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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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It is currently 18 Dec 2018, 09:55

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PostPosted: 27 Mar 2018, 11:08 
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Joined: 23 May 2016, 19:39
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laurie54 wrote:
Kevin did you do your own silk-screen for the writing on the front cover or did you send out?

I tried out a kit called EZ-Screen. It’s great for small jobs like synth panels and stompboxes.


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PostPosted: 21 Apr 2018, 06:51 
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How do reduce the bias? Mine is running around 50mv instead of the recommended 41mv. Both OP tubes balance correctly but bias is high. Increase 15R resistor?


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PostPosted: 21 Apr 2018, 08:40 
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Location: Arizona, USA
Hi, That is how they are controlled. However, verify that the resistor is actually 15 ohms. The LM317s are "programmable" using this method and unless defective can't have that much of an error. Typical tolerance on a 317 is about 2% when used this way. Dumb question....is your meter accurate when making this measurement?

Good listening
Bruce

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PostPosted: 21 Apr 2018, 11:07 
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I used two different meters and both read the same so I am to assume they are correct. Are you saying to increase resistance or the bias controlled by the LM317? The resistors are correct 1% metal film, name brand from Mouser.


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PostPosted: 21 Apr 2018, 14:11 
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HI, Curious. With 15 ohms you should be close to 83 ma for the pair (41 each tube). Are the 1 ohm resistors correct? If it were my build....I would lift the ground side of the LM317 and measure the actual current flowing in ma. If it still shows high then try 18 ohms. I kind of wonder if the LM317s are either seconds or fakes. I can't imagine faking something so cheap though.

Good listening
Bruce

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PostPosted: 21 Apr 2018, 21:58 
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The 18 ohm resistors instead of 15 ohms worked just right. Bias is 41mv each tube. Thanks Bruce.


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PostPosted: 11 Jun 2018, 04:36 
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I have been following this thread with interest and see that many people are having trouble acquiring the 25 ohm cathode balance pot, at least here in Europe. I accidentally fell across this solution: two CCS one for each tubes. This eliminates the need for a balance pot.

Regards
Stig Hansen


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PostPosted: 11 Jun 2018, 10:31 
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Hi That can work, but CTS makes the control and places like Mouser carry it. Using the capacitors introduces another set of non-linear components and each one will in some way alter the sound. Plus it is impossible to get the balance exact with two CCS. Part of the high quality sound is related to the excellent performance at very low signal levels. The better the balance is the better that will be. The sub one watt range is where all the fine details occur in most music with typical speakers. In sensitive systems (like my present ones) this is actually in the 100milli watt and lower range. Without the tubes being really well balanced they will exhibit some differences in response. I grant that it may be small, but when you are building a moderately costly amp, the effort and cost to not get one component is IMO not a great idea. There is an alternative way to balance the tubes. See the attached schematic. Applying a small positive voltage to the grids can do it as well. This works fine for tubes that bias at moderate levels. The issue is that each volt added to the grids adds one to the cathodes and in turn raises the voltage across and dissipation of the CCS. 5-7 volts are normally OK in the Poddwatts. With the bigger amps you need to be cautious not to exceed the limits of the CCS and 3-5 volts are max IMO. There is a side benefit to adding a small positive voltage to the grids in the Poddwatts. If the voltage is the same on each then it will raise the cathode voltage as well. Using about 5 volts it will make sure the cathode voltage never drops below that level on signal swings. This in turn provides more headroom for the CCS and will keep it out of a potentially non-linear range. You will also gain about 1/2 of a watt output power. All late model Poddwatt DMBs have this modification. They still use the 25 ohm control as it is reliable and works perfectly for this application.

Good listening
Bruce
Attachment:
Odd-Block-Series-1A Bias Mod 4 Jan2010b.jpg
Attachment:
Poddwatt DMB Main Circuit August 25, 2013.jpg


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PostPosted: 01 Oct 2018, 14:52 
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Hi, this is my version of the PoddWatt with 4 EL84 and 2 5751.
The sound is excellent, with good bass reproduction and good positioning of the various instruments. With feedback, the frequency goes from approx. 10 Hz to 54 kHz (-3dB), without feedback approx. 20 Hz to 38 kHz (-3dB). I have chosen not to use feedback. I have chosen to use print cards, I think it makes it all the more manageable.
I highly recommend this amplifier.
Br. Stig Hansen.


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PostPosted: 01 Oct 2018, 20:59 
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Hi, Nice case and good looking build. Your data mirrors mine. Frequency response either with or without NFB is exceptional. It is there mostly to insure that some really odd speaker system (with wild impedance swings) will not somehow interact with the circuit resonance that typically occurs at about 65-70K HZ. Even then it is only about 3 db. Usually it is not needed and is rather transparent sonically. It is just enough as I don't like designs that have a lot of it. To my ears such designs do not have the quality of sound that ones that have little or no NFB. Naturally YMMV.

Good listening
Bruce

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