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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: 06 Jul 2010, 19:15 
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surface tension wrote:
I repurposed an IKEA Dokument magazine rack for the chassis

Fantastic work. The head amp looks great!
Cheers

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PostPosted: 06 Jul 2010, 20:07 
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Quote:
If that is all good, and you are using a 12au7 tube, try removing the bypass caps (c2). This action will lower the gain a bit. If all else fails try increasing the value of P1 in increments of 1000 ohms until you have a decent sound

Hi,
sound card is still good :D

I just add 10K trim in series with the 10K(P1).

I forgot to plug in the tube. I notice at 18V, my psu is showing 300mA current draw. this is normal? I assume the ccs at 125mA x2, my is running hotter?

thanks


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PostPosted: 06 Jul 2010, 20:48 
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I added the 10k trim. I adjusted the test point (FET output) down to 10.84V. very subtle difference but does sound slightly better.
I guess I need more resistance there. So for +B 18V, ideal is to have 9V at the Test Point?

I also added 221R to the gates of the 12au7.

I still hear quiet white noise when nothing is played but it's not that bad.


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PostPosted: 06 Jul 2010, 23:36 
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Location: Denver, Colorado
Try unplugging the input and grounding the input signal, just jumper across R1. If it goes quite, then the issue is before the grid of the tube. If you still have hiss, then it may be the fets, the tube, or a bad ground (solder joint)


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PostPosted: 17 Jul 2010, 16:00 
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I built this amp on protoboard, and it was working fine until I swapped tubes to a JJ Electronic from an Electro Harmonix. With the JJ in, the amp was clipping at low volume. I tried adjusting the pots, but to no avail. Now that I gave up on the JJ (which, I gather, has caused other people problems) and went back to the Electro Harmonix, the amp is still clipping. I am using 8 D-cells to power the amp. The voltage measurements on the positive side of the output cap are coming in at just under 6. I am attaching a picture of the clipping that I made using Audacity. Any suggestions?
Attachment:
Amp clipping-1.png

And I'm new to building, so don't be afraid to suggest the most basic fix.


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PostPosted: 18 Jul 2010, 20:36 
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Hi cmshanno, welcome to the forum.
What is the supply voltage with 8-D cells under load?
Cheers

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PostPosted: 19 Jul 2010, 11:54 
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I'm measuring 10.9 volts.


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PostPosted: 19 Jul 2010, 12:15 
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The voltage may be the problem. The heaters are rated for 12.6V with a tolerance of 10%. At 10.9V you may be a little lean and that may be the source of the problem. Can you try adding another D cell to bring the voltage up so it is between 12 and 12.6V. Then you will want to Set the Bias on the NP-100v12 - 12AU7 / IRF510 Head Amp.
Cheers

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PostPosted: 19 Jul 2010, 16:11 
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Thanks, Gio, that seems to have worked. It still clips with the source (iPod) all the way up, but not as bad. And I still can't get the JJ Electric tube to bias correctly—it's registering way above half of the supply voltage with the pots turned all the way to one side. Is there some way to fix this? Should I just add a resistor is series with the pot?

Thanks again!


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PostPosted: 19 Jul 2010, 18:08 
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So what is your supply voltage now?

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