6112 and 6111 Mini-preamp - diyAudio
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Old 20th December 2011, 06:57 AM   #1
TV7 is offline TV7  Australia
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Default 6112 and 6111 Mini-preamp

Had a go at designing a basic line level preamp to give me a taste of 'tube' sound with one 6112 front stage and a 6111 as a twin cathode follower, SY will love this.

Could have gotten away with using only one 6111 but decided on two, making the total complement 4 tubes for stereo operation, and seeing as I would like this portable, a 12V gel battery will power the heaters nicely.

Going by the data sheets I think I have everything close to right, although they could be totally wrong, anyone have any suggestions? Open for opinions on the HV supply from 12VDC.... I'm trying to follow the KISS principle on this one, no SS, single end. Nice and simple with reasonable sound......

Sorry about the photo, no scanner at the moment so used my camera...

Cheers Click the image to open in full size.
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Old 20th December 2011, 10:14 AM   #2
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Woops decided to change the cathode follower resistor to 18K making the bias -2.5 volts and the quiescent current through both tubes 2mA each, or 4mA in total. Plenty for driving those cables hey?
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Old 20th December 2011, 10:48 AM   #3
DF96 is offline DF96  England
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Have you calculated the voltage gain of this circuit? I suspect you will find it is somewhat high for a line stage. You may have trouble with grid current creating distortion, especially in the second stage.
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Old 20th December 2011, 11:11 AM   #4
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Hi DF96, yeah I was going to include a small feedback loop from the cathode follower back to the first grid to ease the amplification. Attached is a pic with the changes....What do you think? I have built just about everything else in the past on 12AX7 phono circuits, hence the high gain....Click the image to open in full size.
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Old 20th December 2011, 12:31 PM   #5
SY is offline SY  United States
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How much gain do you really need? You're burning off quite a bit and challenging stability by wrapping feedback around two gain stages. I suppose that makes phasing easier, but still...
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Old 20th December 2011, 12:55 PM   #6
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Yeah I suppose it would possibly make a nice phono amp at this stage with little modification.... What do you suggest SY? drop those anode values down a little?
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Old 20th December 2011, 01:02 PM   #7
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Well, start by defining what you want. Gain, voltage swings, output impedances. Then you can rationally make decisions about circuit design. It's always best to adjust your aim when you have an actual target.
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Old 20th December 2011, 02:02 PM   #8
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Haha yeah I'm a bit new to this, designing circuits from scratch is completely different to just wiring one up from a schematic. Please bear with me
Basically what I'm looking for is a small tube sounding in line preamp, that can safely satisfy relative high impedance input and a fair whack of current out. I thought I'd throw together a more simplified version, see attached. I'm using the 6112 for voltage gain and the 6111 as a cathode follower because of it's much higher current rating. I know you have a thing for CFs so If you don't like my way of doing it please offer a better solution. I personally think they have great potential.... even if I possibly have it clipping on the output sometimes due to poor design on my part.....Click the image to open in full size.
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Old 20th December 2011, 02:06 PM   #9
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I can honestly say that I don't recall seeing another tube line stage quite like it. The topology is more like what one would find in an low impedance feedback phono stage, or in a solid-state linestage.

By the way, John Broskie has an absolutely fantastic tube circuit design & education website. John has many great novice tutorials published there, including on cathode followers . He also offeres tube circuit CAD software, and sells inexpensive kits of many of the circuits presented. If you never have before encountered John's "Tube CAD Journal" website, and are interested in learning the basics of tube audio circuit design, you're in for a real treat.

http://tubecad.com/
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Last edited by Ken Newton; 20th December 2011 at 02:17 PM.
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Old 20th December 2011, 02:29 PM   #10
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Thanks for the link Ken, I guess you could say that it's unique! haha. Funny thing is I made a 12AX7 preamp several years ago using close to the same design and it sounded terrific. (to my ears anyway)
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