'12AU7 amp - Schematic and MP3'
Author:DennisF (guest: search)
Date: Sun, Aug 22nd, 2004 @ 12:02 ( . )

Came across a old Heathkit signal generator that had a power transformer that provided about 270v B+ which is perfect for a 12AU7 output secton. This amp is essentially an 18 Watt Lite II coupled to a 12AU7 output section. It's different sounding than my Firefly, less compressed.
There are similar amps outthere like the Moonlight and Adam Alperns LunchBox but I really liked the feel of the parallel 12AX7 for the preamp. It's a neat experiment that's fun to play at less oppressive volumes.

Here are the schematic and an MP3-
[link]
[link]

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'12AU7 amp - Schematic and MP3'
Author:Chris Hurley (moderator: 7810 posts )
Date: Sun, Aug 22nd, 2004 @ 14:49 ( . )

On 08/22/2004 @ 12:02, DennisF wrote :
:Came across a old Heathkit signal generator that had a power transformer that provided about 270v B+ which is perfect for a 12AU7 output secton. This amp is essentially an 18 Watt Lite II coupled to a 12AU7 output section. It's different sounding than my Firefly, less compressed.
: There are similar amps outthere like the Moonlight and Adam Alperns LunchBox but I really liked the feel of the parallel 12AX7 for the preamp. It's a neat experiment that's fun to play at less oppressive volumes.
:
: Here are the schematic and an MP3-
: [link]
: [link]
--


I did this kind of stuff for some reason... :)

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'12AU7 amp - Schematic and MP3'
Author:Brendan (guest: search)
Date: Sun, Aug 22nd, 2004 @ 21:14 ( . )

what voltage do you have on the 12au7 cathode

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'12AU7 amp - Schematic and MP3'
Author:DennisF (guest: search)
Date: Mon, Aug 23rd, 2004 @ 06:48 ( . )

10.15 on the cathode, 269 on the plates with a 560 ohm cathode resistor.

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'12AU7 amp - Schematic and MP3'
Author:Phil Rowley (guest: search)
Date: Mon, Aug 23rd, 2004 @ 08:39 ( . )

Dennis, does this amp clean up at all? The sound clip seemed to be all dirty sounds. Obviously we're talking about home usage, so it doesn't need to be very loud...

On 08/23/2004 @ 06:48, DennisF wrote :
:10.15 on the cathode, 269 on the plates with a 560 ohm cathode resistor.
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'12AU7 amp - Schematic and MP3'
Author:Doug H (registered user: 5400 posts )
Date: Mon, Aug 23rd, 2004 @ 09:15 ( . )

Sounds really great, Dennis!! Very "chewy"!

Sounds like you had good success with a full-blown PI-driven PP approach with 12au7's. I'm curious how well it cleans up too.

I was thinking of trying this using a wreck preamp- kind of a "micro-wreck" idea...

Doug

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'12AU7 amp - Schematic and MP3'
Author:Quayhog (guest: search)
Date: Mon, Aug 23rd, 2004 @ 11:41 ( . )

What spec power trannie or or brand did you use? What voltages did you record at the plates at all stages?
I looks real neat, bet it sounds good too.
Jonathan Horne
jhorne@izap.com

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'12AU7 amp - Schematic and MP3'
Author:DennisF (guest: search)
Date: Mon, Aug 23rd, 2004 @ 16:51 ( . )

I will try to record this thing with lower gain settings it does clean up like you would expect with an 18 watter. As you can tell that MP3 was pretty full bore and "chewy" as Doug says. I love the Micro-Wreck idea. I was also thinking an Orange AD-15 would be cool too.
I will repost the schematic with voltages. The power tranny came from a Heathkit signal generator here's the schematic for that:
[link]
The transformer is labelled as 117v AC in 117v AC out. I used the voltage doubler that existed and built from there. I think I used a Hammond 269EX for my Firefly build that would work too. I will double check that model number tonight.

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'12AU7 amp - Schematic and MP3'
Author:Doug H (registered user: 5400 posts )
Date: Mon, Aug 23rd, 2004 @ 20:49 ( . )

On 08/23/2004 @ 16:51, DennisF wrote :
:I will try to record this thing with lower gain settings it does clean up like you would expect with an 18 watter.
--

That's pretty exciting.;-)

I drew a load line for a PP 12au7 stage with Q point at 280v/2.5ma (!). I get about 280v out of the 207vac surplus transformers I use and the 2.5ma looked pretty reasonable for class AB. Anyway, I was surprised to see the grid voltage down around -13v to -14v. With 2 tubes in push-pull you get between 20-30v headroom which is not too shabby, IMO. It's kind of surprising for the 12au7, but that's the way it works out with its characteristic curve. Incidentally, power output was around 1.1W.


:As you can tell that MP3 was pretty full bore and "chewy" as Doug says. I love the Micro-Wreck idea. I was also thinking an Orange AD-15 would be cool too.
--

Absolutely. I want toy with the Orange preamp at some point too. As for the Wreck, I was thinking the flatter freq resp of the 12au7's would make a good foil for a brighter preamp like that. Might work well for a Vox Top Boost too. With the Orange the 12au7's might help it stay nice and "foggy".;-)


Doug


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