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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: 23 Dec 2010, 13:14 
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Joined: 30 Nov 2010, 15:35
Posts: 22
leesam wrote:
Finally had a day off to put my amp together:

Setting the bias i maxed out the trimpots and just barely got it to 6.5V on both channels.

i had same problem. swap the tube.


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PostPosted: 24 Dec 2010, 02:24 
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Joined: 27 Nov 2010, 02:18
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Location: Arroyo Grande, CA, US
gofast wrote:
leesam wrote:
Finally had a day off to put my amp together:

Setting the bias i maxed out the trimpots and just barely got it to 6.5V on both channels.

i had same problem. swap the tube.

Also make sure you're measuring the BIAS at the correct place, the source of the FET, the positive side of the 470uF cap, _NOT_ at the pot/tube/gate node.


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PostPosted: 24 Dec 2010, 02:27 
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Joined: 27 Nov 2010, 02:18
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Location: Arroyo Grande, CA, US
gregn wrote:
Funny, I had the exact same approach with the connectors -- though I only made a connector from tube to main board.

I'm working on an expanded version of this amp, with a mixer and what-not, that will have the pots mounted directly on the board. I haven't decided what I'm going to do with the tube(s) on that one yet.


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PostPosted: 24 Dec 2010, 02:32 
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Joined: 27 Nov 2010, 02:18
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Location: Arroyo Grande, CA, US
Quick question. I'm trying to make sure I understand the math behind the DC blocking cap selection correctly. Please read this forum thread and check my math.

Short version: I'm calculating the input frequency response at .72Hz (yes, seven tenths of a hertz) and the output response at 6.8Hz into my 50ohm headphones. Does this sound right? Am I understanding and doing the math right?

If this is correct, then the "brightness" and lack of low end is NOT due to sizing of the DC blocking caps. Does anyone have any idea what might be causing the brightness?

Second question: Anyone see any reason not to drop R1 from 100kOhm (.72Hz response) to 10kOhm (7.2Hz response), lowering the impedance, drawing more power from the source, raising the SNR? In general, I try to keep my audio as low an impedance as possible for as long as possible to keep the noise down. Anyone see anything wrong with doing that here?


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PostPosted: 24 Dec 2010, 17:14 
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Joined: 17 Apr 2010, 16:07
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Location: Huddersfield ,West Yorkshire,UK
Merry Christmas everyone now get back to work !!

the other half has had me clearing up my wire snips before the relatives come round can you believe it plus i have had to remove the small pillar drill from the kitchen its so cold in the shed !!
Having said that shes has already opened 3 presents already :eek:

PS: i have been working on a Active Bass and Treble circuit as discussed in Roggoms passive tone controls tread it just about ready to let out the magic smoke on boxing day ;)

http://diyaudioprojects.com/Forum/viewtopic.php?f=10&t=358
http://www.sentex.ca/~mec1995/tutorial/xtor/xtor5/xtor5.html

Koogar

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PostPosted: 28 Dec 2010, 17:09 
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Joined: 22 Dec 2010, 18:18
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I have a question regarding the trim pots. I am having a bit of distortion and I am assuming it has to do with the mosfet's source voltage, which is 8.5. The trim pots cannot get it lower than this. A pot basically allows a gradient between pin 1 and 3. On one side we have 12.6 volts and on the other is the tube's pin #1.. which in my case I am seeing 12.5V. How can a potentiometer get 6V out of this? Shouldn't one side be grounded?

Also, should the ground pins from the audio jacks both be joined to the circuits ground?


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PostPosted: 28 Dec 2010, 19:40 
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Joined: 22 Dec 2010, 18:18
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gregn wrote:
I have a question regarding the trim pots. I am having a bit of distortion and I am assuming it has to do with the mosfet's source voltage, which is 8.5. The trim pots cannot get it lower than this. A pot basically allows a gradient between pin 1 and 3. On one side we have 12.6 volts and on the other is the tube's pin #1.. which in my case I am seeing 12.5V. How can a potentiometer get 6V out of this? Shouldn't one side be grounded?

Also, should the ground pins from the audio jacks both be joined to the circuits ground?

When both input and output ground are connected to DC ground, I only have the right channel on and it is pretty heavily distorted at higher volumes. However, if I tie together the input and output grounds and tie the DC ground to the inputs right wire I get pretty decent sound. I have checked over the whole circuit and cannot find any shorts. I definitely have too much voltage at the LM input @ 8.5. I am using a JJ ECC83 -- I'm going to try to put a higher resistance trim pot in there -- I don't think the 10k is enough.

Any suggestions?


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PostPosted: 28 Dec 2010, 22:26 
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leesam wrote:
Setting the bias i maxed out the trimpots and just barely got it to 6.5V on both channels.
gregn wrote:
I'm going to try to put a higher resistance trim pot in there -- I don't think the 10k is enough.

Yes, a higher trim pot is required for some tubes. This was identified a while back in this thread ( viewtopic.php?p=20345#p20345 ) but I did not get a chance to update the project page until just now - sorry for the delay. A 47k or 50k trim pot seems to work with a wide range of tubes.

The project page has been updated: http://diyaudioprojects.com/Solid/12AU7 ... 7-Headamp/
You may have to refresh / reload the page to see the changes.

Let us know how you make out with the larger trim pot.
Cheers

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PostPosted: 29 Dec 2010, 13:03 
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Joined: 22 Dec 2010, 18:18
Posts: 9
Wow... just a stupid ground problem for pin #3 on the tube. Switched out the 10k pots for 500k ones, getting exactly 6.3V at the mosfets. I am extremely happy how this sounds. I did point to point and I have no noise or hum. I am using an HP variable voltage supply currently until I put this into an enclosure.


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PostPosted: 03 Jan 2011, 11:31 
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Joined: 18 Sep 2010, 12:48
Posts: 81
What are the updates in the project page?
;)


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