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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 25 Apr 2010, 22:55 
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Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
I like it, This amp can take on so many looks. It also makes you look at ordinary household items in a new light. Hmm what can I make this into.

You can upgrade the caps if you wish, I do not see the need for a very expensive capacitor in this amplifier as it is not operating in its most optimum range. A nice Wima or Solen will do the trick for the input and I like Nichicon Muse for electrolytic. Give it a listen, and remember the sound will change in a few weeks as the capacitors "burn in". It will get a bit warmer in sound.


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PostPosted: 25 Apr 2010, 23:34 
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Joined: 20 Apr 2010, 13:29
Posts: 8
mxweas wrote:
Made an enclosure for my amp today out of an old car stereo face case:
http://twitpic.com/1ij8ml
http://twitpic.com/1ij8mj

That is beautiful!


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PostPosted: 26 Apr 2010, 01:03 
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Joined: 06 Jun 2008, 18:23
Posts: 5115
Location: Australia
mxweas wrote:
Made an enclosure for my amp today out of an old car stereo face case:
http://twitpic.com/1ij8ml
http://twitpic.com/1ij8mj

Which poor harp gave up his home for the amp? Novel idea though.

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Projects:"retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp | ”najah” - Raw 180W Tripath Class D power amp | "Icon" - Shuguang CV-181Z preamp


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PostPosted: 26 Apr 2010, 21:50 
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Joined: 17 Apr 2010, 10:12
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After listening to the amp for awhile, it sounds abit bright for me. Any thing I can change other than tube rolling to maybe make it more mellow?


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PostPosted: 26 Apr 2010, 22:46 
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Joined: 13 Apr 2010, 10:39
Posts: 37
Location: Dallas, Texas, USA
keechoon wrote:
After listening to the amp for awhile, it sounds abit bright for me. Any thing I can change other than tube rolling to maybe make it more mellow?

I agree - but I changed phones and it's less so. Very interesting little amp.

I was using some cheap lil' Sennheiser CX-300 earbuds - I guess I just don't like earbuds - because they were 'bright' in the extreme.

Now I've switched to some Sennheiser PX-200's and the audio is superb - the 'distribution' to the highs seems to be moderated. I have some nicer Sony's to try...

Not being an expert (or even 'high' intermediate) audio person, I'm trying different types of songs / genres from Dixie Chicks "Brilliance" (old Dixie Chicks - before they were popular), The Doors, Beatles, Death Cab for Cutie, John Lee Hooker .... so, gosh, looks I have a few more hours and a couple of adult beverages to work on it!!!

I built a 'companion' power supply for it: an 18V / 2A laptop supply (in it's former life), and essentially the power regulator and filter that's in the Millet Max - no more hum.

Mike Y
Dallas, Texas


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PostPosted: 27 Apr 2010, 03:21 
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Location: Australia
If it is bass shy it may sound bright. How is the bass??

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PostPosted: 27 Apr 2010, 09:50 
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Joined: 27 Apr 2010, 08:57
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I'm building this amp (my first tube project) and I get sound out of it, but I think there's some kind of problem with the biasing of the tube.
I'm using a 12AT7 instead of the recommended 12AU7 because it was easier for me to get. I'm using a 100 ohm resistor for R2 and I switched to a P1 10k potentiometer after noticing poor performance with the fixed 4.7kohm R3 resistor.

I have added a passive mixer stage that consists of two 100k log pots, each with one end connected to an input and the other to ground, with the wipers connected to the input coupling capacitors (one for each input). I added 100kohm resistors in series with each capacitor, which are connected together at the grid of the tube. I had originally raised the R1 resistor to 1Mohm because I wanted to limit the attenuation of this stage to 1/2 instead of 1/3, but I reduced that it to 100kohm because it was causing problems.

I also added two 1N4002 diodes in series to and from ground after the output coupling capacitors to limit voltage during testing. I think this should be enough to deafen me before it starts clipping, as the diodes should have a voltage drop of 1.6-2.0V and specs on my headphones say the sensitivity is ~100dB/V.

After a few adjustments (setting P1 to 10kohm, reducing R1 to 100kohm) it sounds very good at low volumes. There's some 60Hz hum, but I think that it's from poor regulation on the power supply I'm using (an old AT form factor computer PSU). However, at higher volumes I notice some distortion and clipping, with it being considerably worse at low values of P1/R3. The voltage at the source of the MOSFET is ~9V and the voltage across P1 very low (I think about 400mV). The weirdest part is the voltage across R1, about -360mV. 360mV across the 100kohm R1 resistor means that there is 3.6uA leaking through the grid. Is this in the normal range? I think that this bias at the grid, combined with the increased sensitivity of the 12AT7 might be causing my problem.

Any suggestions would be appreciated. :)


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PostPosted: 27 Apr 2010, 17:27 
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A 12AT7 is going to push the gain to about 20. Also I'm not sure how well that tube works at low voltage.

You will need to used a very clean supply or you will get noise. Also, the high gain of the 12AT7 is going to amplify the noisefloor of your source.

Find a 12AU7 ;)

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PostPosted: 28 Apr 2010, 08:57 
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Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
keechoon wrote:
After listening to the amp for awhile, it sounds abit bright for me. Any thing I can change other than tube rolling to maybe make it more mellow?

Give the capacitors a chance to burn in, the sound should change to be a bit warmer after some hours of play time. If you are at home on any particular day, just leave the amp on while you are at home.

If the amp remains very bright, you can change the input cap from 2.2uF down to 1.0uF, then to .47uF and so on.

Rogers


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PostPosted: 28 Apr 2010, 13:04 
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Joined: 28 Apr 2010, 12:20
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Hi,
I am planning on buing an electric guitar in a week or so and i need some cheap preamp just to hear some sound. I did some thinking and realised that chip amps aren't the best way for me since i am a bit of an audiofile. So I ran into this project and all the parts are avilable in my area.

So I wanted to ask you guys if this preamp is usable as a guitar preamp. (I am planning on feeding it into my PC speakers/livingroom reciever/60 ohm portapros)

Thanks


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