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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: 09 May 2010, 16:51 
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Joined: 25 Apr 2010, 03:14
Posts: 2
I believe you're measuring it at the wrong place. As it has been said, you have to measure the voltage on the Source pin of the Mosfet, which is connected to IN pin on the LM317. This connection is then connected to the + side of the output capacitor, so the easiest way to check the bias is to just measure the voltage on that capacitor (measure the + leg, the - one goes to the headphone. You can measure the - leg later for offset). You can see this in picture number 14, Setting the Bias.

Image


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PostPosted: 09 May 2010, 18:11 
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Joined: 13 Apr 2010, 10:39
Posts: 37
Location: Dallas, Texas, USA
I think you're testing the 'bias' at the tube's pins 6 and 1 (the plate).
That is where the 10k trimpot is located, but the measurement of voltage indicated (1/2 of input
voltage or around 6 or 6.3 volts...) is measured at the FET's Source pin, or alternately, between
the FET and the LM317's input, or pin 1.

I also got around 9.2V and couldn't get lower (at the plate!), But the thing you're looking for
is about 1/2 the input voltage there before the output capacitor (between the FET and the LM317).
I think that is so there is plenty of room to swing the output voltage before clipping, so putting it in the middle of the 12.6V range is ideal.

Mike Y
Dallas, Texas
mxweas wrote:
Roggom, I'm using 10k trim pots, should I be using 100k trim pots?

I've tried looking at the voltage on them and one side has constant 12.6v and this remains at 12.6v regardless of what the trim pots are set at. The other side however will decrease as I adjust the trim pot, but my 10k trimpot will only bring it down to 9.12v. What am I doing wrong?


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PostPosted: 09 May 2010, 20:38 
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Joined: 20 Apr 2010, 02:04
Posts: 13
OH! I completely missed that! Thanks for the info.

Max

EDIT: w00t! it sounds great! I still get clipping if I turn the volume completely up, but thats because my input source is fairly loud.

Thanks Roggom and everyone who helped me out! I love this thing!


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PostPosted: 09 May 2010, 21:28 
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Joined: 20 Apr 2010, 02:04
Posts: 13
http://twitpic.com/1ml43d

thought this was cool looking.


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PostPosted: 10 May 2010, 09:33 
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Joined: 27 Apr 2010, 08:57
Posts: 2
I finally got mine working pretty well. This is what I had to do (in case anyone else has similar issues).

1. I had to build a regulator. I tried using a LM7812 regulator, but it blew into three pieces on the third trial, as soon as I flipped the switch. To replace it, I built a board consisting of a NE5534 op-amp with a zener diode reference and a heatsinked TIP120 on the output (this probably isn't the ideal part, but it's all I had and it can handle a good bit of current). This eliminated the 60Hz noise I had before.
2. Because I am using 12AT7, I had to mess with the P1/R3 resistor values a bunch. In the schematic, a 4.7kohm resistor is specified. I slowly increased this, checking the voltage drop across it and the final output at the source of the mosfet, ending up with a 100k resistor in series with a 100k potentiometer (100k-200k total). This results in 400mV across the resistor and 6.8v at the source of the mosfet. It does clip at higher volumes, but not at normal listening volumes. Overall, I wouldn't recommend substituting a 12AT7 for the 12AU7 like I did. I got it to work in the end, but it's probably a lot more trouble.

I like the end result. It's absolutely silent when no sound is playing and I don't hear any noticeable distortion. Someone who has more experience with audio equipment might be able to nitpick little issues, but I can't hear the difference and it does exactly what I wanted it to do (acts as a high-impedance amplifier stage to my passive mixer).


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PostPosted: 10 May 2010, 23:22 
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Joined: 10 May 2010, 22:54
Posts: 1
Any reason not to substitute 1/4'' stereo jacks for the 3.5mm ones? Most of my headphones use 1/4'', and 1/8'' adapters are easy to come by for the ones that don't.

Also, if I drive this with the headphone output from an iPod, then I'll be putting the iPod's built-in headphone amp in series with this one. In that case, what's the best volume (% of max) to keep the iPod at (or is there one?) if I want to use a pot on the tube amp for volume? Are iPod/iPhone users here pulling a line out off the dock connector?


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PostPosted: 11 May 2010, 21:51 
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Joined: 28 May 2008, 21:53
Posts: 4578
Location: Winnipeg, CANADA
benmont, use whatever connection is most suitable for you.
Cheers

_________________
[ DIY Mains AC Power Cable Cord ] - [ Gobo LM1875 Amp Kit ] - [ Tang Band D4-1 Horn Speaker Kit ] - [ Monoblock Push-Pull KT88 Tube Amp Kit ]


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PostPosted: 13 May 2010, 01:35 
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Joined: 20 Apr 2010, 02:04
Posts: 13
Made another one today with spare parts from the first amp. It's much much cleaner!

http://twitpic.com/1ncjly
http://twitpic.com/1ncjmt

Max


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PostPosted: 13 May 2010, 09:52 
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Joined: 03 Aug 2008, 20:22
Posts: 386
Location: Denver, Colorado
Benmont,
As Gio stated you can use any connectors you wish. As a matter of fact, I used rca or 3.5mm input and 1/4 out on many of the older versions. This prevented me from accidentally hooking the amp output to the output of my sound card. I fried a sound card with the turn on thump in the earlier stages of the amp (had the am hooked up backward.)

Max,

Looks good,
Once you have the circuit down it can take on many shapes. If you use a 6111 submini tube it can even fit in a tin.

Rogers


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PostPosted: 15 May 2010, 13:29 
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Joined: 15 May 2010, 10:01
Posts: 6
Hey y'all

Me and a friend built this amplifier and it worked first time, but there is a horrible hum. To me it sounds like a 50Hz hum (we live in Europe) but we're not feeding 50Hz into the amp. Anyway, the volume control controls the level of the music but the hum is constant. It's the same on both our amps and on both channels. Anybody's got any idea of what's wrong? I have tried two different 12V sources and it's the same with both, I haven't tried a battery so far though.

Well, I sure would be thankful for some help, I really want a tube amp.

/Jonas


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