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Old 1st March 2006, 04:17 PM   #1
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Default 12B4A line driver/preamp

I built some 2A3 monoblocks that I'd like to place near my speakers. Because of where things are placed, I need to run about 15 ft of cabling from the preamp to these.

I currently have a passive pre that obviously will not work.

I'd like to build a 12B4A preamp to drive the cables.

I want to use a preamp transformer (Sowter) that is 4:1 and I think a good match for the valve. I'd also like to reverse the output so phase is preserved.

Using an op point of 80V, 15mA, -10, ra = about 1200ohms. So with the 4:1 transformer, I expect that this will drive the cables at about 75ohms.

Does this make sense?

Thanks
Dan
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Old 1st March 2006, 05:51 PM   #2
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Is the transformer designed to be happy passing 15mA? Somehow, I doubt it. It's not the end of the world, though. Fit a semiconductor CCS in the anode circuit and feed the transformer via a capacitor.
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Old 1st March 2006, 05:59 PM   #3
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Use RG-62U coax. 92 ohm . It has half the capacitance per foot of regular coax (about 15pF per foot vs 30pF). You may not need a line driver. I am using 25 foot runs to my power amp with no driver.
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Old 1st March 2006, 07:24 PM   #4
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Quote:
Originally posted by EC8010
Is the transformer designed to be happy passing 15mA? Somehow, I doubt it. It's not the end of the world, though. Fit a semiconductor CCS in the anode circuit and feed the transformer via a capacitor.
yep 25mA. pretty meaty. I'm trying to stay away from a cap and be able to reverse the inverted output

I also want to use a silver foil to make the long interconnects. So that's why I'm wandering down this path (
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Old 1st March 2006, 08:02 PM   #5
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Fair enough. Carry on.
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Old 1st March 2006, 08:59 PM   #6
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Hi Dan,

Why do you want to drive 75 ohms? That’s only good for video. IMO you don’t need 15 mA through that tube and a 4:1 transformer for a line driver (unless you want several volts into a 600 ohm load). 5 mA – 10 mA will suffice me think and the tube will last a lot longer. But you can experiment with that anyway.

Cheers
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Old 1st March 2006, 10:40 PM   #7
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what kind of interconnect is that?

i have the same setup where my amps are 5 meters away from the rest of my gears. maybe i'm deaf but using a passive DACT fed by my clearaudio phono stage, i didn't have to worry bout the long run.

i also have a 12B4A pre and i just invert the speaker terminals which is free in fact sometimes i even forget

i've have a 5 meter DIY interconnect, OFC copper in teflon, hand braided and a commercial Quantum (it says made in Germany) 5 meter interconnect which is dirt cheap. both are terminated with Eichmann Bullet Plugs.
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Old 2nd March 2006, 07:55 AM   #8
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I came to 75 ohms simply because ra divided by 4 squared equalled that.

For cables, I want to try one of the silver foil interconnect recipes in Allen Wright's cable cookbook.

Dan
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Old 2nd March 2006, 08:38 AM   #9
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Quote:
Originally posted by lt_texan
For cables, I want to try one of the silver foil interconnect recipes in Allen Wright's cable cookbook.
Which are likely to have quite high capacitance. Stick with your low source impedance. Although 12B4A is useable at 5-10mA, it's down in the region where the curves bunch together, so 15mA is a better idea. Carry on with your project and let us know how you get on.
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Old 3rd March 2006, 09:10 AM   #10
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True, but the effect depends largely on the actual load line. And if the next amp has an input impedance of say 47K, I don’t see a problem.

But anyway most of the (AC) current is supped up by the magnetisation current of the transformer at low frequencies. So the difference can be noticeable at low frequencies and at high frequencies due to the cable capacitance. But IMO it is worth a try.

Cheers
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