(300+900) or about 1/4. That would eliminate most of the gain of the tube, increase distortion enormously, and cause a bass roll-off because the coupling cap isn't sized for 300 ohms. You need a headphone amp, many of which are described in this forum.
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Old 8th April 2006, 05:18 AM   #208
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Pulled out the scope and signal generator and took the following pics of the result from my 12B4 preamp.
Attached Images
File Type: jpg 12b4pre.jpg (50.3 KB, 1246 views)
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Old 8th April 2006, 07:48 AM   #209
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You need to set your oscillator. It's not set for 50 : 50 spacing. There must be a pot on the front panel.

I must check out what I get on my 12B4A .
Anybody have any idea if there is any difference between RCA, GE and Zenith brands ?
Thanks,
Cheers.
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Old 10th April 2006, 12:39 AM   #210
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ashok, you're right. i just found the adjustment- it's the same button to select sine/square- it says symmetry (turn for symmetry).

thanks!
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